HOMER INTERACTING PROTEINS

Abstract: A method is provided for identifying a compound that modulates a cellular response associated with Homer and mediated by a cell-surface or an intracellular receptor. A method is further provided for identifying a compound that modulates receptor activated calcium mobilization associated with Homer. A method is provided for identifying a compound that inhibits Homer protein activity based on the crystal structure coordinates of Homer protein binding domain. A method is also provided for identifying a compound that affects the formation of cell surface receptors into clusters. Also provided are nucleic acids encoding Homer proteins as well as Homer proteins, and Homer interacting proteins.

Description: HOMERINTERACTING PROTEINS STATEMENT AS TO FEDERALLY SPONSORED RESEARCH This invention was made with Government support under Grant No. RO1 DAI 0309, RO1 DAI 1742 and KO2 MH01152, awarded by the National Institutes of Health. The government may have certain rights in the invention. FIELD OF THE INVENTION The present invention relates generally to protein-protein interactions and more specifically to molecules involved in mediating receptor-activated or ion channel- mediated intracellular calcium mobilization or concentration. BACKGROUND OF THE INVENTION The mature central nervous system exhibits the capacity to alter cellular interactions as a function of the activity of specific neuronal circuits. This capacity is believed to underlie learning and memory storage, age-related memory loss, tolerance to and dependence on drugs of abuse, recovery from brain injury, epilepsy as well as aspects of postnatal development of the brain (Schatz, C, Neuron, 5:745, 1990). Currently, the role of activity-dependent synaptic plasticity is best understood in the context of learning and memory. Cellular mechanisms underlying activity-dependent plasticity are known to be initiated by rapid, transmitter-induced changes in membrane conductance properties and activation of intracellular signaling pathways (Bliss and Collingridge, Nature, 361 :31. 1993). Several lines of evidence also indicate a role for rapid synthesis of mRNA and protein in long-term neuroplasticity. For example, classical studies of learning and memory demonstrate a requirement for protein synthesis in long-term, but not short-term memory (Flexner, et al, Science, 141:57, 1963; Agranoff, B., Basic Neurochemistry, 3rd Edition, 1981; Davis and Squire, Physiol. Bull, 96:518, 1984), and long-term enhancement of synaptic connectivity, studied in cultured invertebrate neurons (Montarolo, et al, Science, 224:1249, 1986; Bailey, et al, Neuron, 9:749, 1992) or in the rodent hippocampus (Frey, et al, Science, 260:1661. 1993; Nguyen, et al, Science, 265:1104. 1994), is blocked by inhibitors of either RNA or protein synthesis. Importantly, inhibitors of macromolecular synthesis are most effective when administered during a brief time window surrounding the conditioning stimulus indicating a special requirement for molecules that are rapidly induced (Goelet, et al, Nature, 222:419, 1986). Immediate early genes (IEGs) are rapidly induced in neurons by neurotransmitter stimulation and synaptic activity and are hypothesized to be part of the macromolecular response required for long-term plasticity (Goelet, et al, supra; Sheng and Greenberg, Neuron, 4:477, 1990; Silva and Giese, Neurobiology, 4:413, 1994). To identify cellular mechanisms that may contribute to long-term plasticity in the vertebrate brain, differential cloning techniques have been used to identify genes that are rapidly induced by depolarizing stimuli (Nedivi, et al, Nature, 2(_3:713, 1993; Qian, et al, Nature, 261:453, 1993; Yamagata, et al, Neuron, 11:371, 1993; Yamagata, et al, Learning and Memory 1:140, 1994; Yamagata, et al, Journal of Biological Chemistry, 262:16333, 1994; Andreasson and Worley, Neuroscience, 69:781, 1995; Lyford, et al, Neuron, 14:433, 1995). In contrast to the earlier focus on transcription factors, many of the newly characterized IEGs represent molecules that can directly modify the function of cells and include growth factors (Nedivi, et al, supra; Andreasson and Worley, supra ), secreted enzymes that can modify the extracellular matrix, such as tissue plasminogen activator (Qian, et al, supra), enzymes involved in intracellular signaling, such as prostaglandin synthase (Yamagata, et al, supra), and a novel homolog of H-Ras, termed Rheb (Yamagata, et al, supra), as well as a novel cytoskeleton-associated protein, termed Arc (Lyford, et al, supra). The remarkable functional diversity of this set of rapid response genes is representative of the repertoire of cellular mechanisms that are likely to contribute to activity-dependent neuronal plasticity. Pharmaceutical agents often act by modulating signaling between cells or within cells. For example, Prozac alters the reuptake of the neurotransmitter serotonin and enhances aspects of its signaling function in brain. Nonsteroidal antiinflammatory drugs (NSAIDs) act by inhibiting the activity of cyclooxygenase enzyme, which is involved in the signaling pathways of inflammation. Viagra modifies the intracellular guanylate cyclase response to autonomic neurotransmitters in erectile tissues. These, and other precedent setting pharmaceuticals, validate the notion that specific signaling pathways may be targeted for therapeutic development. Cellular mechanisms that modify important intracellular signals can involve changes in intracellular calcium. This type of mechanism is used in brain neurons to adapt to changes in intercellular signaling, and is demonstrated to exert powerful effects on cellular responses induced by glutamate. Similar, though distinct, cellular mechanism may be used to modulate intracellular calcium signals in other tissues including heart, lung, liver and skeletal muscle. Compounds that can modify this mechanism can modulate natural transmitter signals and may exert therapeutic effects. Classical studies demonstrated that activation of receptors on the cell surface evoke changes in the level of specific, diffusable molecules inside the cell. The regulated production of these molecules serves to signal events happening at the membrane surface to intracellular receptors and are therefore termed second messenger signaling pathways. Major second messenger pathways include the phosphoinositide pathway, which regulates intracellular calcium; the adenylate cyclase pathway, which regulates levels of cyclic AMP; the guanylate cyclase pathway, which regulates levels of cGMP; and the nitric oxide pathway which regulates NO. The regulated release of intracellular calcium is essential to the function of all tissues. Each tissue possesses a distinct physiology that is dependent on receptor/transmitter-regulated release of intracellular calcium. For example, synaptic function is modulated in brain neurons by glutamate receptor regulated release of intracellular calcium. Contractility of cardiac and smooth muscle is also regulated by intracellular calcium. Recent reviews of the role of calcium signaling in cellular responses include: Berridge, Nature 386:759 (1997); Berridge, J. Physiol. (London) 499:291 (1997); Bootman et al, Cell 21:367 (1997). Recent studies demonstrate that molecules that function together in signaling networks are frequently clustered together in macromolecular complexes. For example, components of the MAP kinase pathway form a complex of cytosolic kinases with their specific substrates (Davis, Mol. Reprod. Dev. 42:459 (1995)). Similarly, proteins such as AKAP function as scaffolds for specific kinases and their substrates (Lester and Scott, Recent Prog. Horm. Res. .52:409 (1997)). Recently, a multi-PDZ containing protein was identified in Drosophila (termed InaD) that couples the membrane-associated, light-activated ion channel with its effector enzymes (Tsunoda et al, Nature 388:243 (1997)). The biochemical consequence of this clustering is that the local concentrations of molecules that convey the signals between proteins are as high as possible. Consequently, signaling takes place efficiently. The clustering activity of these proteins is essential to normal function of the signaling cascade (Lester and Scott, supra 1997; Tsunoda et al, supra 1997). Accordingly, Accordingly, agents that alter these signaling complexes will modify the response due to transmitter or other form of cellular stimulation in a way that mimics more classical receptor agonists or antagonists. For example, a metabotropic glutamate receptor signaling may be blocked either at the receptor by conventional receptor antagonists or by uncoupling the metabotropic receptor from its intracellular IP3 receptor by agents that block the cross-linking activity of Homer family proteins. The identification of molecules regulating the aggregation of neurotransmitter receptors at synapses is central to understanding the mechanisms of neural development, synaptic plasticity and learning. The most well characterized model for the synaptic aggregation of ionotropic receptors is the neuromuscular junction. Early work showed that contact between the axon of a motor neuron and the surface of a myotube rapidly triggers the accumulation of preexisting surface acetylcholine receptors (Anderson and Cohen, J Physiol 2 .:1S1-112>, 1977; Frank and Fischbach, J Cell Biol 83:143-158, 1979). Subsequent work has shown that agrin, a complex glycoprotein secreted by the presynaptic terminal, activates a postsynaptic signal transduction cascade (reviewed by Colledge and Froehner, Curr Opin Neurobiol 8:357-63, 1998), that leads to receptor clustering by the membrane associated protein rapsyn. SUMMARY OF THE INVENTION Homer proteins, the products of neuronal immediate early genes, selectively bind the carboxy-termini of certain cell-surface receptors (e.g., group 1 metabotropic receptors), certain intracellular receptors and binding proteins (e.g., inositol trisphosphate receptors, ryanodine receptor, Shank proteins, 142). Many forms of Homer proteins contain a "coiled-coil" structure in the carboxy-terminal domain which mediates homo- and heteromultimerization between Homer proteins. The present invention is based on the seminal discovery that Homer plays a significant role in mediating receptor-activated calcium mobilization from internal stores and that Homer proteins regulate aspects of receptor clustering In one embodiment, a method is provided for identifying a compound that modulates a cellular response mediated by a cell-surface receptor. The method includes incubating a test compound and a cell expressing a cell-surface receptor and a Homer protein under conditions sufficient to permit the compound to interact with the cell, and exposing the cell to a cell-surface receptor ligand. A cellular response to the ligand by the cell incubated with the compound is compared with a cellular response of the cell not incubated with the compound wherein a difference in cellular response identify a compound that modulates a Homer-associated cellular response. In another embodiment, a method is provided for identifying a compound that modulates a cellular response mediated by an intracellular receptor. The method includes incubating the compound, and a cell expressing an intracellular receptor and a Homer protein under conditions sufficient to permit the compound to interact with the cell and exposing the cell to conditions that activate the intracellular receptor. A cellular response by a cell incubated with the compound is compared with a cellular response of a cell not incubated with the compound wherein a difference in a cellular response identifies a compound that modulates a Homer-associated cellular response. In yet another embodiment, a method is provided for identifying a compound that modulates receptor activated calcium mobilization in a cell. The method includes incubating the compound and a cell expressing a Homer protein under conditions sufficient to permit the compound to interact with the cell and exposing the cell to conditions sufficient to activate calcium mobilization. The receptor-activated calcium mobilization of a cell incubated with said the compound is compared with the receptor-activated calcium mobilization of a cell not incubated with the compound wherein a difference in calcium mobilization is indicative of an effect of the compound on Homer-associated calcium mobilization. In another embodiment, a method is provided for modulating receptor- mediated calcium mobilization. The method includes exposing a cell expressing Homer protein to a compound in a sufficient amount to modulate the calcium mobilization that typically occurs when a cell is exposed to an amount of ligand sufficient to activate an intercellualr signaling pathway that includes Homer protein. In another embodiment, a method is provided for identifying a compound that inhibits Homer protein activity. The method includes identifying an inhibitor of Homer binding or crosslinking activity and identifying an inhibitor of Homer protein activity that forms covalent or non-covalent bonds with amino acids in a Homer protein binding site, based upon the crystal structure coordinates of Homer protein binding domain, and synthesizing the inhibitor. In one embodiment, a method is provided for identifying a compound that affects the formation of cell surface receptors into clusters. The method includes incubating the compound and a cell expressing a Homer protein and a Homer interacting protein, e.g., a Shank protein, under conditions sufficient to allow the compound to interact with the cell and determining the effect of the compound on the formation of cell-surface receptors into clusters. The formation of cell-surface receptors into clusters of a cell contacted with the compound is compared to the formation of cell-surface receptors into clusters of a cell not contacted with the compound, wherein a difference in the formation of clusters is indicative of a compound that affects formation of cell surface receptors into clusters. In another embodiment, a method is provided for treating a disorder associated with glutamate receptors, including metabotropic and NMDA-type glutamate receptors, in a subject. The method includes administering to a subject in need, a therapeutically effective amount of a compound that modulates Homer protein activity. In another embodiment, a method is provided for treating a disorder associated with Homer protein activity including administering to a subject in need a therapeutically effective amount of a compound that modulates Homer protein activity. The compound may be identified by a method of the invention described herein. In another embodiment, there is provided an isolated nucleic acid encoding Homer protein lb, having the nucleotide sequence as set forth in SEQ ID NO:3 as well as an isolated Homer protein having substantially the same amino acid sequence as set forth in SEQ ID NO:4. In another embodiment, there is provided an isolated nucleic acid encoding Homer protein lc, having the nucleotide sequence as set forth in SEQ ID NO: 5 as well as an isolated Homer protein having substantially the same amino acid sequence as set forth in SEQ ID NO:6. In another embodiment, there is provided an isolated nucleic acid encoding Homer protein 2a, having the nucleotide sequence as set forth in SEQ ID NO: 7 as well as an isolated Homer protein having substantially the same amino acid sequence as set forth in SEQ ID NO: 8. In another embodiment, there is provided an isolated nucleic acid encoding Homer protein 2b, having the nucleotide sequence as set forth in SEQ ID NO:9 as well as an isolated Homer protein having substantially the same amino acid sequence as set forth in SEQ ID NO: 10. In another embodiment, there is provided an isolated nucleic acid encoding Homer protein 3, having the nucleotide sequence as set forth in SEQ ID NO: 11 as well as an isolated Homer protein having substantially the same amino acid sequence as set forth in SEQ ID NO: 12. In another embodiment, there is provided an isolated peptide having the amino acid sequence set forth in SEQ ID NO: 13 an isolated peptide having the amino acid sequence set forth in SEQ ID NO: 14. In yet another embodiment, there is provided an isolated nucleic acid encoding Homer Interacting Protein, having the nucleotide sequence as set forth in SEQ ID NO: 15 or 17 with a deduced amino acid sequence as set forth in SEQ ID NO: 16 or 18, respectively. In another embodiment, there is provided an isolated Homer Interacting Protein having substantially the same amino acid sequence as set forth in SEQ ID NO: 19. In another embodiment, there is provided an isolated Homer Interacting Protein having substantially the same amino acid sequence as set forth in SEQ ID NO:20. In yet a further embodiment, there is provided a substantially purified polypeptide containing a proline rich region that is specifically capable of specifically binding to polypeptides of the Homer family. In still another embodiment, there is provided a transgenic non-human animal having a transgene that expresses a Homer protein, e.g., Homer la, chromosomally integrated into the germ cells of the animal. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. Schematic Representation of EVH1 Domain-containing Proteins. EVH1 domains are found at or near the N-termini of Homer, Ena, Mena, VASP, and WASP proteins. Homer lb/2/3 encode a CC domain which mediates multimerization between various Homer proteins. In ENA, Mena, VASP, WASP, and N-WASP, the EVH1 domain is followed by a central proline rich region of variable length. The proteins are drawn to the scale shown, and the respective amino acid lengths are shown at the right. Figure 2. Structure-Based Alignment of EVH1, PH, and PTB Domain Sequences. A structure-based sequence alignment between EVH1 domains and the β- spectrin PH domain and the IRS-1 PTB domain is shown. Species are indicated by Rn (rat), Hs (human), Mm (mouse), and Dm (Drosophila). Elements of the Homer EVH1 domain secondary structure are represented by arrows (β-strands), cylinders (α-helices), and lines (coils). Conserved residues (among EVH1 domains) are highlighted. The fractional solvent accessibility (FAS) of each residue in Homer la is indicated by ovals. Filled ovals = O≤FAS≤O.l (buried); shaded ovals = 0.1 <FAS<0.4 (partially accessible); open ovals = FAS> 0.4. Mutations in the EVH1 domain of the WASP gene are indicated in lower case letters below the WASP amino acid sequence. Mutations that are associated with the severe WAS phenotype are show in bold letters (Zhu et al, 1997). Sites mutated to more than one residue are indicated by asterisks. Bold asterisks indicate residues that, when mutated, affect the interaction of WASP with WIP (Stewart et al., 1999). Residues of Homer, β-spectrin, and IRS-1 that align well following structural superposition and were used to calculate rms differences in Cα positions between these domains are underlined in the IRS-1 sequence. Gaps are indicated by dashes while continued sequences at amino- and carboxy-termini are indicated by periods. Residue numbering for Homer la is shown above its amino acid sequence. The number of the last included residue of each protein is shown at the end of each row. Figure 3. Ribbon Diagram of the Homer la EVH1 Domain. The amino and carboxy termini are indicated, and elements of secondary structure are labeled to correspond to homologous structures in PH and PTB domains. An additional short region of β-strand between βl and β2 has been labeled βi. Figure 4. Structural Comparison of EVH1, PH, and PTB Domains. Ribbon diagrams (A)-(C) and surface representations (D)-(F) of the Homer 1 EVH1, β- spectrin PH, and IRS-1 PTB domains, respectively, are shown. All molecules are shown in a similar orientation, which is rotated about 45° about the vertical axis from orientations shown in Figure 3. The β-spectrin PH domain is shown with bound inositol trisphosphate (Hyvonen et al, 1995). The IRS-1 domain is shown complexed to a phosphotyrosine-containing peptide derived from the insulin receptor (ECk et al, 1996). Figure 5. Versatile Ligand Recognition by PH-Like Domain. Sterodiagram of a backbone trace of Homer 1 EVHl doamin showing the relative positions of IP3 as bound by the β-spectrin and PLC-δ PH domains, as well as the peptide ligands for the IRS-1 and Numb PTB domains is shown. The orientations of the EVHl domain is similar to that in Figure 4. Ligand positions were determined by superimposing the backbone traces of the EVHl, PH and PTB domains in the program ) (Jones et al., 1991). Figure 6. Mapping of WAS-Causing and Homer Binding Mutations on the EVHl Surface. (A) and (B) Surface representations of the Homer 1 EVHl doamin with sites homologous to positions of WASP mutations (in parentheses) colored according to solvent accessibility. Solvent exposed residues are shown in magenta, and buried or partially buried residues are shown in blue. Residue assignments are based on the sequence shown in Figure 2. WASP EVHl mutations are listed in Table 2. Surface representations of Homer 1 EVHl domain showing the location of residues targeted by site-directed mutagenesis. Mutations that disrupt binding of Homer EVHl to ligands in an in vitro binding assay are shown in red, while those that have no effect on binding are shown in light blue (see Table 3). The orientation of the EVHl domain in panels A and C is identical to that in Figure 4A and D. IN panesl B and D,. the moleucle is rotated about 180 degrees about the vertical axis. Figures 7 through 47 are described in the following table. Figurecs Homer Family Proteins and Homer Interacting Proteins DETAILED DESCRIPTION OF THE INVENTION Homer represents a family of proteins that selectively binds the carboxy- terminus of group 1 metabotropic receptors and is enriched at excitatory synapses (Brakeman et al, 1977). In the adult brain, Homer is rapidly and transiently induced by physiological synaptic stimuli that evoke ion-term potentiation in the hippocampus (Brakeman et al, 1997; Kato et al, 1997), and is also induced in the striatum by dopaminetic drugs of addiction (Brakeman et al, 1997). The first Homer gene identified, now termed Homer la (Brakeman et al, Nature 38(5:2284-288 (1997); GenBank Accession No. U92079), is a member of a family of closely related Homer proteins that are constitutively expressed in brain (Kato et al, 1998; Sun et al, 1998; Xiao et al, 1998). There are now three mammalian genes identified and at least six distinct transcripts expressed in brain (Xiao et al, 1998). All Homer family members, including Homer la, contain an amino-terminal region of about 110 amino acids that binds metabotropic glutamate receptors la and 5 (mGluRla and mGluR5) (Xiao et al, 1998). The region of Homer that interacts with mGluRla or 5 is termed "EVHl domain", based on homology to similar domains in a family of proteins that include Drosophila Enabled (Gertler et al, 1996), mammalian VASP (Haffner et al, 1995) and the Wescott-Aldrige protein (WASP) (Ponting and Phillips, 1997; Symons et al, 1996). The EVHl domain of Homer is conserved at a level of about 80% between Drosophila, rodent and human (Xiao et al, 1998) The Homer family EVHl domain also can bind to intracellular receptors such as the inositol trisphosphate receptor and dyamin III. Binding of Homer proteins in the EVHl region is mediated by an amino acid sequence motif that is rich in proline residues. To explore the proline-rich motif and its role in Homer interactions, a deletion mutation strategy was used. A 50-amino acid deletion at the carboxy-terminal end of mGluR5 destroyed binding to Homer. By contrast, a 41 amino acid deletion of mGluR5 retained full binding activity. The intervening sequence is proline rich and shares sequence similarity with the previously described SH3 ligand sequence (Yu, 1994 ). A series of point mutants based on the known structure-function relationship for SH3 ligands was prepared and binding assays confirmed general characteristics of SH3 ligand binding, but also demonstrated that that the Homer binding site is distinct in the positioning of critical amino acids (Tu et al, 1998). A consensus for binding was determined to be PPXXFR, consistent with the observation that mutation of either of the proline residues or the phenylalanine, or a change in their relative position, interrupted binding. The arginine in the last position was preferred over other tested amino acids, but is not essential. Mutations were identically effective in interrupting binding to each of the Homer family members including Homer la, Vole, 2a/b, 3 and an EVHl fragment (110 amino acids) of Homer 1. Thus, it was concluded that the interaction with mGluR5 was mediated by the Homer EVHl domain. To further explore Homer binding, mutations of mGluR5 were tested using a 250 amino acid carboxy-terminal fragment of the receptor, which had an identical effect on binding when placed in the full length mGluR5 protein (Tu et al, 1998). This exquisite sensitivity of Homer binding to changes in single amino acids within the Homer-ligand site was confirmed in other Homer-interacting proteins including mGluRla (Tu et al, 1998), Shank (Tu et al, 1999), and 142 (see below). To further confirm that the interaction was mediated by a direct interaction at the Homer-ligand site (as opposed to a secondary allosteric effect on a remote binding site), synthetic 10-mer peptides with either the wild type, or F-to-R mutation were prepared. The wild type peptide blocked binding of mGluRla or mGluR5 to each of the Homer family members (Tu et al, 1998). Approximately half of the binding was blocked at a peptide concentration of 3.4 micromolar. By contrast, the F-to-R mutant peptide did not alter binding at concentrations as high as 340 micromolar. Most forms of Homer protein encode a carboxy-terminal domain with a "coiled-coil" structure. This coiled-coil domain mediates homo- and heteromultermization between Homer proteins (Kato et al, 1998; Xiao et al, 1998) and such multimers can be identified in normal brain tissue (Xiao et al, 1998). Homer proteins are enriched in brain tissue fractions from postsynaptic densities and are localized at the ultrastructural level to postsynaptic densities. Homer la differs from the other members of the Homer family in that Homer la is not constitutively expressed and it does not contain a carboxy terminal coiled-coil domain. Experimental data showing that Homer proteins interact with cell-surface receptors and with intracellular receptors, and form multimeric complexes with other Homer proteins indicates an important role for Homer proteins in intracellular signaling. An exemplary polynucleotide encoding a Homer protein is set forth as SEQ ID NO: 1. The term "polynucleotide", "nucleic acid", "nucleic acid sequence", or "nucleic acid molecule" refers to a polymeric form of nucleotides at least 10 bases in length. By "isolated polynucleotide" is meant a polynucleotide that is not immediately contiguous with both of the coding sequences with which it is immediately contiguous (one on the 5' end and one on the 3' end) in the naturally occurring genome of the organism from which it is derived. The term therefore includes, for example, a recombinant DNA which is incorporated into a vector; into an autonomously replicating plasmid or virus; or into the genomic DNA of a prokaryote or eukaryote, or which exists as a separate molecule (e.g., a cDNA) independent of other sequences. The nucleotides of the invention can be ribonucleotides, deoxyribonucleotides, or modified forms of either nucleotide. A polynucleotide encoding Homer includes "degenerate variants", sequences that are degenerate as a result of the genetic code. There are 20 natural amino acids, most of which are specified by more than one codon. Therefore, all degenerate nucleotide sequences are included in the invention as long as the amino acid sequence of a polypeptide encoded by the nucleotide sequence of SEQ ID NO: 1 is functionally unchanged. A nucleic acid molecule encoding Homer includes sequences encoding functional Homer polypeptides as well as functional fragments thereof. As used herein, the term "functional polypeptide" refers to a polypeptide which possesses biological function or activity which is identified through a defined functional assay (e.g., EXAMPLE 3), and which is associated with a particular biologic, morphologic, or phenotypic alteration in the cell. The term "functional fragments of Homer polypeptide," refers to fragments of a Homer polypeptide that retain a Homer activity, e.g., the ability to interact with cell-surface or intracellular receptors or mediate intracellular calcium mobilization, and the like. Additionally, functional Homer fragments may act as competitive inhibitors of Homer binding, for example, biologically functional fragments, for example, can vary in size from a polypeptide fragment as small as an epitope capable of binding an antibody molecule to a large polypeptide capable of participating in the characteristic induction or programming of phenotypic changes within a cell. A functional Homer polypeptide includes a polypeptide as set forth in SEQ ID NO:2 and conservative variations thereof. The terms "conservative variation" and "substantially similar" as used herein denotes the replacement of an amino acid residue by another, biologically similar residue. Examples of conservative variations include the substitution of one hydrophobic residue such as isoleucine, valine, leucine or methionine for another, or the substitution of one polar residue for another, such as the substitution of arginine for lysine, glutamic acid for aspartic acid, or glutamine for asparagine, and the like. The terms "conservative variation" and "substantially similar" also include the use of a substituted amino acid in place of an unsubstituted parent amino acid provided that antibodies raised to the substituted polypeptide also immunoreact with the unsubstituted polypeptide. Also included are other Homer nucleic acid and amino acid sequences, including Homer lb (SEQ ID NOS:3 and 4); Homer lc (SEQ ID NOS:5 and 6); Homer 2a (SEQ ID NOS:7 and 8); Homer 2b (SEQ ID NOS:9 and 10); Homer 3 (SEQ ID NOS:l l and l2). Cell-surface receptors are important intermediaries in intercellular signaling. A "cell-surface receptor" is a protein, usually having at least one binding domain on the outer surface of a cell where specific molecules may bind to, activate, or block the cell surface receptor. Cell surface receptors usually have at least one extracellular domain, a membrane spanning region ("transmembrane") and an intracellular domain. Activation of a cell-surface receptor can lead to changes in the levels of various molecules inside the cell. Several types of cell-surface receptors have been identified in a variety of cell types, including ligand-gated receptors, ligand-gated channels, voltage-activated receptors, voltage-activated channels, ion channels and the like. One class of cell-surface receptor is excitatory amino acid receptors (EAA receptors) which are the major class of excitatory neurotransmitter receptors in the central nervous system. "EAA receptors" are membrane spanning proteins that mediate the stimulatory actions of glutamate and possibly other endogenous acidic amino acids. EAA are crucial for fast excitatory neurotransmission and they have been implicated in a variety of diseases including Alzheimer's disease, stroke schizophrenia, head trauma and epilepsy. EAA have also been implicated in the process of aging In addition, EAA are integral to the processes of long-term potentiation, one of the synaptic mechanisms underlying learning and memory. There are three main subtypes of EAA receptors: (1) the metabotropic or trans ACPD receptors; (2) the ionotropic NMDA receptors; and (3) the non-NMDA receptors, which include the AMPA receptors and kainate receptors. Ionotropic glutamate receptors are generally divided into two classes: the NMDA and non-NMDA receptors. Both classes of receptors are linked to integral cation channels and share some amino acid sequence homology. GluRl-4 are termed AMPA (α-amino -3-hydroxy-5-methylisoxazole-4-propionic acid) receptors because AMPA preferentially activates receptors composed of these subunits, while GluR5-7 are termed kainate receptors as these are preferentially sensitive to kainic acid. Thus, an "AMPA receptor" is a non-NMDA receptor that can be activated by AMPA. AMPA receptors include the GluRl-4 family, which form homo-oligomeric and hetero-oligomeric complexes which display different current-voltage relations and Ca 2+ permeability. Polypeptides encoded by GluRl-4 nucleic acid sequences can form functional ligand-gated ion channels. An AMPA receptor includes a receptor having a GluRl, GluR2, GluR3 or GluR4 subunit. NMDA receptor subtypes include class NR2B and NR2D, for example. Metabotropic glutamate receptors are divided into three groups based on amino acid sequence homology, transduction mechanism and binding selectivity: Group I, Group II and Group III. Each Group of receptors contains one or more types of receptors. For example, Group I includes metabotropic glutamate receptors 1 and 5 (mGluRl and mGluR5), Group II includes metabotropic glutamate receptors 2 and 3 (mGluR2 and mGluR3) and Group III includes metabotropic glutamate receptors 4, 6, 7 and 8 (mGluR4, mGluRό, mGluR7 and mGluR8). Each mGluR type may be found in several subtypes. For example, subtypes of mGluRl include mGluRla, mGluRlb and mGluRlc. Group I metabotropic glutamate receptors represent a family of seven membrane spanning proteins that couple to G-proteins and activate phospholipase C (Nakanishi, 1994). Members of the family include mGluRl and mGluR5. Activation of these receptors results in the hydrolysis of memberane phosphatidylinositol bisphosphate to diacylglycerol, which activates protein kinase C. and inositol trisphosphate, which in turn activates the inositol trisphosphate receptor to release intracellular calcium. (Aramori and Nakanishi, 1992; Joly et al, 1995 Kawabata et al, 1998) Activation of a glutamate receptor on the cell surface results in a cellular response. A "cellular response" is an event or sequence of events that singly or together are a direct or indirect response by a cell to activation of a cell surface receptor. A "cellular response" is also the blockade or activation of selective and non-selective cation channels and potentiation or inhibition of other cell-surface receptor responses. In addition, a "cellular response" may be the activation of an intracellular signaling pathway, including the activation of all steps or any one step in an intracellular signaling pathway. An "intracellular signaling pathway" is a sequence of events that transduces information about an extracellular event into a signal to intracellular receptors or effector molecules such as enzymes. One type of intracellular signaling pathway is a second messenger signaling pathway. It may begin with the activation of receptors on the cell surface, which activation evokes changes in the level of specific, diffusible molecules inside the cell. The regulated production of these molecules serves to signal events to the intracellular receptors and is therefore termed a second messenger signaling pathway. Major second messenger pathways include the adenylate cyclase pathway, which regulates levels of cyclic AMP, the phosphoinositide pathway, which regulates intracellular calcium, guanylate cyclase, which regulates levels of cGMP, and the nitric oxide pathway, which regulates nitric oxide. A cellular response mediated by cell surface receptors can also include calcium mobilization. A compound can modulate cellular responses mediated by cell surface receptors by inhibiting or potentiating the release of calcium from intracellular stores. A compound increases calcium mobilization by increasing the release of calcium from intracellular stores. A compound decreases calcium mobilization by inhibiting of the release of calcium from intracellular stores. Cell-surface receptors are known to mediate cellular responses. Methods for demonstrating cellular responses are well known in the art (e.g. electrophysiological and biochemical methods). (See Examples section for additional methodology). A method is provided for identifying a compound that modulates a cellular response mediated by a cell-surface receptor. The method includes incubating the compound and a cell expressing a cell-surface receptor and a Homer protein under conditions sufficient to permit the compound to interact with the cell. The cell may be any cell of interest, including but not limited to neuronal cells, glial cells, cardiac cells, bronchial cells, uterine cells, testicular cells, liver cells, renal cells, intestinal cells, cells from the thymus and spleen, placental cells, endothelial cells, endocrine cells including thyroid, parathyroid, pituitary and the like, smooth muscle cells and skeletal muscle cells. The cell is exposed to a cell-surface receptor ligand. A "cell surface receptor ligand" is a compound that binds to the binding site of the cell-surface receptor thereby initiating a sequence of events that singly or together embrace a "cellular response". The effect of the compound on the cellular response is determined, either directly or indirectly, and a cellular response is then compared with a cellular response of a control cell. A suitable control includes, but is not limited to, a cellular response of a cell not contacted with the compound. The term "incubating" includes conditions which allow contact between the test compound and the cell of interest. "Contacting" may include in solution or in solid phase. Compounds which modulate a cellular response can include peptides, peptidomimetics, polypeptides, pharmaceuticals, chemical compounds and biological agents, for example. Antibodies, neurotropic agents, anti-epileptic compounds and combinatorial compound libraries can also be tested using the method of the invention. One class of organic molecules, preferably small organic compounds having a molecular weight of more than 50 and less than about 2,500 Daltons. Candidate agents comprise functional groups necessary for structural interaction with proteins, particularly hydrogen bonding, and typically include at least an amine, carbonyl, hydroxyl or carboxyl group, preferably at least two of the functional chemical groups. The candidate agents often comprise cyclical carbon or heterocyclic structures and/or aromatic or polyaromatic structures substituted with one or more of the above functional groups. The test agent may also be a combinatorial library for screening a plurality of compounds. Compounds such as peptides identified in the method of the invention can be further cloned, sequenced, and the like, either in solution of after binding to a solid support, by any method usually applied to the isolation of a specific DNA sequence Molecular techniques for DNA analysis (Landegren et al, Science 242:229- 237, 1988) and cloning have been reviewed (Sambrook et al, Molecular Cloning: a Laboratory Manual. 2nd Ed.; Cold Spring Harbor Laboratory Press, Plainview, NY, 1998, herein incorporated by reference). Candidate compounds are obtained from a wide variety of sources including libraries of synthetic or natural compounds. For example, numerous means are available for random and directed synthesis of a wide variety of organic compounds and biomolecules, including expression of randomized oligonucleotides and oligopeptides. Alternatively, libraries of natural compounds in the form of bacterial, fungal, plant and animal extracts are available or readily produced. Additionally, natural or synthetically produced libraries and compounds are readily modified through conventional chemical, physical and biochemical means, and may be used to produce combinatorial libraries. Known pharmacological agents may be subjected to directed or random chemical modifications, such as acylation, alkylation, esterification, amidification, etc., to produce structural analogs. Candidate agents are also found among biomolecules including, but not limited to: peptides, saccharides, fatty acids, steroids, purines, pyrimidines, derivatives, structural analogs or combinations thereof. A variety of other agents may be included in the screening assay. These include agents like salts, neutral proteins, e.g. , albumin, detergents, etc. that are used to facilitate optimal protein-protein binding and/or reduce nonspecific or background interactions. Reagents that improve the efficiency of the assay, such as protease inhibitors, nuclease inhibitors, antimicrobial agents and the like may be used. The mixture of components are added in any order that provides for the requisite binding. Incubations are performed at any suitable temperature, typically between 4 and 40°C. Incubation periods are selected for optimum activity, but may also be optimized to facilitate rapid high-throughput screening. Typically between 0.1 and 10 h will be sufficient. In another embodiment, a method is provided for identifying a compound that modulates a cellular response mediated by an intracellular receptor. An "intracellular receptor" is a protein that binds particular intracellular molecules. Intracelluar receptors include ryanodine receptors and inositol trisphosphate receptors, for example, an "inositol trisphosphate receptor" is a receptor that binds the compound inositol 1,4,5 trisphosphate, which is an important intracellular second messenger. Inositol 1,4,5 trisphosphate is released from phosphatidyl inositol bisphosphate by the action of a specific phospholipase C enzyme (PLC) and binds to and activates a calcium channel in the endoplasmic reticulum (ER). A compound can modulate a cellular response mediated by an intracellular receptor by inhibiting or potentiating the release of calcium from intracellular stores, for example, a compound increases calcium mobilization by increasing the release of calcium from intracellular stores. A compound decreases calcium mobilization by inhibiting of the release of calcium from intracellular stores. The method of the invention includes incubating the compound and a cell expressing an intracellular receptor and a Homer protein under conditions sufficient to permit the compound to interact with the cell, exposing the cell to conditions that activate said intracellular receptor, and comparing a cellular response in a cell incubated with said compound with the response of a cell not incubated with said compound. Methods for determining cellular responses mediated by intracellular signals are well known to one of skill in the art (e.g., biochemical assays) and provided in the Examples as well. A method is also provided for identifying a compound that modulates receptor-activated calcium mobilization. The term "calcium mobilization" means a change in the amount or concentration of free calcium (Ca +2 ) sequestered in the endoplasmic reticulum, sarcoplasmic reticulum or mitochondria of a cell. The method includes incubating the compound and a cell expressing a Homer protein under conditions sufficient to permit the compound to interact with the cell and exposing the cell to conditions sufficient to activate calcium mobilization. Then, the cellular response of the cell exposed to the compound is compared to the cellular response of a cell not exposed to the compound. A difference in a cellular response is indicative of a compound that modulates receptor-activated calcium mobilization in a cell. In another embodiment of the invention, a method is provided for modulating receptor-mediated calcium mobilization in a cell including exposing a cell to a compound in a sufficient amount to modulate the calcium mobilization that normally occurs when a cell is exposed to an amount of ligand sufficient to activate an intracellular signaling pathway. Those of skill in the art will understand that "the calcium mobilization that normally occurs" depends on the cell type and on the ligand activating the intracellular pathway (Berridge, 1997 supra; Berridge, 1998 supra; Bootman, 1997 supra). Methods of measuring free calcium flux are well known in the art (e.g., imaging methodology using calcium-sensitive dyes such as fura-2 and the like). A ligand which activates the intracellular signaling pathway may be an agonist or antagonist of metabotropic glutamate receptors. The terms "agonist" and "antagonist" are meant to include compounds that bind to the receptor and, respectively, activate or block activation of the receptor. Known agonists of metabotropic glutamate receptors include glutamate, quisqualate, Ibotenate, homocysteine sulfmate and the neurotoxin β-N-methylamino-L-alanine. Antagonists of metabotropic glutamate receptors include MCPG. Known agonists of the NMDA type glutamate receptor include glutamate and NMDA and known antagonists include MK-801 and APV. Another embodiment of the invention includes a method of identifying a compound that inhibits Homer protein activity. The method relies on functional properties of the Homer EVHl and coiled-coil binding domains that can be used to establish high-throughput screens for molecules that influence these and other functional properties of Homer family members. Homer protein activity may be blocked, partially or completely, by interfering with a protein or other molecule in the intracellular signaling pathway though which Homer proteins act. For example, Homer activity can be modulated, for example, by modulating Homer protein expression, by modifying the activity of the Homer EVHl domain, by modification of the activity of the Homer CC domain, by modification of Homer crosslinking activity, and the like. Homer activity can also be modulated with by interfering with the expression or activity of Homer Interacting Protein 142, Homer Interacting Protein 130, NR2D, ACK-2, Shank proteins, ryanodine, inositol trisphosphate, and hlnaD, and the like Homer proteins function as a regulated adapter network that cross-links interacting proteins. Cross-linking is determined by the binding properties of the Homer EVHl domain, which recognize a unique pro line-rich ligand with a core sequence consensus of PXXF. This Homer ligand is present in all identified proteins that naturally associate with Homer, and the ability of Homer proteins to bind can be disrupted by single amino acid changes in this motif. Cross-linking activity of Homer proteins has demonstrated effects on glutamate receptor signaling and this action is due to the formation of signaling complexes that link cell-surface receptors with intracellular receptors. Cross-linking by Homer proteins may also have consequences on receptor trafficking or other cellular functions of the interacting proteins. Development of agents that modulate activity of the Homer EVHl domain is furthered by knowledge of the crystal structure of Homer protein. The method includes designing inhibitors of Homer protein that form non-covalent bonds with amino acids in the Homer binding sites based upon the crystal structure co-ordinates of Homer protein binding domain; synthesizing the inhibitor; and determining whether the inhibitor inhibits the activity of Homer protein. The "Homer protein binding domain" is a conserved sequence of amino acids in the amino-terminal region of the that interacts with other proteins. All Homer proteins possess a conserved region of about 175 amino acids at their amino-termini. The 110 terminal amino acids in this region interact with the carboxy-termini of other proteins, for example metabotropic glutamate receptors, inositol trisphosphate receptors, Shank, and the like. The carboxy-termini region of the proteins to which the Homer protein binding domain may bind usually contains an amino acid sequence that contains a high number of proline residues. One aspect of the invention resides in the obtaining of crystals of Homer protein of sufficient quality to determine the three dimensional (tertiary) structure of the protein by X-ray diffraction methods. The knowledge obtained concerning Homer proteins may be used in the determination of the three dimensional structure of the binding domain of Homer proteins. The binding domain can also be predicted by various computer models. Upon discovering the three-dimensional protein structure of the binding domain, small molecules which mimic the functional binding of Homer protein to its ligands can be designed and synthesized This is the method of "rational" drug design. Another approach to "rational" drug design is based on a lead compound that is discovered using high thoughput screens; the lead compound is further modified based on a crystal stucture of the binding regions of the molecule in question. Accordingly, another aspect of the invention is to provide material which is a starting material in the rational design of drugs which mimic or prevents the action of Homer proteins. The term "crystal structure coordinates" refers to mathematical coordinates derived from mathematical equations related to the patterns obtained on diffraction of a monochromatic beam of X-rays by the atoms (scattering centers) of a Homer protein molecule in crystal form. The diffraction data are used to calculate an electron density map of the repeating unit of the crystal. The electron density maps are used to establish the positions of the individual atoms within the unit cell of the crystal. The crystal structure coordinates of the Homer protein binding domain are obtained from a Homer protein crystal having orthorhombic space group symmetry P2 1 2 1 2j with a = 33.79, b = 51.40, and c = 66.30 Angstroms. The coordinates of the Homer protein binding domain can also be obtained by means of computational analysis. The term "selenomethione substitution refers to the method of producing a chemically modified form of the crystal of Homer. The Homer protein is expressed by bacterial in meida that is depleted in methionine and supplement in selenomethionine. Selenium is thereby incorporated into the crystal in place of methionine sulfurs.. The location(s) of selenium are determined by X-ray diffraction analysis of the crystal. This information is used to generate the phase information used to construct three-dimensional structure of the protein. The term "heavy atom derivatization" refers to the method of producing a chemically modified form of the crystal of Homer. A crystal is soaked in a solution containing heavy metal atom salts or organometalhc compounds, which can diffuse through the crystal and bind to the surface of the protein. The location(s) of the bound heavy metal atom(s) are determined by X-ray diffraction analysis of the soaked crystal. This information is used to generate the phase information used to construct three-dimensional structure of the protein. Those of skill in the art understand that a set of structure coordinates determined by X-ray crystallography is not without standard error. The term "unit cell" refers to the basic parallelipiped shaped block. The entire volume of a crystal may be constructed by regular assembly of such blocks. The term "space group" refers to the arrangement of symmetry elements of a crystal. The term "molecular replacement" refers to a method that involves generating a preliminary model of an Homer crystal whose structure coordinates are not known, by orienting and positioning a molecule whose structure coordinates are known. Phases are then calculated from this model and combined with observed amplitudes to give an approximate Fourier synthesis of the structure whose coordinates are known. The crystal structure coordinates of Homer protein may be used to design compounds that bind to the protein and alter its physical or physiological properties in a variety of ways. The structure coordinates of the protein may also be used to computationally screen small molecule data bases for compounds that bind to the protein. The structure coordinates of Homer mutants (e.g., missense mutations, deletion mutations, and the like, obtained by site-directed mutagenesis, by exposure to mutagenic agents, through selection of naturally occurring mutants, etc.) may also facilitate the identification of related proteins, thereby further leading to novel therapeutic modes for treating or preventing Homer-mediated conditions. A potential inhibitor is designed to form hydrogen bonds with tryptophan 24 , phenylalanine 74 , threonine 66 , threonine 68 , glutamine 76 , alanine 78 , threonine 70 , and valine 85 of the Homer binding domain. A method is also provided for identifying a compound that affects the formation of cell surface receptors into clusters. The method includes incubating the compound and a cell expressing a Homer protein and a Homer Interacting protein, such as a Shank protein, a Homer Interacting Protein, and the like, under conditions sufficient to allow the compound to interact with the cell, determining the effect of the compound on the formation of cell-surface receptors into clusters, and comparing the formation of cell-surface receptors into clusters in cells contacted with the compound with the formation of cell surface receptors into clusters in cells not contacted with the compound. Shank proteins are a novel family of proteins found at the postsynaptic density (PSD) and which are capable of binding to other proteins. Shank proteins contain multiple protein interaction domains, including ankyrin repeats, SH3 domain, PDZ domain, at least one proline rich domain and at least one SAM domain. The PDZ domain of Shank mediates binding to the carboxy-terminus of guanylate kinase associated protein (GKAP), and this interaction is important in neuronal cells for the synaptic localization of Shank proteins. Shank proteins also interact with Homer proteins and therefore Shank and Homer may serve as a protein bridge that links specific proteins that bind to Homer and specific proteins that bind to Shank. Exemplary Shank proteins include Shank la, Shank lb and Shank 3, and cortactin binding protein, and the like. A compound can affect the formation of cell-surface receptors into clusters by either stimulating the formation of cell-surface receptors into clusters or by inhibiting the recruitment of cell-surface receptors into clusters. When the effect is "inhibition", cell-surface clustering is decreased as compared with the level in the absence of the test compound. When the effect is "stimulation", cell-surface clustering is increased as compared to a control in the absence of the test compound. A method is further provided for treating a subject with a disorder associated with metabotropic receptors or ion channel receptors comprising administering to the subject a therapeutically effective amount of a compound that modulates Homer protein activity. In yet another embodiment, a method is provided for treating a subject with a disorder associated with Homer protein activity, comprising administering to the subject a therapeutically effective amount of a compound that modulates Homer protein activity. Essentially, any disorder that is etiologically linked to a glutamate receptor, an inositol trisphosphate receptor, a ryanodine receptor, a Shank protein , 142 (or other Homer interacting proteins) or to a Homer protein could be considered susceptible to treatment with an agent that modulates Homer protein activity. The disorder may be a neuronal cell disorder. Examples of neuronal cell disorders include but are not limited to Alzheimer's disease, Parkinson's disease, stroke, epilepsy, neurodegenerative disease, Huntington's disease, and brain or spinal cord injury/damage, including ischemic injury. The disorder may also be a disorder of a cardiac disorder, a disorder of musculature, a renal disorder, a uterine disorder or a disorder of bronchial tissue. The disorder may be epilepsy, glutamate toxicity, a disorder of memory, a disorder of learning or a disorder of brain development. Detection of altered (decreased or increased) levels of "Homer protein activity" can be accomplished by hybridization of nucleic acids isolated from a cell of interest with a Homer polynucleotide of the invention. Analysis, such as Northern Blot analysis, are utilized to quantitate expression of Homer, such as to measure Homer transcripts. Other standard nucleic acid detection techniques will be known to those of skill in the art. Detection of altered levels of Homer can also accomplished using assays designed to detect Homer polypeptide. For example, antibodies or petides that specifically bind a Homer polypeptide can be utilized. Analyses, such as radioimmune assay or immunohistochemistry, are then used to measure Homer, such as to measure protein concentration qualitatively or quantitatively. Treatment can include modulation of Homer activity by administration of a therapeutically effective amount of a compound that modulates Homer or Homer protein activity. The term "modulate" envisions the suppression of Homer activity or expression when Homer is overexpressed or has an increased activity as compared to a control. The term "modulate" also includes the augmentation of the expression of Homer when it is underexpressed or has a decreased activity as compared to a control. The term "compound" as used herein describes any molecule, e.g., protein, nucleic acid, or pharmaceutical, with the capability of altering the expression of Homer polynucleotide or activity of Homer polypeptide. Treatment may inhibit the interaction of the EVHl domain of Homer with its target protein , may increase the avidity of this interaction by means of allosteric effects, may block the binding activity of the coiled-coil doamin of Homer or influence other functional properties of Homer proteins . Candidate agents include nucleic acids encoding a Homer, or that interfere with expression of Homer, such as an antisense nucleic acid, ribozymes, and the like. Candidate agents also encompass numerous chemical classes wherein the agent modulates Homer expression or activity. Where a disorder is associated with the increased expression of Homer, nucleic acid sequences that interfere with the expression of Homer can be used. In this manner, the coupling of cell-surface and intracellular receptors can be inhibited. This approach also utilizes, for example, antisense nucleic acid, ribozymes, or triplex agents to block transcription or translation of Homer mRNA, either by masking that mRNA with an antisense nucleic acid or triplex agent, or by cleaving it with a ribozyme in disorders associated with increased Homer. Alternatively, a dominant negative form of Homer polypeptide could be administered. When Homer is overexpressed, candidate agents include antisense nucleic acid sequences. Antisense nucleic acids are DNA or RNA molecules that are complementary to at least a portion of a specific mRNA molecule (Weintraub, 1990, Scientific American, 262:40). In the cell, the antisense nucleic acids hybridize to the corresponding mRNA, forming a double-stranded molecule. The antisense nucleic acids interfere with the translation of the mRNA, since the cell will not translate a mRNA that is double-stranded. Antisense oligomers of about 15 nucleotides are preferred, since they are easily synthesized and are less likely to cause problems than larger molecules when introduced into the target cell. The use of antisense methods to inhibit the in vitro translation of genes is well known in the art (Marcus-Sakura, 1988, Anal.Biochem., 172:289). Use of an oligonucleotide to stall transcription is known as the triplex strategy since the oligomer winds around double-helical DNA, forming a three-strand helix. Therefore, these triplex compounds can be designed to recognize a unique site on a chosen gene (Maher, et al, 1991, Antisense Res. and Dev., 1(3):227; Helene, C, 1991, Anticancer Drug Design, 6(6):569). Ribozymes are RNA molecules possessing the ability to specifically cleave other single-stranded RNA in a manner analogous to DNA restriction endonucleases. Through the modification of nucleotide sequences which encode these RNAs, it is possible to engineer molecules that recognize specific nucleotide sequences in an RNA molecule and cleave it (Cech, 1988, J.Amer.Med. Assn., 260:3030). A major advantage of this approach is that, because they are sequence-specific, only mRNAs with particular sequences are inactivated. There are two basic types of ribozymes namely, tetrahymena-type (Hasselhoff, 1988, Nature, 334:585) and "hammerhead"-type. Tetrahymena-type ribozymes recognize sequences which are four bases in length, while "hammerhead"-type ribozymes recognize base sequences 11-18 bases in length. The longer the recognition sequence, the greater the likelihood that the sequence will occur exclusively in the target mRNA species. Consequently, hammerhead-type ribozymes are preferable to tetrahymena-type ribozymes for inactivating a specific mRNA species and 18-based recognition sequences are preferable to shorter recognition sequences. When a disorder is associated with the decreased expression of Homer, nucleic acid sequences that encode Homer can be used. An agent which modulates Homer expression includes a polynucleotide encoding a polypeptide of SEQ ID NO:2, 4, 6, 8, 10 or 12, or a conservative variant thereof. Alternatively, an agent of use with the subject invention includes agents that increase the expression of a polynucleotide encoding Homer or an agent that increases the activity of Homer polypeptide. In another embodiment of the invention, there is provided a transgenic non- human animal having a transgene that expresses Homer la chromosomally integrated into the germ cells of the animal. Animals are referred to as "transgenic" when such animal has had a heterologous DNA sequence, or one or more additional DNA sequences normally endogenous to the animal (collectively referred to herein as "transgenes") chromosomally integrated into the germ cells of the animal. The transgenic animal (including its progeny) will also have the transgene fortuitously integrated into the chromosomes of somatic cells. Various methods to make the transgenic animals of the subject invention can be employed. Generally speaking, three such methods may be employed. In one such method, an embryo at the pronuclear stage (a "one cell embryo") is harvested from a female and the transgene is microinj ected into the embryo, in which case the transgene will be chromosomally integrated into both the germ cells and somatic cells of the resulting mature animal. In another such method, embryonic stem cells are isolated and the transgene incorporated therein by electroporation, plasmid transfection or microinjection, followed by reintroduction of the stem cells into the embryo where they colonize and contribute to the germ line. Methods for microinjection of mammalian species is described in United States Patent No. 4,873,191. In yet another such method, embryonic cells are infected with a retrovirus containing the transgene whereby the germ cells of the embryo have the transgene chromosomally integrated therein. When the animals to be made transgenic are avian, because avian fertilized ova generally go through cell division for the first twenty h in the oviduct, microinjection into the pronucleus of the fertilized egg is problematic due to the inaccessibility of the pronucleus. Therefore, of the methods to make transgenic animals described generally above, retrovirus infection is preferred for avian species, for example as described in U.S. Patent No. 5,162,215. If microinjection is to be used with avian species, however, a recently published procedure by Love et al, (Biotechnology, 12, Jan 1994) can be utilized whereby the embryo is obtained from a sacrificed hen approximately two and one-half h after the laying of the previous laid egg, the transgene is microinj ected into the cytoplasm of the germinal disc and the embryo is cultured in a host shell until maturity. When the animals to be made transgenic are bovine or porcine, microinjection can be hampered by the opacity of the ova thereby making the nuclei difficult to identify by traditional differential interference-contrast microscopy. To overcome this problem, the ova can first be centrifuged to segregate the pronuclei for better visualization. The "non-human animals" of the invention are murine typically (e.g., mouse). The "transgenic non-human animals" of the invention are produced by introducing "transgenes" into the germline of the non-human animal. Embryonal target cells at various developmental stages can be used to introduce transgenes. Different methods are used depending on the stage of development of the embryonal target cell. The zygote is the best target for microinjection. The use of zygotes as a target for gene transfer has a major advantage in that in most cases the injected DNA will be incorporated into the host gene before the first cleavage (Brinster et al, Proc. Natl. Acad. Sci. USA 82:4438-4442, 1985). As a consequence, all cells of the transgenic non-human animal will carry the incorporated transgene. This will in general also be reflected in the efficient transmission of the transgene to offspring of the founder since 50% of the germ cells will harbor the transgene. The term "transgenic" is used to describe an animal which includes exogenous genetic material within all of its cells. A "transgenic" animal can be produced by cross-breeding two chimeric animals which include exogenous genetic material within cells used in reproduction. Twenty- five percent of the resulting offspring will be transgenic i.e., animals which include the exogenous genetic material within all of their cells in both alleles. 50% of the resulting animals will include the exogenous genetic material within one allele and 25% will include no exogenous genetic material. In the microinjection method useful in the practice of the subject invention, the transgene is digested and purified free from any vector DNA e.g. by gel electrophoresis. It is preferred that the transgene include an operatively associated promoter which interacts with cellular proteins involved in transcription, ultimately resulting in constitutive expression. Promoters useful in this regard include those from cytomegalovirus (CMV), Moloney leukemia virus (MLV), and herpes virus, as well as those from the genes encoding metallothionin, skeletal actin, P-enolpyruvate carboxylase (PEPCK), phosphoglycerate (PGK), DHFR, and thymidine kinase. Promoters for viral long terminal repeats (LTRs) such as Rous Sarcoma Virus can also be employed. Constructs useful in plasmid transfection of embryonic stem cells will employ additional regulatory elements well known in the art such as enhancer elements to stimulate transcription, splice acceptors, termination and polyadenylation signals, and ribosome binding sites to permit translation. Retroviral infection can also be used to introduce transgene into a non-human animal, as described above. The developing non-human embryo can be cultured in vitro to the blastocyst stage. During this time, the blastomeres can be targets for retro viral infection (Jaenich, R., Proc. Natl. Acad. Sci USA 73:1260-1264, 1976). Efficient infection of the blastomeres is obtained by enzymatic treatment to remove the zona pellucida (Hogan, et al. (1986) in Manipulating the Mouse Embryo, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y). The viral vector system used to introduce the transgene is typically a replication-defective retro virus carrying the transgene (Jahner, et al, Proc. Natl. Acad. Sci. USA 82:6927-6931, 1985; Van der Putten, et al, Proc. Natl Acad. Sci USA £2:6148-6152, 1985). Transfection is easily and efficiently obtained by culturing the blastomeres on a monolayer of virus-producing cells (Van der Putten, supra; Stewart, et al, EMBO J. (5:383-388, 1987). Alternatively, infection can be performed at a later stage. Virus or virus-producing cells can be injected into the blastocoele (D. Jahner et al, Nature 2z 28:623-628, 1982). Most of the founders will be mosaic for the transgene since incorporation occurs only in a subset of the cells which formed the transgenic nonhuman animal. Further, the founder may contain various retro viral insertions of the transgene at different positions in the genome which generally will segregate in the offspring. In addition, it is also possible to introduce transgenes into the germ line, albeit with low efficiency, by intrauterine retroviral infection of the midgestation embryo (D. Jahner et al, supra). A third type of target cell for transgene introduction is the embryonal stem cell (ES). ES cells are obtained from pre-implantation embryos cultured in vitro and fused with embryos (M. J. Evans et al. Nature 222:154-156, 1981; M.O. Bradley et al, Nature 302: 255-258, 1984; Gossler, et al, Proc. Natl Acad. Sci USA 83: 9065-9069, 1986; and Robertson et al, Nature 322:445-448, 1986). Transgenes can be efficiently introduced into the ES cells by DNA transfection or by retro virus-mediated transduction. Such transformed ES cells can thereafter be combined with blastocysts from a nonhuman animal. The ES cells thereafter colonize the embryo and contribute to the germ line of the resulting chimeric animal. (For review see Jaenisch, R., Science 240: 1468-1474, 1988). "Transformed" means a cell into which (or into an ancestor of which) has been introduced, by means of recombinant nucleic acid techniques, a heterologous nucleic acid molecule. "Heterologous" refers to a nucleic acid sequence that either originates from another species or is modified from either its original form or the form primarily expressed in the cell. "Transgene" means any piece of DNA which is inserted by artifice into a cell, and becomes part of the genome of the organism (i.e., either stably integrated or as a stable extrachromosomal element) which develops from that cell. Such a transgene may include a gene which is partly or entirely heterologous (i.e., foreign) to the transgenic organism, or may represent a gene homologous to an endogenous gene of the organism. Included within this definition is a transgene created by the providing of an RNA sequence which is transcribed into DNA and then incorporated into the genome. The transgenes of the invention include DNA sequences which encode Homer protein-sense and antisense polynucleotides, which may be expressed in a transgenic non-human animal. The term "transgenic" as used herein additionally includes any organism whose genome has been altered by in vitro manipulation of the early embryo or fertilized egg or by any transgenic technology to induce a specific gene knockout. As used herein, the term "transgenic" includes any transgenic technology familiar to those in the art which can produce an organism carrying an introduced transgene or one in which an endogenous gene has been rendered nonfunctional or "knocked out". Antibodies of the invention may bind to Homer proteins or Homer interacting proteins provided by the invention to prevent normal interactions of the Homer proteins and Homer Interacting proteins. Binding of antibodies to Homer proteins or Homer Interacting Proteins can interfere with cell-signaling by interfering with an intracellular signaling pathway. Binding of antibodies can interfere with Homer protein binding to extracellular receptors, e.g., to NMDA receptors, to metabotropic receptors, and the like. Binding of antibodies can interfere with Homer protein binding to intracellular receptors, e.g., inositol trisphosphate receptors, and the like. Furthermore, binding to Homer proteins or to Homer Interacting Proteins can interfere with cell-surface receptor clustering mediated by Homer family proteins. The antibodies of the invention can be used in any subject in which it is desirable to administer in vitro or in vivo immunodiagnosis or immunotherapy. The antibodies of the invention are suited for use, for example, in immunoassays in which they can be utilized in liquid phase or bound to a solid phase carrier. In addition, the antibodies in these immunoassays can be detectably labeled in various ways. Examples of types of immunoassays which can utilize antibodies of the invention are competitive and non-competitive immunoassays in either a direct or indirect format. Examples of such immunoassays are the radioimmunoassay (RIA) and the sandwich (immunometric) assay. Detection of the antigens using the antibodies of the invention can be done utilizing immunoassays which are run in either the forward, reverse, or simultaneous modes, including immunohistochemical assays on physiological samples. Those of skill in the art will know, or can readily discern, other immunoassay formats without undue experimentation. The term "antibody" as used in this invention includes intact molecules as well as fragments thereof, such as Fab, F(ab')2, and Fv which are capable of binding to an epitopic determinant present in an invention polypeptide. Such antibody fragments retain some ability to selectively bind with its antigen or receptor. Methods of making these fragments are known in the art. (See for example, Harlow and Lane, Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory, New York (1988), incorporated herein by reference).Monoclonal antibodies are made from antigen containing fragments of the protein by methods well known to those skilled in the art (Kohler, et al, Nature, 256:495, 1975). Antibodies which bind to an invention polypeptide of the invention can be prepared using an intact polypeptide or fragments containing small peptides of interest as the immunizing antigen. For example, it may be desirable to produce antibodies that specifically bind to the N- or C-terminal domains of an invention polypeptide. The polypeptide or peptide used to immunize an animal is derived from translated cDNA or chemically synthesized and can be conjugated to a carrier protein, if desired. Commonly used carrier proteins which may be chemically coupled to the immunizing peptide include keyhole limpet hemocyanin (KLH), thyroglobulin, bovine serum albumin (BSA), tetanus toxoid, and the like. Invention polyclonal or monoclonal antibodies can be further purified, for example, by binding to and elution from a matrix to which the polypeptide or a peptide to which the antibodies were raised is bound. Those of skill in the art will know of various techniques common in the immunology arts for purification and/or concentration of polyclonal antibodies, as well as monoclonal antibodies (See, for example, Coligan, et al, Unit 9, Current Protocols in Immunology, Wiley Interscience, 1994, incorporated by reference). The antibodies of the invention can be bound to many different carriers and used to detect the presence of an antigen comprising the polypeptides of the invention. Examples of well-known carriers include glass, polystyrene, polypropylene, polyethylene, dextran, nylon, amylases, natural and modified celluloses, polyacrylamides, agaroses and magnetite. The nature of the carrier can be either soluble or insoluble for purposes of the invention . Those skilled in the art will know of other suitable carriers for binding antibodies, or will be able to ascertain such, using routine experimentation. There are many different labels and methods of labeling known to those of ordinary skill in the art. Examples of the types of labels which can be used in the present invention include enzymes, radioisotopes, fluorescent compounds, colloidal metals, chemiluminescent compounds, phosphorescent compounds, and biolumi- nescent compounds. Those of ordinary skill in the art will know of other suitable labels for binding to the antibody, or will be able to ascertain such, using routine experimentation. Another technique which may also result in greater sensitivity consists of coupling the antibodies to low molecular weight haptens. These haptens can then be specifically detected by means of a second reaction. For example, it is common to use such haptens as biotin, which reacts with avidin, or dinitrophenyi, puridoxal, and fluorescein, which can react with specific antihapten antibodies. In using the monoclonal and polyclonal antibodies of the invention for the in vivo detection of antigen, e.g., Homer, the detectably labeled antibody is given a dose which is diagnostically effective. The term "diagnostically effective" means that the amount of detectably labeled antibody is administered in sufficient quantity to enable detection of the site having the antigen comprising a polypeptide of the invention for which the antibodies are specific. The concentration of detectably labeled antibody which is administered should be sufficient such that the binding to those cells having the polypeptide is detectable compared to the background. Further, it is desirable that the detectably labeled antibody be rapidly cleared from the circulatory system in order to give the best target-to-background signal ratio. As a rule, the dosage of detectably labeled antibody for in vivo treatment or diagnosis will vary depending on such factors as age, sex, and extent of disease of the individual. Such dosages may vary, for example, depending on whether multiple injections are given, antigenic burden, and other factors known to those of skill in the art. The following examples are intended to illustrate but not to limit the invention in any manner, shape, or form, either explicitly or implicitly. While they are typical of those that might be used, other procedures, methodologies, or techniques known to those skilled in the art may alternatively be used. EXAMPLES Homer la is an IEG and is the original member of a family of proteins that function together as a regulated adapter system that is hypothesized to control the coupling of membrane receptors to intracellular pools of releasable calcium. Homer proteins function at excitatory synapses to couple membrane group 1 metabotropic glutamate receptors (mGluR) to endoplasmic reticulum-associated inositol trisphosphate receptors (IP3R) (Brakeman et al, 1997; Tu et al, 1998; Xiao et al, 1998). Current studies suggest a broader role for Homer proteins in calcium signaling and receptor trafficking. The Shank family of proteins was identified based on their association with Homer (Naisbitt et al, 1999; Tu et al, 1999). Shank, together with Homer, appears to be part of both the NMDA and group 1 mGluR signaling complexes. By virtue of its interaction with Shank, Homer provides a mechanism to couple NMDA Ca2+ influx to intracellular Ca2+-induced Ca2+ release pools. The inventors have identified additional Homer-interacting proteins that provide insight into the role of Homer in trafficking of group 1 mGluR (e.g., SEQ ID NOS: 16, 18, 20). Because these Homer-dependent cellular processes are regulated by the IEG form of Homer (Homer la), mechanisms by which Homer proteins can modulate Ca2+ dynamics of mGluR and NMDA receptors, as well as regulate receptor trafficking are defined. Homer family proteins possess an N-terminal EVHl domain that mediates interactions with mGluRs, IP3R, Shank and other novel proteins. The EVHl domain has been determined to bind the proline rich motif PPXXFR (Tu et al, 1998). The present invention provides the crystal structure of the Homer EVHl domain. In complementary studies, genetic approaches were used to identify critical residues in both the EVHl domain and the ligand that modulate the affinity of the Homer- mGluR (and other Homer-interacting proteins) interaction. This information is essential to an understanding of the integrative cellular actions of Homer proteins,. Together, these studies define the molecular basis of specificity of EVHl interaction with its ligands, and provide insight into how the EVHl interaction is regulated. This patent application includes a description of several Homer-interacting proteins that are part of the signaling network that is controlled by Homer (e.g., SEQ ID NOS: 16, 18, 20). Yeast two-hybrid screens and searches of NCBI protein data bases identified a set of known and novel candidate interacting proteins for Homer include the ryanodine receptor, NMDA receptor subunit NR2D, human InaD and novel interacting proteins termed 142 and 130. As described below, current data indicate that agents can be developed that specifically modulate the crosslinking activity of Homer for these various receptors and thereby provide novel theraputics that regulate the output of these receptors on cellular function. Homer acts in several ways to regulate cellular function. Homer and Homer- related proteins function as an adapter system to couple membrane receptors to intracellular pools of releasable Ca. This "signaling" function of Homer is documented in Xiao et (1998), Tu et al (1998 and 1999) Naisbett et al (1999), as well as by studies of the novel Homer-interacting protein termed 142 (see below). By virtue of its crosslinking activity, Homer proteins play a role in synaptogenesis and spatial targeting/trafficking of GluRs to other postsynaptic structural proteins. This function of Homer is supported by observations in Tu et al (1999) and Naisbett et al (1999). Initial cloning of Homer; a novel brain Immediate Early Gene (IEG) Homer was cloned in a differential screen of seizure-stimulated hippocampus. Prior work, in which IEG induction was examined in brain provided a detailed understanding of time course and tissue distribution of the IEG response (Cole et al, 1989; Saffen et al, 1988; Worley et al, 1990), and suggested a paradigm to maximally induce novel IEG mRNAs (Lanahan and Worley, 1998; Worley et al, 1990). Once cloned, in situ hybridization was used to screen for IEGs that were regulated in other paradigms that activate neurons including LTP stimulation in the hippocampus (Brakeman et al, 1997) and acute administration of cocaine. In these models, Homer was one of the most highly induced of all of the IEGs (Brakeman et al, 1997). Initial characterization of Homer was challenging in that the mRNA was nearly 7 kb, while the best deduced open reading frame was only 186 aa, and was located near the 5' end of the cDNA (Brakeman et al, 1997). The 3' UTR was over 5 kb. The ORF was confirmed by in vitro transcription and translation of the cDNA, and rabbit polyclonal antisera were generated against bacterially expressed fusion proteins. With these antibodies, we were able to demonstrate that the protein was rapidly and transiently induced in the hippocampus following a seizure (Brakeman et al, 1997). This confirmed the deduced ORF and assured us that the cDNA was indeed translated in brain. Homer selectively binds group 1 metabotropic receptors and is enriched at synapses In an effort to discover the function of Homer, a yeast 2-hybrid technique (Chevray and Nathans, 1992; Fields and Song, 1989) was used to screen a cDNA library prepared from rat hippocampus and cortex. The full length Homer IEG was used as bait. Among -30 confirmed interacting cDNAs, one encoded the C-terminal 250 aa of mGluR5. We initially confirmed that the proteins bind using a GSTHomer in a pulldown assay with either fragments of mGluR5, or full length mGluR5 expressed in heterologous cells (HEK293 cells) (Brakeman et al, 1997). Homer protein also bound to mGluRla, but not mGluR2, 3, 4, or 7. This was an interesting clue to the function of Homer since mGluRl and mGluR5 (termed group 1 metabotropic receptors) couple to phospholipase C and active hydrolysis of phosphoinositides to generate inositol trisphosphate and diacylglycerol (Nakanishi et al, 1994). mGluRla and 5 also share sequence similarity in their long, cytosolically disposed C-terminus. Other metabotropic glutamate receptors (termed group 2 and 3) inhibit adenylate cyclase activity, and have short C-termini that lack homology to group 1 receptors. We proceeded to test whether Homer and mGluR5 naturally associate in brain and confirmed that these proteins co-immunoprecipitate from detergent extracts of hippocampus (Brakeman et al, 1997). The next major clue was provided by the observation that Homer immunoreactivity was enriched at excitatory synapses (Brakeman et al, 1997). In brain, Homer protein was associated with dendrites and showed a punctate pattern consistent with a localization in spines. The binding properties and cellular distribution of Homer suggested a role at the excitatory synapse. Homer is a member of a family of closely related proteins that are enriched at the excitatory synapse A search of the NCBI sequence data base identified several ESTs that showed strong homology to Homer, but were clearly distinct in that they encoded additional C-terminal sequence (Brakeman et al, 1997). Using a combination of screening strategies, a family of 12 cDNAs was identified from rat, mouse, Drosophila, and human (Xiao et al, 1998). All of these cDNAs encoded proteins with a similar protein structure and were deduced to be the products of 3 independent mammalian genes (termed Homer 1, 2, 3) and 1 Drosophilia gene. Like Homer IEG (now termed Homer la), all new family members contain an N-terminal, ~110 amino acid domain that binds mGluRla/5 ((Xiao et al, 1998). The region of Homer that interacts with mGluRla/5 is termed an EVHl domain based on its modest homology (20-25% identity) to domains in a family of proteins that include Drosophilia Enabled [Gertler, 1996], mammalian VASP (Haffner et al, 1995) and the Wiscott-Aldridge protein (WASP) (Ponting and Phillips, 1997; Symons et al, 1996). The EVHl domains of Homer proteins from Drosophilia, rodent and human are conserved at a level of 80% identity (Xiao et al, 1998). Other than the IEG Homer la, all new forms of Homer encode an additional C-terminal domain with predicted coiled-coil (CC) structure. As the nomenclature suggests, Homer 1 gene encodes both the IEG form (Homer la) and splice forms that encode CC domains termed Homer lb and lc. The lb and lc splice forms differ in their inclusion of an approximately 10 amino acid sequence located between the EVHl and CC domains. (Homer family members that encode CC domains are also referred to as CC-Homers to distinguish them from Homer la, which lacks a CC domain.) Similarly, Homer 2 encodes two CC-Homer splice forms termed Homer 2a and 2b, which also differ by a short internal sequence between EVHl and CC domains. Homer 3 encodes a single form. The CC domains are less conserved than the EVHl domain (-40% identity between rat Homer 1, 2 and 3) but they are able to specifically bind to themselves and to CC-domains of other Homer family members (Xiao et al, 1998). Homer CC domains do not interact with other representative CC-domain proteins in GST pulldown assays, and a yeast 2- hybrid screen of brain cDNA with the CC-domain of Homer 1 identified multiple copies of Homer 1, Homer 2 and Homer 3, but not other CC domains (Xiao et al, 1998). As evidence that Homer proteins can naturally self-multimerize, we demonstrated that Homer lb/Homer 3 heteromultimers co-immunoprecipitate from brain (Xiao et al, 1998). These observations indicate that the Homer CC domains mediate specific self-association. In contrast to Homer la, all CC-containing Homer family members are constitutively expressed in brain (Xiao et al, 1998)]. This was confirmed using both Northern blot and in situ hybridization assays which compared expression with Homer la in the same material. mRNA and protein expression of Homer Vole, Homer 2 and Homer 3 are unchanged in hippocampus following a seizure while Homer la mRNA and protein are induced at least 10 fold. Antibodies were generated that specifically recognize each of the CC-Homers. Antibodies were raised against synthetic C-terminal peptide sequences. Because Homer lb and lc possess identical C-termini, the C-terminal antibodies recognize both splice forms. Similarly, C-terminal Homer 2 antibodies recognize both Homer 2a and 2b. Accordingly, when using these antibodies to detect Homer proteins, we refer to the immunoreactivity as Homer lb/c or Homer 2a/b. We used these antibodies to determine that Homer lb/c and 3 are enriched in a detergent resistant fraction of the postsynaptic density (PSD) (Xiao et al, 1998). Homer 2a/b is also enriched in synaptic fractions, but is relatively more soluble than Homer lb/c and Homer 3. Like Homer la, each of the CC-Homers co-immunoprecipitates with group 1 mGluRs from brain (Xiao et al, 1998). Immunogold electron microscopy (EM) demonstrated that Homer lb/c and Homer 3 are ultrastructurally localized at the PSD (Xiao et al, 1998). These observations suggest that CC-Homer proteins function as multivalent adapter complexes that bind mGluRs at postsynaptic sites. Homer la functions as a natural dominant negative protein. The fact that Homer la lacks a CC domain suggested that it may function as a natural dominant negative to disrupt cross-linking of CC-Homers. In this model, the EVHl domain of Homer la can bind and compete for the same target proteins as CC- Homers (such as mGluR5), but because Homer la lacks the CC-domain, it cannot self-associate and cannot cross-link. To test the dominant negative hypothesis, we generated a transgenic mouse that constitutively expressed Homer la in brain neurons under the control of a modified Thy-1 promoter [Aigner, 1995 #200]. We confirmed transgene expression in hippocampus, cerebellum and cortex in two independent lines (Xiao et al, 1998). The level of transgene expression in the hippocampus was similar to natural Homer 1 a expression induced by a seizure. In contract to the natural Homer 1 a, however, the transgene was constitutively expressed in the unstimulated mouse. A prediction of the dominant negative hypothesis is that the ability to co- immunoprecipitate mGluR with Homer lb/c or Homer 3 antibodies should be diminished in the transgenic mouse. As one of the controls for this experiment, we demonstrated by western blot that levels of expression of mGluRla, mGluR5 and Homer lb/c, 2a/b, 3 were unchanged in the transgenic mouse brain. We then performed IP experiments and observed the anticipated result; the co- immunoprecipitation of mGluR5 with CC-Homers from hippocampus was reduced in the transgenic mouse (Xiao et al, 1998). Similar co-immunoprecipitations of mGluRla with Homer 3 from cerebellum was also reduced. As an additional control, we demonstrated that the ability to co-immunoprecipitate Homer lb/c with Homer 3 was not altered in the transgenic mouse. This was the predicted result since the association between these proteins is mediated by their CC domains, and this interaction is not altered by the Homer la EVHl domain. These observations support the hypothesis that Homer la functions as a natural dominant negative to regulate CC- Homer-dependent cross-linking. Homer binds a proline rich sequence that is -50 aa from the C-terminus of group 1 mGluRs. When we initially characterized the interaction between Homer and mGluR5, we anticipated that Homer might bind the free C-terminus. This surmise was based on the precedent of PDZ proteins such as PSD95 and GRIP, which bind the free C- terminus of NMDAR2 (Kornau, 1995) and AMPA receptors (Dong et al, 1997). Homer was noted to encode a GLGF sequence like the PDZ domain. Additionally, in GST pulldown assays that used brief washes, we noted a modest reduction of binding when the C-terminal 4 or 10 aa were deleted from mGluR5 [Brakeman, 1997 #99]. (In retrospect, this modest reduction of binding may be due to Homer pulldown of Shank which does bind the free C-terminus of mGluR5, but appears to be lower affinity than Homer-mGluR5 binding; see below.) However, with more standard wash conditions, it became clear that the 4 and 10 aa C-terminal deletion mutants of mGluR5 continued to bind avidly to Homer. We continued the deletion strategy until we found that a 50 aa C-terminal deletion of mGluR5 destroyed binding to Homer. By contrast, a 41 aa deletion of mGluR5 retained full binding activity. We noted that the intervening sequence was proline rich and shared sequence similarity with the previously described SH3 ligand sequence [Yu, 1994 #166] We prepared a series of point mutants based on the known structure-function relationship for SH3 ligands. Binding assays confirmed general characteristics of SH3 ligand binding, but also demonstrated that that the Homer binding site is distinct in the positioning of critical amino acids (Tu et al., 1998). A consensus for binding was determined to be PPXXFR, consistent with the observation that mutation of either of the prolines or the phenylalanine, or a change in their relative position, interrupted binding. The arginine in the last position is preferred over other amino acids, but is not essential. Mutations were identically effective in interrupting binding to each of the Homer family members including Homer la, lb/c, 2a/b, 3 and an EVHl only fragment (HOaa) of Homer 1. Thus, we conclude that the interaction with mGluR5 is mediated by the Homer EVHl domain. Mutations of mGluR5 were initially tested in the context of a 250aa C- terminal fragment, but were also determined to have an identical effect on binding when placed in the full length mGluR5 protein (Tu et al, 1998). This exquisite sensitivity of Homer binding to changes in single amino acid within the Homer-ligand site has been confirmed in other Homer-interacting proteins including mGluRla (Tu et al, 1998), Shank (Tu et al, 1999) and 142 (see below). To further confirm that the interaction was mediated by a direct interaction at the Homer-ligand site (as opposed to a secondary allosteric effect on a remote binding site), we prepared synthetic 10 mer peptides with either the wild type, or F-to-R mutation, and demonstrated that the wild type peptide blocked binding of mGluRla or mGluR5 to each of the Homer family members [ (Tu et al, 1998)]. Approximately half of the binding was blocked at a peptide concentration of 3.4 micromolar. By contrast, the F-to-R mutant peptide did not alter binding at concentrations as high as 340 micromolar. Homer binds the IP3 receptor. Armed with a consensus sequence that predicted binding to Homer, we searched the NCBI data base for other proteins that might bind Homer. A Homer- ligand site was identified in the IP3R, dynamin III, a human alpha adrenergic receptor and the ryanodine receptor [ (Tu et al, 1998)]. Each of these interactions were determined to be consistent with the known topology of the candidate interacting protein, assuming that Homer proteins are cytosolic. We were able to confirm a biochemical interaction of Homer with the IP3R and dynamin III using GST pull down assays. More importantly, we demonstrated that the IP3R co- immunoprecipitates with each of the Homer lb/c, 2a/b and 3 from detergent extracts of cerebellum [ (Tu et al, 1998) ]. Homer appears to be associated with a substantial portion of IP3R in the cerebellum, since a cocktail of the three Homer antibodies is able to specifically (compared to a cocktail of preimmune serums) co- immunoprecipitate -50% of the total IP3R in detergent extracts (CHAPS). CC-Homers function to link mGluR5 and IP3R in a signaling complex. Based on the prior observations, we examined the hypothesis that CC-Homers might cross-link mGluR and IP3R. This notion was appealing in that the IP3R is part of the signaling network that is activated upon glutamate stimulation of mGluRl/5. Signaling complexes had previously been described including; AKAP proteins which function as scaffolds for specific kinases and their substrates [Lester, 1997 #149], and the Drosophila protein InaD which couples the membrane light activated channel with its down stream effector enzyme, phospholipase C [Tsunoda, 1997 #147]. Unlike these other examples of signaling complexes, however, Homer would need to form a bridge between receptors in two different membranes. Functional mGluRs are in the plasma membrane while the IP3R is localized primarily to intracellular endoplasmic reticulum (ER). In support of the notion that ER and plasma membranes can come in close apposition in neurons, we noted that Dr. Kristin Harris (Harvard) described the presence of smooth ER (SER, or spine apparatus) in the spines of hippocampal and cerebellar neurons (Tu et al, 1998). Remarkably, the SER forms close appositions with the plasma membrane that were uniquely localized to the lateral margin of the PSD. These sites are precisely where the group 1 mGluRs are localized (Baude et al, 1993; Lujan et al, 1997; Nusser et al, 1994). The IP3R is present in spines of cerebellar Purkinje neurons where it is associated with the spine apparatus (Satoh et al, 1990). (Interestingly, in hippocampal neurons, the RYR is present in the spine apparatus while the IP3R appears to be restricted to the dendritic shaft [reviewed in (Narasimhan et al, 1998)]. Homer lb/c and 3 are also enriched in the cytosol at the lateral margin of the PSD [ (Xiao et al, 1998)]. Thus, available anatomic evidence supported the notion that synaptic mGluRs come in close apposition with SER-associated IP3Rs at sites that are enriched for CC-Homers. As a first test of the hypothesis that CC-Homers cross-link mGluR and IP 3R, we asked whether we could detect a trimolecular complex of mGluR, Homer and IP3R in brain. Indeed, IP3R antibody specifically co-immunoprecipitateed Homer and mGluRla from cerebellum (Narasimhan et al, 1998). Since IP3Rs are not known to directly interact with mGluRla, this result supported the hypothesis that Homer bridges these proteins to form a trimolecular signaling complex. A further prediction of the "Homer hypothesis" is that Homer la should uncouple the putative mGluR-CC- Homer-IP3R complex. To test this, we monitored the effect of Homer la expression on glutamate-induced intracellular calcium release. Plasmids expressing Homer la or Homer lb were transfected along with green fluorescent protein (gene gun) and identified Purkinje neurons were stimulated with quisqualate. A patch electrode containing the Ca2+ detector Fura-2 was attached to the soma and a holding potential of -60 mV was applied. Tetrodotoxin and picrotoxin were included in the bath to block synaptic input and EDTA/MgCl was included to assure that measured Ca2+ increases in the cell were generated from intracellular stores. Under these conditions, quisqualate-induced Ca2+ increases are due to mGluRl -evoked release from IP3R pools (Roche et al, accepted). Expression of Homer lb did not alter the induced Ca2+ transient compared to cells transfected with an empty vector. By contrast, neurons transfected with Homer la showed a Ca2+ transient that was reduced in amplitude and delayed in time to peak (Tu et al, 1998). This result is consistent with the notion that the IP3 generated by mGluRla activation of phospholipase C is less effective in releasing Ca2+ from the IP3R pools in neurons expressing Homer la, and is anticipated if Homer la disrupts the physical linkage between mGluRla and IP3R. Released IP3 must diffuse further, thereby resulting in a lower effective concentration of IP3 at the receptor. CC-Homers alter trafficking of mGluRla/5 in heterologous cells. We initiated studies to examine the effect of Homer on mGluR5 expression. When wild type mGluR5 was expressed in heterologous cells (HEK293, COS or HeLa) the receptor reached the plasma membrane surface where it was diffusely localized. This was also true when mGluR5 was co-expressed with Homer la. However, we noted that co-expression of mGluR5 with Homer lb resulted in intracellular inclusions of mGluR5 (Roche et al, accepted). This effect of Homer lb was dependent on the amount of transfected plasmid and was most obvious when equal amounts of Homer lb and mGluR5 plasmids were co-transfected. There was a trend for higher level expression of mGluR5 when co-transfected with Homer lb. When ratios of transfected plasmids were titrated so that total mGluR5 expression was the same (comparing expression with or without Homer lb), a substantial portion of the total mGluR5 was associated with the intracellular pool when co-expressed with Homer lb. In these cells, relatively less reached the plasma membrane compared to mGluR expressed alone, or co-expressed with Homer la. We further noted that at earlier times after transfection of Homer lb and mGluR5, mGluR5 showed an enrichment in perinuclear organelles with a reticular pattern throughout the cell that resembled the ER. To assess the nature of the CC-Homer-dependent cellular accumulation, we compared the distribution of mGluR5 with the ER specific maker BIP [ B (Roche et al, accepted)]. Staining with BIP antibodies revealed extensive ER present in both transfected and untransfected cells and co-localization with mGluR5. We also noted that the perinuclear organelles were not present within non-transfected cells and therefore appeared to be ER-derived structures unique to cells overexpressing mGluR5 and Homer lb. These observations suggest that Homer lb, but not Homer la, causes mGluR5 to be retained in the ER. As an additional assay for ER retention, we examined the status of the carbohydrates present on mGluR5 in cells co-expressing Homer la or Homer lb. If Homer lb caused mGluR5 to be retained within the ER, then mGluR5 should contain immature, high mannose carbohydrates which are sensitive to digestion with the enzyme Endoglycosidase H (Endo H). Alternatively, if mGluR5 had successfully traveled through the ER and cis Golgi, it would possess mature, complex carbohydrates which would be Endo H resistant. Mature carbohydrates would be anticipated if mGluR5 was on the cell surface or if it was sequestered in a post-Golgi intracellular compartment such as endosomes. We determined that mGluR5 is Endo H resistant when expressed alone or with Homer la (Xiao et al., 1998). However, when expressed with Hlb, mGluR5 is Endo H sensitive, consistent with the hypothesis that expression of Hlb leads to the retention of group I mGluR in the ER. The subcellular localization of the group II metabotropic glutamate receptor mGluR2 was the same whether expressed alone or with Hlb. In addition, we used a series of mGluR5 constructs containing point mutations within the Homer binding site and found that mutations that disrupt mGluR5/Homer interactions in vitro also prevented ER retention of mGluR5 co-expressed with Hlb [ (Takei et al, 1994)]. mGluR5 P1125L, which does not bind to Homer in vitro (Tu et al, 1998), was not retained in the ER when co-expressed with Hlb. In contrast, mGluR5 S1126F, which does bind Homer in vitro, was ER retained when co-expressed with Hlb. Other point mutations in adjacent residues were analyzed and the results were consistent with in vitro binding studies [summarized in [ B (Ikeda et al, 1995)], demonstrating that mGluR5 is retained within the ER by Hlb only when its Homer binding site is intact. While these experiments were performed in heterologous cells, we also noted enrichment of the group I metabotropic receptor mGluRla in the ER of Purkinje cells [ (Kammermeier et al, submitted). Since Purkinje neurons express particularly high levels of CC-Homers [ (Xiao et al, 1998)], this suggests Homer proteins may naturally regulate receptor trafficking through the ER. In this model, Homer la would be permissive for transfer through the ER Golgi system to insertion into the postsynaptic membrane. The ability of CC-Homers to alter the spatial distribution and metabolism of ER associated proteins may also impact the IP3R. IP3Rs in Purkinje neurons are associated with dense stacks of ER (Satoh et al, 1990) and this stacking morphology has been shown to be regulated by neural activity (Takei et al, 1994). Since a substantial portion of IP3R in cerebellum is associated with CC-Homers, it is possible that the ability of CC-Homer to crosslink interacting proteins on two adjacent membranes plays a regulatory role in ER morphology and function. Experiments in Aims 2 and 3 will examine this hypothesis. Homer Modulates mGluR coupling to ion channels. Group 1 mGluRs modulate ionic currents by activating pertussis toxin- sensitive and -insensitive G proteins (Naisbitt et al, 1999). Modulation of Ca2+ currents by heterologously expressed group 1 mGluRs in superior cervical ganglion (SCG) neurons proceeds through multiple pathways involving both the a and βg - subunits of G proteins. We examined the effect of Homer on mGluR coupling to Ca2+ and M-type potassium channels in SCG neurons. CC-Homers, including lb, 2b and 3 produced a similar reduction of the effect of group 1 mGluRs (Kim et al, 1997; Naisbitt et al, 1999; Naisbitt et al, 1997; Takeuchi et al, 1997). By contrast, Homer la or an engineered short form of Homer 2 did not block group 1 mGluR effects, but were able to partially reverse the effect of the CC-Homers. Homer interacts with Shank suggesting a role synaptogenesis and NMDAR function. To gain further insight into the physiological function of Homer, we characterized a novel family of proteins that were identified based on their interaction with Homer la in a yeast 2-hybrid screen of a brain cDNA library. These Homer- interacting proteins were determined to be identical to the Shank family of PSD proteins that interact with GKAP and the PSD-95 complex (Tu et al, 1999). Shank proteins are specifically enriched at excitatory synapses and co-localize with NMDA receptors in primary neuronal cultures (Naisbitt et al, 1999). Shank proteins appear to be recruited to excitatory synapses by virtue of their interaction with GKAP, a synaptic protein that binds to the guanylate kinase domain of PSD-95 (Kim et al, 1997; Naisbitt et al, 1999; Naisbitt et al, 1997; Takeuchi et al, 1997). In addition to the PDZ domain which binds GKAP, Shank contains domains that mediate self- multimerization and interaction with cortactin (Golshani et al, 1998). Shank also directly interacts with Homer [ (Lujan et al., 1997)]. Homer and Shank proteins co- localize at the PSD of CA1 pyramidal neurons [ (Tu et al, 1999)], and native Homer- Shank complexes were identified in brain using GST pull down assays of Shank with GKAP [ (Otani and Connor, 1998)]. Additionally, Homer and Shank co- immunoprecipitate from brain [ (Aniksztejn et al, 1991; Ben-Ari et al, 1992)]. These observations indicate that Shank and Homer naturally associate in brain. Biochemical studies indicate that the Shank-Homer interaction is mediated by the EVHl domain of Homer which binds to a single Homer-ligand site present in the proline-rich domain of Shank proteins [ (Tu et al, 1999)]. A quaternary complex of Homer/Shank/GKAP/PSD-95 is assembled in heterologous cells, with Homer and PSD-95 co-localizing in large clusters [ (Berridge, 1998)]. Thus, Shank provides a molecular bridge that links the NMDA receptor complex with Homer and its associated proteins. The Homer-Shank interaction also produces clustering of group 1 mGluRs [ (Satoh et al, 1990; Villa et al, 1992)]. Clustering molecules have previously been identified for a variety of receptors and ion channels (Selig et al, 1995), but Shank- Homer are the first clustering proteins for group 1 mGluR. It is notable that the mechanism of clustering involves a linkage of mGluRs with the previously defined NMDA receptor scaffold. Thus the Shank-Homer interaction could be relevant to synaptogenesis, by docking mGluRs to a preestablished "core" of NMDA receptors. In support of such a mechanism, functional NMDA receptors appear to precede the emergence of metabotropic receptors in the hippocampus and cerebellum (Xiao et al, 1998). Homer proteins, in association with Shank, could function to localize and cluster the mGluRs in proximity to NMDARs, and may contribute to the perisynaptic localization of group 1 metabotropic receptors (Lujan et al, 1997). By linking NMDA and mGluR signaling pathways, the Shank-Homer interaction might also contribute to examples of glutamate receptor crosstalk for which physical proximity of molecules may be important, such as activation of phospholipase C (Beneken et al, submitted) or protein kinase C (Aniksztejn et al, 1991; Ben-Ari et al, 1992). Additionally, the Homer/Shank/GKAP/PSD-95 assembly may mediate physical association (and perhaps functional coupling) of the NMDAR with IP3R/RYR and intracellular Ca2+ stores. Consistent with such a functional interaction, recent studies indicate that NMDA receptor-dependent increases in spine Ca2+ may derive from intracellular stores by a mechanism of Ca2+ -induced Ca2+ release (CICR) [(Emptage et al, 1999) and reviewed by (Svoboda and Mainen, 1999)]. Both IP3R and ryanodine receptor channels possess CICR properties (Berridge, 1998), and are similarly localized in dendrites and spines of specific neuronal types (Satoh et al, 1990; Villa et al, 1992). The physical proximity of glutamate receptors with calcium pools may underlie synergistic effects of mGluRs on NMDA-dependent responses as reported in studies of LTP [(Bashir et al, 1993; Bortolotto et al, 1994) but see also ((Selig et al, 1995)], and is consistent with the reduction of LTP in group 1 mGluR mutant mice [(Prehoda et al, 1999) but see also (Conquet et al, 1994)]. The proposed model for Shank and Homer-dependent clustering requires that Homer be multivalent in order to cross-link Shank/GKAP/PSD95 to IP3R/RYRs and to mGluRs. This is achieved by multimerization of constitutively expressed CC-Homers (Xiao et al, 1998). In this context, the monovalent Homer la IEG product appears to function to uncouple proteins that are linked via the constitutively expressed CC- Homer multimers, and thereby dynamically regulate the assembly of this postsynaptic network. Cocaine-induced increases in Homer la may thus modulate both mGluR and NMDA Ca2+ responses in spines. Homer EVHl domain crystal structure. To investigate the structural basis of interactions between EVHl domains and ligands, we determined the high-resolution crystal structure of the EVHl domain from rat Homer 1 ( ). Methods of protein purification and crystallization are described in our manuscript (Niebuhr et al, 1997; Tu et al, 1998). This structure revealed that the EVHl module is homologous to both the plextrin homology (PH) domain and the phosphotyrosine binding (PTB) domain. ( legend next page) At the same time we were working to solve the structure of Homer 1 EVHl, Dr. Wendel Lim's group (at UCSF) solved the structure of the related EVHl protein termed Mena (20% identical to Homer EVHl domain) (Prehoda et al, 1999). Comparison of the Mena and Homer coordinates confirmed that these are related proteins despite the low degree of amino acid identity. The Mena crystal was solved with a 6mer peptide and identified a putative ligand binding surface. Both of our groups determined that co-crystals were not formed with longer synthetic peptides. One issue that concerned us regarding the putative ligand-binding site on Mena was that the affinity of the 6mer used for Mena was 100 fold less than that of a 10 mer (Prehoda et al, 1999). The measured affinity of the 6mer was -600 micromolar. Additionally, within the EVHl family, Homer is one of the most divergent members (Prehoda et al, 1999). One important difference between Mena and Homer EVHl binding, is the orientation of the phenylalanine relative to the polyprolines. The optimal ligand for Mena is FPPPP while the consensus ligand for Homer is PPXXF. This may be important since the F is the single most critical side chain for the interaction when tested with larger peptides for both EVHl domains (Niebuhr et al, 1997; Tu et al, 1998). In the Mena structure, the F side chain is not placed in a clear hydrophobic pocket (the ring appears to coordinate an arginine) and superposition of the ligand coordinates in Homer EVHl is even less obviously stabilized. To examine the predictive power of the Mena co-crystal for the ligand binding activity of Homer EVHl, we tested a series of missense mutations that targeted sites anticipated to contact the prolines of the ligand (PPXXF) sequence. Based on the homology of the EVHl domain with the PTB domain, we also tested sites on Homer that would be critical if Homer mimicked the peptide binding surface of the PTB domain. This PTB ligand site is remote from the putative Mena EVHl ligand site. Our mutation analysis also tested a series of mutants selected based on the homology between Homer and WASP. Genetic data from patients with Wiscott Aldrich syndrome defined a series of mutations in the EVHl domain that map to sites that are distinct from both the PTB and the putative Mena ligand sites. Our selection of the mutational substitutions was based on the Homer EVHl structure. Substituted amino acids were selected to be sufficiently conservative as not to disrupt the primary structure. A total of 30 missense mutants of the Homer EVHl domain were expressed in HEK293 cells and assayed for binding to either mGluRla or Shank3 using GST pulldown assays. Surface-exposed mutations within the region homologous to the peptide binding site of PTB domains had no affect on peptide binding. Similarly, mutations based on the WASP data were also ineffective in disrupting binding. By contrast, certain of the mutants based the Mena ligand site did disrupt Homer EVHl binding. Despite ambiguities involved with interpreting the effects of any single mutation, the nature and distribution of the effects of site-directed mutations in the Homer EVHl domain on Homer-ligand interactions strongly implicate the Mena ligand region as mediating natural ligand binding by the Homer EVHl domain. One interesting finding from our analysis of mutant Homer EVHl binding is that certain mutations disrupt binding specifically to mGluRla, but not to Shank3 (and visa versa). One interpretation of this finding is that there are determinants of binding in addition to the core PPXXF motif. An important implication of this observation is that differences in critical determinants of Homer binding to its various targets may be exploited to develop pharmaceuticals that can selectively disrupt interactions with a particular target. 142 Interacts with Homer. 142 (SEQ ID NOS: 17 and 18) encodes a novel protein that was first identified in a Y2H screen of brain cDNA with the Homer EVHl domain. Current information indicates that 142 functions with Homer at the excitatory synapse. We have generated 142 specific antisera and can demonstrate robust co-immunoprecipitation of 142 with Homer from brain. ImmunoEM analysis demonstrates that 142 is localized to the postsynaptic density. The predicted domain structure of 142 indicates that it shares certain properties with Shank including a N-terminal structural domain (a band 4.1 domain in 142), a single PDZ domain, and a central proline rich domain with a single Homer-ligand site. Additionally, there is a C-terminal type 1 PDZ ligand motif. We have identified a related sequence in the data base (KIAA sequence has several errors with frame shifts) suggesting that 142 may represent a gene family. Current studies indicate a functional interaction of 142, Homer and mGluRs. We have performed a yeast 2-hybrid screen of the 142 PDZ domain and find it binds β-Pix (also termed Cool-1) (Allen et al, 1998). β-Pix is a guanine nucleotide exchange factor (GEF) for Racl/CDC42. This interaction appears robust using GST pulldown assays and we have recently confirmed the interaction using co- immunoprecipitation assays from brain. Biochemical assays indicate that the PDZ domain of 142 binds its own C-terminus (may be intra or inter molecular). Based on these observations, 142 functions as a scaffold/cytoskeletal regulatory protein that responds to specific signals and may link between mGluR activation and Rac- dependent cytoskeletal remodeling. This biochemical association may play a role in mGluR trafficking or synaptic remodeling. An additional functional consequence of the Homer 142 interaction is indicated by the demonstrated association of β-Pix with p21 activated kinase (Pak) (Tu et al, 1999). Paks are a family of kinase that can signal both locally and more distally to the nucleus. A mutation of Pak3 has recently been linked to mental retardation (Tu et al, 1998), confirming the importance of this regulated kinase to cognitive function. Accordingly, 142 appears to be part of a novel signaling pathway for the mGluRs that may be regulated by Homer proteins. In preliminary studies, we observe that 142 co-immunoprecipitates with Homer from brain. Antibodies for 142 also co-immunoprecipitates mGluRl from brain. In parallel studies, we observed the interaction between 142 and β-Pix ( ). These observations indicate the involvement of Homer in the function of I42/β-Pix and identify another signaling pathway that can be manipulated by agents that modulate Homer binding function. ii) Ultrastructural localization of I42/β-Pix/Pak at synapses: We have performed preliminary immunoEM with 142 Ab and observes that it is associated with the PSD region. The methods and approach are identical to our studies of Shank (Naisbitt et al, 1999). This observation indicates that 142 is enriched at the excitatory synapse together with Homer, Shank and glutamate receptors. Ryanodine receptor (RYR) and Homer. The RYR encodes a potential Homer binding site near the N-terminus (Bhat et al, 1999) and using GST pulldown assays we observe that GSTHomer binds to the relevant fragment of RYRl. Importantly, we have demonstrated that the RYR co- immunoprecipitates with Homer from detergent extracts of skeletal muscle. The interaction between RYR and Homer is understood to be consistent with the function of Homer proteins to regulate the coupling of membrane receptors with intracellular calcium pools. Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons of the midbrain and this is mediated by mGluRl release of intracellular Ca2+ from RYR sensitive CICR pools (Bhat et al, 1999). RYR have recently been implicated as an important source of NMDAR-induced calcium rise in the post synaptic spine (Emptage et al, 1999). Since Shank is part of the NMDA receptor signaling complex (Naisbitt et al, 1999) and binds Homer, it is compelling to evaluate the possible interaction between RYR and Homer. NMDA Receptor Type 2D (NR2D) and Homer. Independent Y2H screens of adult cortex and cerebellum identified several clones of the NMDA receptor type 2D (NR2D). NR2D has not been as extensively studied as NR2B but is expressed in developing cerebellum and interneurons in the forebrain (Dunah et al, 1998; Goebel and Poosch, 1999). NMDAR that include the NR2DR have slower channel properties (Cull-Candy et al, 1998; Okabe et al, 1998; Vicini et al, 1998). The C-terminus of NR2D is highly proline rich consistent with our observation that Homer binds a specific proline rich sequence. Thus, in the case of NR2D, Homer proteins form a direct coupling to CICR pools. This direct coupling would contrast with NMDAR that include NR2B which appear to couple to Homer indirectly via PSD95-GKAP-Shank (Naisbitt et al, 1999). In both cases, modification of Homer crosslinking activity will alter the intracellular release of calcium due to glutamate receptor activation. Because of the differences in the binding properties of the EVHl domain of Homer to its different targets, it is anticipated that agents that specifically disrupt the linkage of NR2B or NR2D can be developed. Mammalian InaD like molecule interaction with Homer. We have identified two distinct novel members of a family of proteins with similarity to the recently reported human InaD (Philipp and Flockerzi, 1997) and Drosophila Discs Lost [DLT (Bhat et al, 1999)]. These proteins encode 5 and 4 PDZ domains, respectively, and a proline rich region that is shared in all clones that is presumed to mediate interaction with Homer. DLT has been demonstrated to be essential for establishment of epithelial cell polarity and binds to the C-terminus of Neurexin IV [DLT (Bhat et al, 1999). We currently refer to our clones as rat InaD. In current studies, we observe that full length myc-tagged rlnaD co-immunoprecipitates with Homer 2 from co-expressing HEK293 cells. 130 interaction with Homer. 130 is a novel member of the family of abl binding proteins. Related proteins function as adaptor proteins that regulate cell growth [Ziemnicka-Kotula, 1998 #392; Biesova, 1997 #393] and are hypothesized . 130 encodes a SH3 domain and a Homer binding site. Accordingly, Homer is anticipated to link this protein to other Homer- interacting proteins including metabotropic glutamate receptors and IP3R. (See SEQ ID NOS: 15, 16, 19 and 20). Cdc42-associated tyrosine Kinase- 2 (ACK-2) interaction with Homer ACK-2 is a non-receptor tyrosine kinase that is regulated by the Rho-related GTP-binding protein Cdc42 [Yang, 1999 #391]. ACK-2 is activated by signals that result from cell adhesion, by for example activation of the integrin receptor. One cellular consequence of ACK-2 activation is down stream activation of c-Jun kinase. Our observation that ACK-2 interacts with Homer indicates that this signaling pathway can be linked to other membrane receptors by Homer, and identifies another signaling cascade that can be manipulated by agents that alter Homer crosslinking function. EXAMPLE 1 Identification and Sequencing of Homer Family Members Low stringency screens of phage cDNA libraries and EST Database searches were performed to identify Homer family members. cDNA libraries were screened using the rat Homer la coding region as a probe. Screens of mouse and rat brain cDNA libraries identified two isoforms of Homer- 1 (Homer- lb and Homer- lc). Searches of EST Databases identified a mouse EST sequence (ID#442801) which is about 73% homologous to a portion of 5' coding region of Homer-I cDNA sequence. Based on the EST used RT-PCR (Forward: 5'-GAC AGC AGA GCC AAC ACC GTG-3'; Reverse: 5'-GTC TGC AGC TCC ATC TCC CAC-3') to amplify the corresponding region from various mouse tissues. The PCR products (-330 bp) consisted of two different sequences, one of which contains an additional insertion of 33 bp. A mixture of these two cDNA fragments were used as probes to screen an adult mouse brain cDNA library. Out of 10 6 clones screened, five clones hybridized well to the probe. Sequence analysis of these clones indicated that they are five partial cDNA clones representing two isoforms of a Homer-2 gene. These clones are identical to the isoforms amplified by RT-PCR. The 5' region of Homer-2 was cloned using 5'-RACE technique. Total RNA from E14.5 mouse brain was reverse-transcribed using the reverse primer described above. Another gene-specific primer (5'-CAC GGT GTT GGC TCT GCT GTC-3') was used in the amplification of the 5' region of Homer-2. The sequence authenticity of the 5' RACE clones was further confirmed by sequencing a partial mouse EST clone #441857. A search of the EST Database allowed the identification of several human EST's corresponding to mouse and rat Homer- lb, Homer-2a and 2b cDNA sequences. RT-PCR was used to clone the human Homer- lb and Homer 2a and 2b coding regions. A 5' degenerate primer (5'-ATG GG(A/G/C) GA(A/G) CA(A G) CC(T/C/G) AT(T/C) TTC-3') was designed based on an amino-terminal seven residue amino acid sequence (MGEQPIF) that is conserved among human EST clone #HCE003, mouse, rat, and Drosophila Homer homologue sequences. The 3' primers (5 '-GAG GGT AGC CAG TTC AGC CTC-3') for human Homer-1 and human Homer-2 (5'-GTT GAT CTC ACT GCA TTG TTC-3') were made from the sequences of human EST clones #562862 and #HIBAB15 respectively. Human Homer-lb and Homer-2a and 2b were amplified from new born human frontal cortex. The sequences of human Homer lb, Homer 2a and Homer 2b were derived from sequencing several PCR clones and EST clones and are shown in SEQ ID NO's:3, 7 and 9. Human and mouse Homer-3 were identified by searching EST Database, using Homer-1 and Homer-2 sequences. Two full-length human Homer-3 clones were identified (Clone ID #284002 and #38753) and sequenced. Numerous mouse Homer-3 clones were found and one of them (Clone ID #1162828 ) contains an almost full-length coding region. Also identified were several Drosophila EST sequences exhibiting significant homology at the amino acid level to the N-terminal region of Homer family members. The sequence presented in SEQ ID NO: 11 is derived from Clone #LD3829. Expression Constructs Mammalian expression constructs were made by cloning cDNA into Sail and Notl sites of pRK5 (Genentech), so that the cDNA was fused in-frame to an N terminal c-Myc tag. GST-fusion constructs were made by cloning Homer cDNA into the Sail and Notl sites of pGEX4T-2 (Pharmacia). The full-length coding regions of mouse Homer-lb, rat Homer-lc, mouse Homer-2b and human Homer-3 were engineered with Sail and Notl sites at the 5' and 3' ends by PCR using high fidelity DNA polymerase Pfu (Stratagene). Various truncations of Homer- lb/c and Homer-2b coding regions were made by PCR with specific Primers containing Sail and Notl sites. All the PCR-based constructs were sequenced to confirm the sequences and in-frame fusion. The sequence of Homer la was used to screen cDNA libraries prepared from rat and mouse brain for related gene products. Homer la sequence was also used to search GenBank data bases. Several related rodent and human sequences were identified. cDNAs that are most closely related to Homer la appear to represent alternative splice forms. This inference is based on nucleotide sequence identity of their 5'UTRs and the first 175 amino acids of the open reading frames (ORF). The presumptive novel splice variants, termed Homer lb and lc, are completely divergent from Homer la after residue 175 of the ORF and they possess entirely distinct 3'UTRs. comparison at the point of sequence divergence indicates that Homer la encodes a unique eleven amino acid carboxy terminus of the ORF and about 5 kb 3' UTR region. The unique eleven amino acid carboxy-terminal sequence of Homer la does not possess a recognizable motif. In Homer lb and lc, an additional 168 and 180 amino acids are present that are predicted to possess coiled-coil (CC) secondary structure (Lupas , Trends Biochem. Sci 21:375 (1969)). While the 3'UTR sequence of Homer la includes multiple AUUUA repeats which are implicated in destabilizing mRNAs of intermediate early genes (IEG) (Shaw and Kamen, Cell 46:659 (1986)), the 3'UTR sequence of Homer lb and lc does not include this motif. The only difference between Homer lb and lc is the inclusion in Homer lc of a twelve amino acid sequence insertion at residue 177, between the conserved amino-terminus and the CC domain. Thus, Homer lb and lc appear to be formed by a splicing event that substitutes a relatively long and unique carboxy-terminus of the ORF and shorter 3'UTR sequence that lacks the characteristic IEG motif. Multiple independent isolates of rat and mouse Homer lb and lc were identified and sequenced to confirm their natural expression in brain. Further searches identified cDNA sequences that appear to represent two additional Homer genes, termed Homer 2 and Homer 3. The sequences of two splice forms of Homer 2 and one Homer 3 sequence is presented (See Figures section). The predicted size of the protein products and general domain structure are similar to Homer lb and lc. Like Homer lb and lc, each of the Homer 2 and Homer 3 proteins contain about 120 amino acids at the amino-terminal that is highly similar to the amino-terminal domain of Homer la. The degree of amino acid identity in these regions is about 88% between Homer 1 and Homer 2 and about 86% between Homer 1 and Homer 3. Many of the amino acid differences are conservative. In contrast to the high degree of conservation in amino-terminal region, the carboxy-terminal regions of Homer 2 and 3 are only about 22% identical to Homer lb, but like Homer lb and lc are predicted to possess a CC secondary structure. The CC domains of all Homer family members exhibit significant homology (about 40- 45% amino acid similarity) to the CC regions of myosin heavy chain (Strehler et al, J Mol Biol 120:291 (1986)), kinesin heavy chain (Yang et al, Cell 56:879 (1989)) and dynactin (Gill et al, J Cell Biol 115:1639 (1991)). The distinct splice forms of Homer 2, termed Homer 2a and Homer 2b, are differentiated by an eleven amino acid insertion at residue 131 in Homer 2b. Human Homer 1, 2 and 3 are mapped to chromosomes 5, 15 and 19, respectively by the Human Genome Project. Drosophila Homer possess the basic domain structure of mammalian Homers. The amino-terminus is highly homologous to that of mammalian Homer and the carboxy terminus is predicted to form a CC secondary structure. EXAMPLE 2 Generation and Characterization of Homer Antisera Rabbit polyclonal antibodies were generated against synthetic peptides derived from the unique carboxy termini of Homer lb/c, Homer 2a/b and Homer 3. Synthetic carboxy-terminal peptides of Homer 1, 2 or 3 were conjugated to thyroglobulin with glutaraldehyde and used to immunize rabbits according to a previously published protocol (Martin et al, Neuron, 2:259 1992). Peptide sequences used are contained in Homer- lb and lc: IFELTELRDNLAKLLECS; Homer-2a and 2b: GKIDDLHDFRRGLSKLGTDN; and Homer-3: RLFELSELREGLARLAEAA. Detergent (2% SDS) extracts from rat cortex, hippocampus, and cerebellum were separated on 8% SDS-PAGE gels and transferred to nitrocellulose membranes. Blots was probed with polyclonal anti-Homer sera. Specificity was tested by incubating the antiserum with 10 μg/ml of relevant peptide at room temperature for 10 m prior to use. Rabbit polyclonal antiserum was also generated against the full length GST-Homer la fusion protein, as described previously (Brakeman, et al, Cell 87:227 1997). This antiserum recognizes all Homer 1 isoforms. Unpurified antibodies were tested for their sensitivity and specificity in detecting heterologously expressed, full length Homer proteins with amino-terminal c-myc tags. Each Homer protein was selectively detected on Western blot by the appropriate Homer antibody in soluble extracts of transfected HEK293 cells. The myc-tagged Homer proteins migrated with an apparent molecular mass of 50 kDa. There was no cross reactivity between antibodies for one Homer form and other family members. EXAMPLE 3 In Vitro Interaction of Homer Proteins with Cell-Surface mGlu Receptors To examine the interaction of Homer proteins with mGluRl and mGluR5, HEK293 cells were transiently transfected (using calcium phosphate) with full length mGluRla and mGluR5 constructs in pRK5 (Brakeman et al, 1997). Cell lysates were made 24-48 h post-transfection. GST fusion proteins bound to glutathione agarose were prepared of Homer la, Homer lc, Homer 2b, Homer 3 and two amino terminal fragments of Homer 2 according to the following procedure. GST fusion constructs were prepared by polymerase chain reaction with specific primers that included Sail and Notl sequences and subcloned into pGEX4T-2 vector (Pharmacia Biotech, Uppsala, Sweden). Constructs were confirmed by sequencing. GST-fusion proteins were expressed in BL21 bacterial strains. Bacteria were harvested and lysed in PBS, 1% Triton XI 00, 2 mM phenylmethylsulfonyl fluoride (PMSF) and pelleted at 13,000 rpm (Sorvall SS-34) at 4°C for 5 m Proteins were purified by incubating 1 ml bed volume glutathione-sepharose (GST) beads (Sigma USA) with bacterial supernatant at 4°C for 10 m, washing twice with PBS and PBS plus 1% Triton X-100. Protein was eluted with 10 mM glutathione and dialyzed against PBS at 4°C. Protein concentrations were measured by BCA (Pierce, Illinois). Cell lysates of the transfected cells were incubated with equivalent amounts of various Homer-GST fusion proteins at 4 °C for 2 h, washed with PBS and 1% Triton X-100. Proteins were eluted in 2% SDS sample buffer and separated on 8% or 2.5% SDS-PAGE gels and probed with appropriate antibody. It has been previously demonstrated that the amino-terminal 131 amino acids of Homer la is sufficient to bind group I metabotropic glutamate receptors (Brakeman et al, Nature 386: 284 (1997)). In view of the high degree of sequence conservation in this region of Homer family members, the possibility that they would also bind group I receptors was examined. GST fusion proteins were prepared of Homer la, Homer lc, Homer 2b Homer 3 and two amino-terminal fragments of Homer 2. The fusion proteins were bound to glutathione agarose and assayed for binding to full length mGluR5 or full length mGluRla expressed in HEK293 cells. These studies show that mGluR5 bound GST Homer la. mGluR5 also bound to all full length Homer constructs and to a Homer 2 amino-terminal fragment of about 141 residues but not to GST alone. The relative binding in the three assays were comparable for each of the three Homer types. A Homer 2 deletion mutant that includes only the amino-terminal 92 residues did not bind mGluR5. Similar binding of Homer proteins to mGluRl was also observed. EXAMPLE 4 In Vivo Interaction of Homer Proteins with Cell-Surface mGlu Receptors To examine if Homer proteins are naturally associated with group I metabotropic receptors in the brain, immunoprecipitation studies were performed. Rat or mouse brain tissues were sonicated (3 x 10 s) in PBS (-200 mg/ml wet weight) containing 1% Triton-XlOO with protease inhibitors and centrifuged for 10 m at 15,000g. Three μl of antiserum directed against Homer lb, Homer lc, Homer 2a, Homer 2b or Homer 3 was added to 60 μl of tissue extract and incubated for 1-1/2 h at 4°C and then washed three times with PBS/Triton. Preimmune and peptide- blocked antisera were used as negative controls. Binding in tissue samples was analyzed by gel electrophoresis and western blot analysis. Proteins were eluted in 2% SDS loading buffer. mGluRlα monoclonal antibody was obtained from PharMingen (San Diego CA). Rabbit polyclonal mGluR5 antibody was a gift from Dr. Richard Huganir, Johns Hopkins School of Medicine. Homer family members are naturally associated with group I metabotropic receptors in brain. This analysis was performed using cerebellum since all three Homer family members are expressed in this tissue. Detergent extracts of whole adult rat cerebellum were incubated with antibodies to Homer lb/c. Homer 2a/b or Homer 3 and immunopreciptates were blotted with a mouse monoclonal antibody to mGluRlα. mGluRlα co-immunoprecipitates with each of the antisera directed against Homer proteins. The predominate band after electrophoreses corresponded to the monomer form of mGluRlα (about 150 kDA) and other bands corresponding to multimers of mGluRlα are also observed. EXAMPLE 5 In Vitro Interaction of Homer Proteins with Intracellular Inositol Trisphosphate Receptors To demonstrate that Homer proteins interact in vivo with inositol trisphosphate receptors immunoprecipitation studies were performed using brain tissue. Rats or mice were sacrificed by decapitation and the cerebella were dissected immediately. Cerebella were sonicated in TE buffer (50 mM Tris, 1 mM EDTA, pH 7.4) containing 1% CHAPS and protease inhibitor cocktail (-100 mg wet weight/ml). The homogenate was centrifuged at 90,000 rpm, 20 m, 4°C in a TLA 100.3 rotor. 100 μl of the cerebellar extract was used for each immunoprecipitation assay with the following antibodies: 3 μl of crude Homer 1, Homer 2 or Homer 3 antibodies (Xiao et al, in press); 20 μg of affinity purified inositol trisphosphate antibody (gift from Alan Sharp). Antibodies and extract were incubated for 30 m at 4°C, then 60 μl of 1 :1 protein A or protein G (for goat antibody) sepharose slurry was added. The antibody/extract/beads were incubated for an additional 90 m at 4°C. After washing 3x10 m in TE-CHAPS buffer, the proteins were eluted from the beads with 30 μl of 4% SDS loading buffer and analyzed by SDS-PAGE and immunoblot. Results from these studies showed that the inositol trisphosphate receptor specifically co-precipitates with antisera directed against Homer 1, Homer 2 and Homer 3. EXAMPLE 6 Calcium Mobilization is Decreased by Transient Expression of Homer Protein Without a Coiled-Coil Domain To demonstrate that Homer cross-links metabotropic glutamate receptors and inositol trisphosphate receptors to provide or enhance a functional signaling complex, calcium mobilization was examined in cells transient expressing truncated forms of Homer protein. The truncated Homer protein used lacks the coiled-coil domain and is unable to form a bridge linking the mGluR at the cell surface with intracellular inositol trisphosphate receptors. The truncated form of Homer protein resembled Homer la with the exception of 11 residues at the carboxy-terminal. This form of Homer results in enhanced expression of Homer protein as compared with transfection of Homer la in heterologous cells. The Homer protein was introduced into Purkinje cells in primary cerebellar cultures and glutamate induced effects on calcium mobilization was measured. Embryonic mouse cerebellar cultures were prepared and maintained according to the method of Schilling et al. (Schilling et al, Neuron 7:891 1991). At 4-5 DIV, cultures were transfected with plasmids coding for E-GFP (Clontech) and either full-length Homer lb or an IEG form of Homer 1. The IEG form of Homer 1 was a 186 amino acid amino-terminal fragment of Homer lb. Plasmids were purified by cesium banding. Three combinations of the plasmids were transfected. Group I (control), 20μg of E-GFP and 40μg of pRK5 vector; group II, 20μg of E-GFP and 40μg of ρRK5 Homer 1 IEG; group III: 20μg of E-GFP and 40μg of pRK5 Homer lb. Plasmid DNA was mixed with gold particles (0.6 micron), and coated onto plastic tubing. DNA was then ballistically transfected into cells according to the manufacturer's protocol (Helios Gene Gun System, BIO-RAD). After transfection, cultures were returned to the incubator and maintained for an additional 2 days for a total of 7-8 DIV at the time of use for imaging experiments. Patch electrodes were attached to the somata of GFP-expressing Purkinje cells and a holding potential of -60 mV was applied. Micropressure electrodes (lμm tip diameter) were filled with quisqualate (100 μm in external saline) and were positioned -20 μm away from large-caliber dendrites. Test pulses were delivered using positive pressure (6 psi, 1 sec). Cells were bathed in a solution that contained (in mM) NaCl (140), KC1 (5), EGTA (0.2), MgCl 2 (0.8), HEPES (10), glucose (10), tetrodotoxin (0.005), and picrotoxin (O.l), adjusted to pH 7.35 with NaOH, which flowed at a rate of 0.5 ml/m The recording electrode contained CsCl (135), HEPES (10), fura-2 K 5 salt (0.2), and Na 2 -ATP (4), adjusted to pH 7.35 with C s OH. Patch electrodes yielded a resistance of 3-5 MΩ when measured with the internal and external salines described above. Fura-2 ratio imaging of intracellular free Ca 2 +, was accomplished by measuring the background corrected fluorescence ratio at 340 and 380 nm excitation using a cooled CCD camera system, as previously described (Linden et al, JNeurosci 15:5098 1995). Exposure times were 200 msec per single wavelength image. Experiments were conducted at room temperature. Enhanced GFP is weakly excited by illumination in the 380-400 nm spectrum. Based upon the bandpass characteristics of our 340HT15 and 380HT10 excitation filters and the absorption spectrum of enhanced GFP (Clontech), we estimate that <1% of the signal at 340 nm excitation and <5% of the signal at 380 nm excitation is contributed by GFP, even in those cells where the fura/GFP loading ratio is smallest. This could lead to a small (<5%) systemic underestimation of free calcium concentration that should distribute randomly across experimental groups. Calcium mobilization in the absence of influx was measure by ratio imaging fura-2 in Purkinje cells bathed in Ca +2 -free external saline and stimulated with a micropressure pulse of quisqualate, a metabotropic glutamate receptor agonist (Linden, Neuron 17:483 1996). The resultant Ca +2 transient is triggered by an mGluR and inositol trisphosphate pathway since it is completely blocked by either an mGluR antagonist ((+)-MCPG, 500μM in the bath) or a novel specific inositol trisphosphate receptor-associated ion channel blocker, xestospongin C (1 μM in the internal saline). Purkinje cells transfected with a truncated form of Homer showed mGluR-evoked Ca +2 responses with a decreased amplitude (170 ± 9 nM, mean ± SEM, n=30 cells) and an increased latency (10.5 ± 1.8 sec) as compared with cells transfected with Homer lb (244 ±_17 nM, 4.2 ± 0.9 sec, n=23) or an empty vector control (239 ± 19 nM, 4.5 ± 1.1 sec, n=15). The decay phase of the Ca +2 response appeared somewhat slower in neurons transfected with the truncated form. While the total Ca +2 flux appeared similar in cells transfected with truncated and complete Homer proteins and in empty vector controls, the measurement could not be made because the tail of the Ca +2 response was abbreviated due to the constraints of the image buffer capacity. EXAMPLE 7 Determination of the Crystal Structure of Homer Protein The crystal structure of Homer protein and a Homer protein binding site were determined. Results of these experiments are presented in Table (a) Protein Expression and Purification Residues 1-120 of rat Homer la were expressed in Escherichia coli BL21 cells as a C-terminal fusion to glutathione-S-transferase (GST-laENH) as previously described (Tu et al, Neuron 21:717 1998). Selenomethionine-substituted (SeMet) GST-laEVH was prepared by expression in the methionine auxotrophic strain B834 (DE3) (Νovagen). 5 mL of an overnight culture grown at 37°C in LB media supplemented with 100 μg/mL ampicillin (Sigma) was added to 4L M9 minimal media (Gibco BRL) supplemented with 100 μg/mL ampicillin, 0.05 mg/mL alanine, aspartic acid, glutamic acid, phenylalanine, glycine, histidine, isoleucine, lysine, asparagine, proline, glutamine, arginine, serine, threonine, valine, tryptophan, tyrosine, L-selenomethione , 1 μ/mL thiamine (Sigma), 2mM MgSO 4 , 1% glucose, 100 μM CaCl 2 . Cells were grown to an A 60 o of 0.5 at which time IPTG (Calbiochem) was added to a final concentration of 0.2 mM. Cells were grown for an additional 3 hours, harvested by centrifugation, and resuspended in lx PBS/1% Triton. Pepstatin A and leupeptin (Boehringer-Mannheim) were added to a final concentration of 1 μg/mL, and PMSF (Life Technologies) was added to 0.5 mM. Cells were lysed by sonication and centrifuged at 13,000 rpm in an SS-34 rotor to pellet cell debris. The cleared lysate was added to a 5 mL glutathione-agarose (Sigma) column. The column was washed in succession with twenty column volumes of lx PBS/1% Triton, twenty column volumes of lx PBS, and ten column volumes of cleavage buffer (50 mM Tris [7.4], 150 mM NaCl, 2.5 mM CaCl 2 ,. 50 mM β-mercaptoethanol). All buffers were degassed. A 50% slurry of glutathione-agarose beads loaded with fusion protein was incubated with 20 U of biotinylated Thrombin (Novagen) for 16 h at room temperature. The released cleavage product (la-EVH) was collected, and the biotinylated Thrombin was removed with streptavidin-agarose beads (Novagen). la-EVH was further purified by cation-exchange chromatography using a Resource S column (Amersham-Pharmacia). (b) Crystallization and Data Collection Crystals of native and SeMet protein were grown in hanging drops by the method of vapor diffusion (Wlodawer et al, Proc Natl Acad Sci USA 12:711 1975). lμl of a 9 mg/mL native or SeMet protein solution was mixed with a 1:1 dilution of reservoir buffer (30% PEG 3350, 87 mM MgSO 4 , 50 mM HEPES, pH 7.3) with distilled water and equilibrated over I mL of reservoir buffer. All crystallization trials for the SeMet protein were set up under anaerobic conditions to minimize potential problems due to oxidation. Two different crystal forms were observed for both the native and the SeMet protein. Crystals in the orthorhombic space group P2 1 2 1 2 1 , (unit cell dimensions a = 33.79 A, b = 51.40 A, c = 66.30 A) typically grew to a size of 0.5 mm x 0.03 mm x 0.03 mm. Crystals in the trigonal space group P3 2 21 (unit cell dimensions a = b = 49.94 A, c = 80.91 A) grew to a size of 0.4 mm x 0.1 mm x 0.1 mm. All data used for phasing and refinement were collected from a single trigonal SeMet crystal soaked in mother liquor plus 10% (v/v) ethylene glycol for approximately three minutes prior to flash freezing in a gaseous nitrogen stream at -180°C. X-ray diffraction data suitable for multiwavelength anomalous dispersion (MAD) phasing were collected at four wavelengths at or near the Se absorption edge. These data were collected at beamline X4A of the National Synchrotron Light Source at Brookhaven National Laboratory using an R-AXIS IV image plate detector. Nonoverlapping oscillations (2°) at φ and φ + 180° were measured over a 90° rotation of the crystal, interleaving the four wavelengths. All data were processed and scaled using the DENZO/SCALEPACK programs (Otwinowski and Minor, Meth Enzymol 276:307 1997). Data collection statistics are shown in Table 1. (c) Structure Solution and Refinement The expected two selenium sites were determined and refined using the program SOLVE (Terwilliger and Berendzen, Ada Crystallogr D53:5711997: Terwilliger and Eisenberg, Ada Crystallogr A39:813 1983) and initial Se scattering factors from (Hall et al, Cell 91 :85 1997). Values for the refined Se scattering factors as determined by SOLVE are shown in Table 1. The electron density maps calculated with the experimental MAD phases as determined by SOLVE were improved by solvent flattening and histogram matching using DM (Collaborative Computational Project, 1994). An initial model of residues 1-105 was built into 1.8 A experimental electron density maps using the program O (Jones et al, Ada Crystallogr A47:110 1991). After one round of simulated annealing with bulk solvent correction and positional and B-factor refinement using CNS (Briinger et al, Ada Crystallogr D54:905 1998), residues 106-111 were built into 2F 0 -F C maps. The model was refined against the maximum-likelihood target (Pannu and Read, Ada Crystallogr A52:659 1996) using data to 1.7 A Bragg spacing collected at 0.9879 A. Eight rounds of model building and water addition alternated with B-factor and positional refinement yielded the current model, which includes residues 1-111 and 88 water molecules. No electron density was observed for residues 112-120. This model has a crystallographic R value of 25.3% and a free R value of 28.4%. The solvent content is ca. 40.6%), with one molecule per asymmetric unit. Fractional solvent accessibility for each residue was calculated in X-PLOR (Briinger, X-PLOR, Version 3.1: A system for X-ray crystallography and NMR (New Haven, CT: Yale Univ. 1992). (d) Determination of Homer Site by Site Directed Mutagenesis Point mutants of N- terminally myc-tagged, full-length Homer lb and lc and Homer 1 EVHl were made using the QuikChange TM Site-Directed Mutagenesis Kit (Stratagene). Expression constructs were transiently transfected into HEK293 cells using calcium phosphate methods. About 24-48 h post-transfection, cell lysates were prepared in lx PBS/1% Triton X-100 (Sigma) and protease inhibitors. GST pull-down assays were performed by mixing 100 μl of cell lysate with GST-mGluR5 or GST-Shank3 (residues 1143-1408) (Tu et al, in press) bound to glutathione-agarose, incubating at 4°C for 2 h, and washing with lx PBS and lx PBS/1% Triton X-100. Bound products were eluted with 100 μl 2x SDS loading buffer and detected by SDS-PAGE and immunoblot using anti-myc antibody 9EI0 (Invitrogen) and ECL reagents (Amersham). EXAMPLE 8 Homer Expression is Upregulated in Certain Brain Regions in Response to Electrically Induced Seizures Rat Homer la was cloned based on its rapid upregulation in hippocampal granule cell neurons following electrically-induced seizure (MECS; see Brakeman et al, Nature 186:284 1997) The expression of other members of the Homer family was examined in the brain following seizure. Radio-labeled riboprobes were prepared using unique sequences for Homer la, Homer lb, Homer lc, Homer 2a, Homer 2b and Homer 3. Probes used did not distinguish between the splice forms of Homer lb and lc or Homer 2 and 2b. (a) In Situ Hybridization Anti-sense and sense cRNA probes were generated from each mouse Homer plasmid by in vitro transcription in the presence of [ S]UTP, as previously described (Lyford et al, Neuron 14:433 1995). Probe for Homer-la (Xiao, 1998; GenBank # ) was derived from nucleotides 1342 to 2140, for Homer lb/c (Xiao, 1998; GenBank # ) from nucleotides 785 to 1396, for Homer-2a/b (Xiao, 1998; GenBank # ) from nucleotides 486 to 1561, and for Homer-3 (GenBank # ) from nucleotides 371 to 2123. Probe (about 10 6 cpm in 75 μl hybridization buffer) was applied to each slide. Coverslipped slides were then incubated in humidified chambers overnight at 56°C. Following completion of wash steps, slides were air dried and exposed to Kodak Biomax MR film for 2-3 days. The anatomic distribution in unstimulated animals reveals that expression of Homer la is similar to the expression of Homer lb and Homer lc. High levels of expression of Homer la are observed in the hippocampus, striatum and cortex. In the cortex, there is laminar expression with the highest levels in the superficial and deep layers. Expression of Homer 2a and 2b is enriched in the thalamus, olfactory bulb and principle neurons of the hippocampus in contrast to the cortex where low levels of expression of Homer 2a and 2b are observed. Homer 3 is expressed primarily in the cerebellum and hippocampus. In situ hybridization studies demonstrate the dramatic induction of Homer la in response to MECS. In the hippocampus, induction of expression is estimated to be greater than 20-fold compared to hippocampus from unstimulated animals. MECS induced an increase in Homer lb and lc expression of about 1.5 fold as determined by blot analysis. Expression of Homer 2 and Homer 3 is not altered in response to MECS. EXAMPLE 9 Formation of Multimeric Complexes of Homer Proteins. The CC secondary structure is implicated in protein-protein interactions (Lupas, 1996 supra). Therefore, the possibility that this domain might confer the ability to form homo- or hetero-multimers between Homer family members was examined. For examining the coiled-coil interaction of Homer family members, myc-tagged Homer- lc and Homer-2b were transfected into HEK293 cells and cell extracts were made 2-3 days post-transfection. Cell lysates were treated as described above. First, the ability of full length, bacterially-expressed GST fusion proteins of Homer to bind full length myc-tagged Homer proteins expressed in HEK293 cells was tested, myc Homer lc bound Homer lb, Homer 2b, Homer 3, Homer lb and Homer 2b carboxy-terminal CC domain, but not Homer la or Homer 2-amino-terminus. This is consistent with the notion that the CC domain is important in the interaction , since Homer la and Homer 2-amino-terminus doe not encode the CC domain. To test the specificity of the CC domain interactions, GST fusions of dynein IC-la and dynein IC-2c were generated. The CC domains of these proteins show modest sequence to Homer family CC domains and bind to the CC domain of dynactin (Gill, 1991 supra). None of the myc-tagged Homer family members bound to either dynein IC-la or dynein IC-2c. To determine whether Homer family members naturally form multimers in brain, immunoprecipitates of cerebellum were examined. Extracts immunoprecipitated with Homer lb/c antibody contained Homer 3, while extracts immunoprecipitated with Homer 3 contained Homer lb/c. While it is possible that these co-immunoprecipitated Homer family members are associated by means other than their CC domains, the fact the amino-terminus of Homer is monovalent and cannot form extended concatomers supports a model of multimerization mediated by the CC domains. Homer 2 was not detected as a multimer with either Homer 1 or Homer 3 in these immunoprecipitation experiments. EXAMPLE 10 Homer Family Proteins are Enriched at Brain Synaptic Fractions and are Expressed in Certain Peripheral Tissues The distribution and localization of Homer family proteins was examined at the using immunochemical methods. Tissue extracts were assayed using immunoblot analysis and tissue localization was examined using immunohistochemistry at the light and ultrastructural levels. (a) Immunoblot Analysis Immunoblot staining of SDS (2%) extracts of various brain regions were examined to assess the distribution of Homer proteins in the brain. Homer lb/c antibody detected a single band of about 47 kDa in cortex, hippocampus and cerebellum. These regions have similar levels of expression. The Homer 2a/b antibody detected a single major band in each of cortex, hippocampus and cerebellum. Less intense, higher apparent molecular mass bands were detected at about 60 and about 80 kDa. Homer 3 immunoblots showed low level expression in cortex and hippocampus and intense staining of a single band in cerebellum (47 kDa). Immmunostaining was completely blocked by preincubating the antibody with lOμg/ml of the relevant peptide antigen. (b) Immunohistochemistry For light microscopy, rats were deeply anesthetized with sevoflurane and perfused through the aorta with 250 ml of saline followed by 400 cc each of 4% paraformaldehyde in 0.1 % phosphate buffer (pH 6.5) and 4% paraformaldehyde in 0.1% phosphate buffer (pH 8.5). The rat was allowed to postfix for 1 hr. at room temperature and then prefused with 15% sucrose in 0.1 % phosphate buffer (pH 7.4). The brain was removed and sectioned at 40 μm on a freezing sliding block microtome and collected in PBS. Tissue was stained with an immunoperoxidase technique, as follows. Brain sections were incubated in PBS containing 0.3% H 2 0 2 and 0.25% Triton X-100 for 30 m and then washed 3x5 m in PBS. Sections were incubated in a buffer "PGT" containing 3% normal goat serum (Colorado Serum Co.) and 0.25%) Triton X-100 in PBS for 1 hr. and then transferred to the primary antiserum diluted 1 :750 in the same PGT buffer. Sections were gently shaken for 48 h at 4°C, washed 4x5 m in PBS and then incubated for 1 hr. at room temperature in a goat anti-rabbit IgG conjugated to horseradish peroxidase (Biosource International) diluted 1 : 100 in PGT. Sections were washed 4x5 m in PBS and incubated for 6 m at room temperature in 0.05% diaminobenzidine dihydrochloride (DAB:Sigma) and 0.01 % H 2 0 in 0.1 M phosphate buffer. Sections were washed in PBS, mounted onto gelatin chrome-alum subbed slides, dehydrated in a series of graded ethanol, cleared in xylene and coverslipped with DPX (BDH Limited). Immunohistochemistry was performed to determine the cellular localization of Homer lb/c and Homer 2a/b and Homer 3 in rat brain. Light microscopic examinations indicated that all three Homer proteins are enriched in Purkinje neurons. Immunoreactivity is present in the cytoplasmic region of the soma and extends prominently into the dendritic arbor. The nucleus is not stained. Little or no staining is detected in the contiguous granule cell layer. A similar light microscopic pattern of cellular localization was detected for Homer 3. Homer 2 immunostaining in cerebellum also showed staining in Purkinje neurons, but appeared technically less differentiated. (c) Electron Microscopy For EM, a postembedding immunogold method as described previously (Wang, et al, J Neurosci 18:1148 1998) was used and modified from the method of (Matsubara, et al, J Neurosci 16:4457 1996). Briefly, male Sprague-Dawley rats were perfused with 4% paraformaldehyde plus 0.5% glutaraldehyde in 0.1 M phosphate buffer. Two hundred micrometer parasagittal sections of the rostral cerebellum (folia III-V) were cryoprotected in 30% glycerol and frozen in liquid propane in a Leica EM CPC. Frozen sections were immersed in 1.5% uranyl acetate in methanol at -90°C in a Leica AFS freeze-substitution instrument, infiltrated with Lowicryl HM 20 resin at -45°C, and polymerized with UV light. Thin sections were incubated in 0.1% sodium borohydride plus 50 mM glycine in Tris-buffered saline/0.1% Triton X-100 (TBST), followed by 10% normal goat serum (NGS) in TBST, primary antibody in 1% NGS/TBST, lO nm immunogold (Amersham) in 1% NGS/TBST plus 0.5% polyethylene glycol, and finally staining in uranyl acetate and lead citrate. Primary antibodies were used at dilutions of 1 :500 for Homer lb and 1:100-1:400 for Homer 3. Immunogold EM of Purkinje neurons of the cerebellum was performed to determine whether Homer family proteins are associated with synaptic structures. Homer lb/c showed striking localization to the region of the postsynapic spine. Gold particles are densely concentrated in the region of the postsynaptic density (PSD). A very similar distribution is noted for Homer 3 immunoreactivity. It is noted that rather than being concentrated directed over the PSD or the contiguous plasma membrane, the majority of the gold particles appear to be present in the cytoplasm immediately subjacent to these structures. Peripheral Tissues Homer proteins are expressed in peripheral tissues. In detergent extracts of heart and kidney, a single band at 47 kDa immunoreactive to Homer lb and lc is detected. In extracts of liver, a complex of three bands ranging from about 44 to 47 kDa is detected. In heart, liver, skeletal muscle and intestine, bands immunoreactive to Homer 2a and 2b are detected. Homer 3 immunoreactive bands are detected in extracts of lung and thymus. Subcellular Distribution To examine the subcellular distribution of Homer proteins, a biochemical fractionation of rat forebrain was performed and fractions were analyzed by Western blotting with Homer antibodies. Fractions were blotted for mGluR5, BIP and synaptophysin to monitor anticipated enrichment of fractions. Homer lb/c, 2a/b and 3 were present in the crude nuclear pellet (PI), the medium spin crude synaptosomal pellet (P2), and the high speed microsomal pellet (P3). BIP is a 78 kDa ER resident protein (Munro and Pelham, Cell 48:899 (1987)). and was enriched in both the P3 and the S3 fractions. While Homer lb/c and Homer 3 were not abundant in the soluble (S3) fraction, Homer 2 was enriched in the S3 fraction. The P2 fraction was subfractionated after hypotonic lysis. The 25,000 x g pellet (LP1), which is enriched in PSDs (Huttner et al, J Cell Biol 26:1374 (1983)), showed enriched presence of mGluR5. The high speed pellet (165,000 x g; LP2) showed the anticipated enrichment in the synaptic vesicle protein synaptophysin (P38). Each of the Homer proteins was enriched in the LP1 fraction relative to LP2. the final soluble fraction (LS2) was uniquely enriched in Homer 2. EXAMPLE 11 Transgenic Mouse Model Demonstrates that Expression of Homer la Selectively Blocks Binding of Homer lb/c to mGluR5 In Vivo N-terminal myc-tagged full-length Homer la ORF was cloned into the expression vector pT2 (Gordon, et al, Cell 50:445 1987; Aigner, et al, Cell 83:269 1995). Transgenic mice were generated at the University of Alabama Transgenic Facility. Expression of the transgene protein was assayed by western blot with rabbit polyclonal antisera that recognizes all Homer 1 isoforms (pan-Homer 1 antibody) and myc antibody. Homer la is unique within the family of Homer related proteins in that it is dynamically regulated and it lacks the CC domain. Accordingly, it was hypothesized that the IEG would bind to group 1 metabotropic receptors and disrupt the formation of multivalent complexes of Homer and mGluR. To examine this hypothesis, a transgenic mouse was generated that expresses Homer la under the control of a modified Thy-1 promoter (Gordon et al, 1987, supra), which drives neuron-specific expression in postnatal brain (Aigner et al, 1995, supra). Transgenic mice expressed Homer la at high levels in cortex, hippocampus, cerebellum and thalamus/brainstem relative to levels in wild type litter mate controls. The pattern of Homer la transgene expression is consistent with the previously reported activity of this promoter (Gordon et al, 1987, supra). As expected, antibodies for both Homer lb/c and Homer 2a/b co-immunoprecipitated mGluR5 from detergent extracts of wild type forebrain. By contrast, Homer lb/c antibody did not co-immunoprecipitate mGluR5 from transgenic mice. The effect of Homer la transgene expression was selective in that it did not disrupt the co-immunoprecipitation of Homer 3 with Homer lb/c. The latter observation is consistent with the notion that the Homer lb/c-Homer 3 interaction is mediated by the CC domain and is predicted not to be altered by Homer la expression. Homer la was not part of the complex co-immunoprecipitated with Homer lb/c, consistent with the notion that the CC is necessary for association with the complex. The effect of the Homer la transgene in blocking the in vivo coupling of mGluR5 and Homer lb/c was additionally selective in that Homer 2 antibody co- immunoprecipitated mGluR5 similarly from extracts of wild type and transgenic mice. Thus Homer la appears to selectively disrupt the interaction of Homer lb/c with mGluR5 but not Homer 2 with mGluR5. Homer 3 is less highly expressed in forebrain than Homer lb/c or Homer 2a/b and co-immunoprecipitates of mGluR5 with Homer 3 antibody were less clean. Accordingly, it could not be determined in these experiments whether Homer la also competes with Homer 3. Identical results were obtained in tow independent mouse lines that express Homer la transgene. The Homer la expressing transgenic mice have not been behaviorally characterized but appear normal in size and gross motor activity. EXAMPLE 12 Yeast Two-Hybrid Screen To examine the physiological functions of Homer, a novel family of proteins was identified based on its ability to interact with Homer family proteins in a yeast two-hybrid screen of a brain cDNA library. Homer la was subcloned into pPC97 (Chevray and Nathans, Proc. Natl. Acad. Sci. U.S.A., .8.9:5789 (1992)) and used to screen a random primed cDNA library prepared from seizure-stimulated rat hippocampus and cortex cloned in pPC86 (Chevray and Nathans, 1992, id.) as described previously (Brakeman et al, Nature, 386:284 (1997)). The same library was rescreened using the PDZ domain of Shank 3 (amino acid residues 559-673) cloned into pPC86. The Shank 3 PDZ domain was also tested for interaction with mGluR constructs in pPC86. mGluR5 constructs included a wild type C-terminal 241 amino acid fragment and a four amino acid carboxy-terminal deletion of the same fragment. Using Homer as "bait" in a yeast two-hybrid screen of a rat cortex and hippocampus cDNA library, multiple cDNA isolates of two novel genes were obtained. Sequencing and full length cloning identified these as distinct members of a gene family, termed Shank 1 and 3 (Naisbitt et al, in press). Shank family proteins are closely related to a previously described protein, termed Cortactin Binding protein (CortBP-1; Du et al, Mol. Cell Biol, 18:5838 (1998)). EXAMPLE 13 Interactions Between Homer Proteins and Shank Proteins In Vitro and In Vivo To characterize the interaction between Homer proteins and Shank proteins, the Shank cDNAs isolated from the yeast two-hybrid screen (Example 10)) were expressed in HEK293 for GST pulldown assays with GST-Homer la. The interaction between Homer and Shank proteins was further characterized by co- immunoprecipitation assays. (a) Expression Constructs Shank expression constructs were prepared as described (Naisbitt et al, in press). Site directed point mutants of Shank were generated using Quik Change (Stratagene). GST fusion constructs were prepared by polymerase chain reaction (PCR) using Pfu Polymerase (Stratagene) with specific primers that included Sail and Notl sequences. After digestion with Sall/Notl, PCR products were subcloned into pGEX4T-2 vector (Pharmacia Biotech, Uppsala, Sweden) or N-myc-tagged pRK 5 vector (modified from Genentech). All constructs were confirmed by sequencing. GST- fusion proteins were expressed in BL21 E. coli strains (GIBCO, BRL). Bacteria were harvested and lysed in PBS, 1% Triton X-100, 2 mM phenylmethylsulfonyl fluoride (PMSF) and pelleted at 13,000 rpm (Sorvall SS-34) at 4°C for 5 m. Proteins were purified by incubating 1 ml bed volume glutathione-sepharose beads (Sigma) with bacterial supernatant at 4 °C for 10 m. and washed twice with PBS and PBS plus 1% Triton X-100. Bound proteins were eluted with 10 mM glutathione and dialyzed against PBS at 4°C. Protein concentrations were measured by BCA (Pierce, Illinois). mGluR5 constructs and mutants are described in Tu et al, Neuron 21:111 (1998). (b) GST pulldown and Co-immunoprecipitation Assays Expression constructs were transiently transfected into HEK293 cells using the calcium phosphate method. Cells were lysed 24-48 h post-transfection with PBS plus 1 % Triton X-100. GST pull down assays were performed by mixing 100 μl cell lysates with beads charged with GST fusion proteins (1-3 μg/50 μl bed vol.) at 4°C for 2 h followed by washing once with PBS, once with PBS plus 1% Triton X-100. Bound proteins were eluted with 100 μl 2 x SDS loading buffer and detected by SDS- PAGE and immunoblotting using ECL reagents (Amersham). GST pull down assays of mGluRla and mGluR5 from brain lysates were performed by sonicating rat cerebellum or cortex in 50mM Tris, ImM EDTA, 1% CHAPS (Sigma), 0.5% deoxycholic acid (Sigma) and proteinase inhibitors with GST-proteins and these tissue extracts were then processed as above. For immunoprecipitation from COS7 cells, transfected cells were extracted in RIPA (see Naisbitt et al, in press). Soluble extracts were precipitated with 2 μg control non-immune IgG, Myc or Shank 1 (56/e) antibodies (Naisbitt et al, in press). Extracts of forebrain crude synaptosomes for immunoprecipitation were prepared using deoxycholic acid as described previously (Dunah et al, Mol. Pharmacol. 53429 (1998)). Forebrain P2 fraction was extracted in 1% deoxycholic acid, dialyzed over night into 0.1 % Triton X-100, 50 mM Tris, pH 7.4. Concurrently, 5 g of each antibody was pre-incubated overnight with 10 μl bed volume protein A- sepharose. After centrifugation at 100,000g for 1 h, 50 μg of extract was incubated with antibody-protein A in 100 μl 0.1% Triton X-100, 50 mM Tris, pH 7.4 for 2 h at 4°C. Pellets were washed 4 times with 1 ml incubation buffer, and bound proteins were analyzed by immunoblotting. Antibodies Shank antibodies were raised in rabbits immunized with GST- fusions of Shank 3 residues 1379-1740 and 1379-1675 (Covance, Denver, PA). Similar bands were seen on rat brain immunoblots with both antisera. GKAP, PSD 95 and Shank 1 (56/e) antibodies are described in (Naisbitt et al, in press). Homer antibodies are described above. Anti-mGluR la monoclonal antibody is from Pharmingen and rabbit polyclonal mGluR5 antiserum was obtained from Dr Richard Huganir (Johns Hopkins University). Shank cDNAs derived from the yeast two-hybrid screen were expressed in HEK293 cells for GST pulldown assays with GST-Homer la. Each of the Shank polypeptides specifically bound Homer la. Based on the finding that the Homer EVHl domain binds a specific proline-rich motif, three potential Homer binding sites (or Homer "ligands") that are conserved in Shank 1, 2, 3 and CortBP-1 were identified. (Naisbitt et al, in press). To define the Homer binding site on Shank family proteins, three deletion fragments of Shank 3 that included, respectively, amino acid residues 559-908, amino acid residues 1143-1408, and amino acid residues 1379-1740 were testing for their ability to bind to Homer lb, Homer lc, Homer 2 and Homer 3 in GST pulldown assays. Similar binding specificity was detected with each of the Homer proteins. Only Shank3 fragment 1143-1408 bound to Homer. This region contains the amino acid sequence that most closely resembles the Homer ligand peptide consensus (LVPPPEEFAN; residues 1307-1316). A similar sequence is present in Shankl (PLPPPLEFSN 1563-1572; see Naisbitt et al, in press). CortBP possesses two similar sites; (PLPPPLEFAN; residues 813-822) and (FLPPPESFDA residues 878-887). Fragments of Shank3 containing amino acid residues located nearer the amino-terminal of the protein such as Shank 3 fragment 559-908 (which includes the PDZ domain and the first proline-rich motif) did not bind to Homer, but did bind to GKAP (Naisbitt et al, in press). Similarly, Shank3 fragment 1379-1740, which includes the carboxy-terminal proline-rich sequence and the SAM domain, did not bind to Homer, though it is capable of binding itself and cortactin (Naisbitt et al, in press). These studies identify the Homer binding site as being distinct from either the PDZ domain that binds GKAP, or the proline-rich binding site that binds cortactin and which is located nearer to the carboxy-terminal (Naisbitt et al, in press). To confirm the site of Homer interaction, site directed point mutants of the putative Homer ligand in Shank3 were assessed for their ability to bind to GST-Homer lc. Full length wild type Shank 3, Shank3(P1311L), and Shank3(F1314C) were expressed in HEK293 cells and assayed for binding to GST- Homer lc. Compared to wild type Shank 3, both point mutants showed dramatically reduced binding to Homer., These experiments provide further confirmation that the Homer ligand in Shank3 is the principle site of interaction. It has been previously demonstrated that amino acids 1-110 of the Homer EVHl domain are necessary and sufficient for binding to Homer ligands (Brakeman et al, 1997, supra; Tu et al, 1998, supra). To confirm that the EVHl domain of Homer mediates interactions with Shank, a series of point mutants of the Homer 1 EVHl domain were generated. Mutations that disrupted binding to mGluR5 disrupted binding to Shank 3 in an identical manner, indicating Homer binds both proteins via a similar EVHl -dependent mechanism (Beneken et al, in press). To confirm the interaction between Homer and Shank in a mammalian cell context, co-immunoprecipitation experiments were performed in heterologous cells. COS7 cell were transfected with Myc tagged-Homer lb, Shank 1, or Shank 1 plus myc-Homer lb. Detergent extracts of cells were subjected to immunoprecipitation and blotted with myc, shank, or control (non-immune IgG) antibodies. Homer lb was used in these experiments because it expresses more efficiently in mammalian cells than Homer la. There is co- immunoprecipitation of Homer with Shank antibody and of Shank with myc antibody only from cells expressing both Shank and myc-Homer lb. To demonstrate the in vivo relevance of the Homer-Shank interaction, co- immunoprecipitation experiments were performed using detergent extracts of rat brain. Detergent extracts of rat forebrain fractions were immunoprecipitated with Shank and control (non-immune) antisera. Immunoprecipitates were blotted for Homer, Shank and GRIP antibodies. Antibodies raised against a fusion protein of Shank 1 immunoprecipitated Homer lb and lc proteins as well as Shank from rat forebrain. GRIP was not co-immunoprecipitated with Shank and neither Shank or Homer were precipitated by non-immune IgGs. Furthermore, another Shank antibody, generated against Shank 3 fragment 1379-1675, co-immunoprecipitated Homer lb and lc extracted from both cerebellum and cortex. EXAMPLE 14 Homer and Shank Mediate Clustering of Cell-Surface Receptors Shank proteins may link Homer proteins with components of a cell-surface clustering complex, such as the NMDA clustering complex. COS7 cells were transfected using the Lipofectamine method (GIBCO-BRL) on poly-lysine coated coverslips for clustering experiments, as described in Naisbitt et al (in press) and Kim et al. (Neuron 17:103 1996). Primary antibodies were used as follows: GKAP C9589, 1 μg/ml (Naisbitt et al, in press); Shank 56/e 0.5 μg/ml (Naisbitt et al, in press), PSD-95, 1:1000 diluted guinea pig serum (Kim et al, Neuron 378:85 1995). Cy3 and (fluoroscein isothiocyanate conjugate (FITC)- conjugated secondary antibodies (Jackson Immunoresearch) were used at dilutions of 1 :500 and 1 : 100 respectively. Yeast two-hybrid screens were performed as described in Example 10. A yeast two-hybrid screen of the same rat brain cDNA library was performed using the PDZ domain of Shank3 as bait. From this screen, two identical clones of the carboxy-terminus of GKAP-3/SAPAP3 were isolated. In a reciprocal screen, Naisbitt et al. (in press) isolated multiple clones of Shankl, 2 and 3 using GKAP as bait. This result provides independent confirmation of the specificity of the interaction between the Shank and GKAP/SAPAP families of proteins. The cDNA from the yeast two-hybrid screen encoding the carboxy-terminal 347 amino acids of GKAP-3 was expressed with an amino-terminal myc tag in HEK293 cells and tested for binding to GST fusion constructs of Shank3 and other PDZ containing proteins. The GST fusion of Shank3 fragments containing just the PDZ domain (residues 559-673) was sufficient to bind GKAP3, while a Shank3 construct lacking the PDZ domain (residues 665-908) failed to bind. Additionally, PDZ domains of GRIP and SAP 102 failed to pull down GKAP3, demonstrating the specificity of the Shank-GKAP interaction. The above findings suggest that Homer, Shank and GKAP may assemble into a ternary complex. To explore this further, GST pull-down assays were performed using rat brain extracts. The carboxy-terminal 76 amino acids of GKAP la, containing the Shank PDZ-binding sequence -QTRL, was fused to GST [GST-GKAP(carboxy- terminal)]. GST-GKAP(carboxy-terminal) specifically pulled down both Shank and Homer lb and lc, but not GKAP1 or several other proteins (Naisbitt et al, in press). Since GKAP binds directly to Shank but not to Homer (Naisbitt et al, in press), the results suggest that the GKAP pulldown of Homer is mediated by Shank. These findings corroborate the co-immunoprecipitation experiments of Shank and Homer from brain extracts and confirm that Homer is associated with Shank in a native complex. Since Shank proteins may link Homer proteins with components of the NMDA clustering complex, co-clustering of these proteins in transfected COS cells was assessed. In cells co-expressing Homer lb and PSD-95, both proteins showed a diffuse distribution in the cytoplasm. This is not surprising, since Homer and PSD-95 do not interact directly. When cells were transfected with Shank 1 and GKAP in addition to Homer and PSD-95, Homer and PSD-95 redistributed into plaque-like clusters in which both proteins were exactly co-localized. By contrast, co-clustering of Homer and PSD-95 was not observed following co-transfection of Homer and PSD-95 with either Shankl or GKAP alone. Thus, Homer and PSD-95 co-cluster only upon co-expression of Shank and GKAP. Therefore, Shank and GKAP may mediate the formation of a quaternary protein complex containing PSD-95 and Homer (see also Naisbitt et al, in press). Other types of macromolecular complexes may also form when Homer and Shank proteins interact. Cells expressing Homer lb and Shank 1 (without GKAP or PSD-95) exhibited a redistribution of Homer lb into a reticular filamentous pattern, as well as into clusters; in both kinds of structures Shank and Homer immunoreactivities were co-localized. These findings provide further evidence for an interaction between Homer and Shank, and suggest that Homer lb and Shank can co-assemble into higher order macrocomplexes. This result is consistent with the biochemical properties of Shank that include its ability to self- multimerize and bind cortactin (Naisbitt et al, in press). Since Shank, GKAP, and PSD-95 are components of NMDA receptor-associated complex (Naisbitt et al, in press), the identification of Homer as a Shank-binding protein invokes a molecular link between the NMDA receptor complex and Homer-associated synaptic proteins such as mGluRla and 5 and the inositol trisphosphate receptor. Group 1 Metabotropic Receptors Based on the observations in heterologous cells that Shank clusters with Homer lb and that Shank together with GKAP can mediate the co-clustering of Homer and PSD-95 Shank may mediate clustering of group 1 metabotropic glutamate receptors (mGluRs). Co-expression of Shankl and mGluR5 in COS cells did not result in obvious clustering of either protein. Similarly, Homer and mGluR5 do not form co-clusters. Co-expression of the three proteins Homer, Shank 1, and mGluR5, however, resulted in conspicuous co- clustering of mGluR5 with Shank 1. Clustering of mGluR5 in these triply transfected cells was dependent on the ability of Homer to bind the receptor since a point mutant of mGluR5 that does not interact with Homer failed to co-cluster with Shank. Thus, both Homer and Shank are required to mediate the clustering of mGluR5. EXAMPLE 15 The Shank 3 PDZ Domain Binds the Carboxy-Terminus of Group 1 Metabotropic Receptors Directly at a Site Distinct from the Homer Binding Site The Shank PDZ domain shows selective binding to the GKAP carboxy- terminus (Naisbitt et al, in press). The carboxy-terminal sequence of GKAP (-QTRL) finds similarities with that of the group 1 mGluRs (mGluRla -SSSL; mGluR5 -SSTL) and therefore it was determined whether the PDZ domain of Shank can directly bind the carboxy-terminus of group 1 mGluRs. GST-pulldown assays were performed using extracts from heterologous cells expressing a recombinant mGluR5 carboxy-terminal 241 amino acid peptide. The mGluR5 carboxy-terminal tail bound two partially overlapping constructs of Shank 3 that included the PDZ domain (559-908; and 559-673), but not a construct from which the PDZ domain was deleted (amino acids 665-908). Binding of mGluR to the Shank3 PDZ domain was qualitatively similar to mGluR5 binding to Homer lc and Homer 2. Negative controls included absence of binding of mGluR to SAP102 PDZ1-3 and GRIP PDZ 4-6. Furthermore, a deletion mutant of the mGluR5 polypeptide that lacked the carboxy- terminal four amino acids failed to bind to the PDZ domain of Shank3. Identical interactions between Shank PDZ and mGluR5 C-terminal tail were detected in a yeast two-hybrid analysis. These studies indicate that the PDZ domain of Shank 3 can bind the carboxy-terminus of group 1 metabotropic receptors via a PDZ-mediated interaction with the carboxy-terminal sequence -S S/T L. To confirm that Shank3 PDZ domain can bind full length native mGluRs, GST pull down assays were performed with detergent extracts of forebrain or cerebellum. The PDZ domain of Shank 3 bound specifically to mGluRla and mGluR5 from cerebellum and forebrain, respectively. (Cerebellum predominantly expresses mGluRl, while forebrain expresses predominantly mGluR5.) While it is possible that the Shank3 PDZ pulldown of mGluRs from brain extracts is indirect, via Shank PDZ pulling down a GKAP-Shank-Homer-mGluR complex, this extended complex is unlikely given the more modest ability of GST-GKAP to pull down Homer. These studies suggest that Shank may interact with the cytoplasmic tail of mGluRla/5 both directly, via its PDZ domain, and indirectly, via Homer. The inability of Shank 1 to cluster mGluR5 in the absence of Homer indicates that the direct PDZ-dependent Shank-mGluR interaction is contingent upon a co-incident Homer interaction. Both modes of interaction with mGluR may be involved in mGluR clustering by Shank and contribute to physiological regulation. EXAMPLE 16 Shank and Homer Co-Localization at Specific Post Synaptic Densities Immuno Electron Microscopy A postembedding immunogold method (Petralia et al., Nature Neurosci 2:31 1999; Zhao et al, J Neurosci 18:5517 1998) was used. Male Sprague-Dawley rats was perfused with 4% paraformaldehyde plus 0.5% glutaraldehyde in 0.1 M phosphate buffer (PBS). Parasagittal sections (250 μm) of the hippocampus were cryoprotected in 30% glycerol and frozen in liquid propane in a Leica EM CPC. Frozen sections were immersed in 1.5% uranyl acetate in methanol at -90 °C in a Leica AFS freeze-substitution instrument, infiltrated with Lowicryl HM 20 resin at -45 °C, and polymerized with UV light. Thin sections were incubated in 0.1% sodium borohydride plus 50 mM glycine in Tris-buffered saline/0.1% Triton X- 100 (TBST), followed by incubations in 10% normal goat serum (NGS) in TBST, primary antibody in 1% NGS/TBST, 10 nm immunogold (Amersha ) in 1% NGS/TBST plus 0.5% polyethylene glycol, and finally staining with uranyl acetate and lead citrate. For double labeling, the first primary antibody (e.g., Shank; Shank3 1379-1675 antigen) and corresponding immunogold-conjugated antibody (10 nm gold) were applied, sections were exposed to paraformaldehyde vapors at 80°C for one hour, and the second primary (Homer lb and lc) and secondary (20 nm gold; Ted Pella/BBI International) antibodies were applied the following day. Controls (showing little or no gold labeling) included absence of the primary antibody for single labeling and absence of the second primary antibody for double labeling. Primary antibodies were used at dilutions of 1 :100-1:300 for Shank and 1:400 for Homer lb and lc. An antibody generated against a carboxy-terminal region of Shank 3 (amino acids 1379-1675) was used to examine the ultrastructural distribution of the Shank proteins in brain. This antibody recognizes multiple bands on brain immunoblots, including major bands of -160-180 kD and -210 kD in forebrain and cerebellum, similar to those seen with other Shank antibodies (see Naisbitt et al, in press). The different size bands presumably derive from the multiple Shank genes and splice variants. All Shank immunoreactivity is blocked by incubation of the Shank antibody with the Shank fusion protein antigen. Immunogold electron microscopy revealed intense Shank immunoreactivity at the PSD of CAl pyramidal neurons. Gold particles were distributed over the entire region of the PSD. In the same preparations, Homer lb /lc was found to co-distribute with Shank. In all profiles with immunostaining for both Shank and Homer, gold particles were present over the PSD but also extended into the region subjacent to the PSD. This distribution is similar to the distribution of NMDA receptors associated with the postsynaptic membrane (Petralia et al, 1999, supra) and distinct from the distribution of mGluR5 which are most prevalent in the perisynaptic membrane region just outside the PSD (Lujan et al, Eur J Neurosci 8:1488 1996). This spatial localization is consistent with the idea that Shank 3 and Homer interact with components of both the NMDA receptor and metabotropic receptor signaling complexes. This family of proteins that interact with Homer are identical to the Shank family of postsynaptic density (PSD) proteins that interact with GKAP and PSD-95 complex (Naisbitt et al, in press). 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It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. Table 1. Data Collection, Phase Calculation, and Refinement Statistics MAD Data Collection Statistics Wavelength (λ) 09879 09793 09790 09611 Unique reflections 24051 24179 24226 24481 Redundancy 6.2 6.2 6.2 6.3 Completeness (%) 96.8 97.2 97.3 98.4 Signal (<I>/σ<I>) a 21.5 (2.3) 21.0 (2.1) 20.9 (2.0) 20.4 (1.9) Rsym (%) 8.1 8.7 9.1 8.8 Overall figure of merit 0.71 MAD Structure Factor Ratios b and Anomalous Scattering Factors 0 Wavelength (λ) 0.9879 0.9783 0.9790 0.9611 0.9879 0.033 0.040 0.032 0.026 0.9793 0.047 0.029 0.044 0.9790 0.063 0.036 0.9611 0.050 f (e) -4.87 -9.96 -8.06 -4.15 f' (e) 0.47 3.77 6.28 4.12 Refinement Statistics Rcryst (%) 25.3 Rfree (%) 28.4 Average B (A 2 ) 24.8 protein/31.7 solvent No. of water molecules 88 RMSD bond lengths (A) 0.0126 RMSD bond angles (°) 1.745 RMSD B values (A 2 ) 0.837/1.487 bonds/angles main chain 1.021/1.594 bonds/angles side chains "Values in parentheses are for the highest resolution shell (1.73-1.70A). Rsym - 100 x Σ 11 - <I> I / ΣI where I is the integrated intensity of a given reflection. b RMS (Δ I F I ) /RMS ( I F | ) where Δ | F | is the Bijvoet difference at one wavelength (values on the diagonal) or the dispersive differences between two wavelength (values off the diagonal). c Anomalous components of the Se scattering factors as a function of wavelength as determined by SOLVE (Terwilliger and Eisenberg, 1983). d All rounds of refinement included data for which | F | >2.0σ. R value = Σ | F p (obs) - F p (calc) I /ΣFp(obs), where F p is the structure factor amplitude. The free R value was calculated from 10% of the data that was excluded from the refinement (Briinger, Amino Acid Residues and the Homer Binding Domain Mutation Expression Level Binding 3 (Western Blot) Homer 2 EVH WT ++ + ~ - F7A - ND F7R + + S8L + N23A ++ + S28A + V34M ++ + S35V ++ + D39A ++ R42E ++ R42A ++ + R46A ++ R46C ++ + I48A ++ + N58A ++ + N64G ++ + F67S + K69A ++ + Q72A ++ + F74A ++ + F74L ++ + F90S ++ + E93K + + H95A ++ + L96S + + F109C + (-) indicates substantially reduced binding relative to wild-type (+). Table 2. WASP EVHl Mutations Table 2A - βl region WASP Residue/Mutation Homer Residue Exposed L39M Metl C43W Pro 5 L46P Ser8 T48I ArglO E133K His 95 Buried/partially buried T45M Phe7 A47D Thr9 A49E Ala 11 Table 2B - β3 region WASP Residue/Mutation Homer Residue Exposed S82P/F Arg42 Buried/partially buried F84L Val44 R86C/H/P/L Arg46 G89D Ser49 Table 2C - Other mutations WASP Residue/Mutation Homer Residue Exposed P58L Pro 18 E131K Glu93 Buried/partially buried H68P Ser28 V75M Ser35 Y107S/C Phe67 G125R Gly87 F128S Phe90 A134T/V Leu 96 Other A56V - W97C - Homer Sequence Listing SEQ ID Sequence No. 1 Human Homer la (nucleic acid) Human Homer la (amino acid) Human Homer lb (nucleic acid) Human Homer lb (amino acid) Homer lc (nucleic acid) Homer lc (amino acid) Human Homer 2a (nucleic acid) Human Homer 2a (amino acid) Human Homer 2b (nucleic acid) 10 Human Homer 2b (amino acid) 11 Human Homer 3 (nucleic acid) 12 Human Homer 3 (amino acid) 13 peptide binding — core region: PPXXFR 14 peptide binding — extended region: ALTPPSPFRD 15 Homer interacting protein: rat 130 (nucleic acid) 16 Homer interacting protein: rat 130 (amino acid) 17 Homer interacting protein: rat 142 (nucleic acid) Homer interacting protein: rat 142 (amino acid) 19 Homer interacting protein: human 130 (nucleic acid) 20 Homer interacting protein: human 130 (amino acid) 21 Homer interacting protein: human 142 (nucleic acid) 22 Homer interactin protein: human 142 (amino acid) 23 Mouse Homer la (nucleic acid) 24 Mouse Homer la (amino acid) 25 Mouse Homer lb (nucleic acid) 26 Mouse Homer lb (amino acid) 27 Mouse Homer 2a (nucleic acid)

Claims: What is claimed is: 1. A method for identifying a compound that modulates a cellular response mediated by a cell-surface receptor comprising: (a) incubating the compound and a cell expressing a cell-surface receptor and a Homer protein under conditions sufficient to permit the compound to interact with the cell; (b) exposing the cell to a cell-surface receptor ligand; and (c) comparing a cellular response to said ligand in a cell incubated with said compound with the response of a cell not incubated with said compound, thereby identifying a compound that modulates a cellular response. 2. The method of claim 1, wherein said cell-surface receptor is a glutamate receptor. 3. The method of claim 2, wherein the glutamate receptor is a Group I metabotropic glutamate receptor or ionotropic glutamate receptor. 4. The method of claim 3, wherein the metabotropic glutamate receptor is selected from the group consisting of metabotropic glutamate receptor la and metabotropic glutamate receptor 5. 5. The method of claim 3, wherein the ionotropic glutamate receptor is selected from the group consisting of NMDA glutamate receptor of the class NR2B and NMDA glutamate receptor of the class NR2D. 6. The method of claim 1, wherein said Homer protein is selected from the group consisting of Homer la, Homer lb, Homer lc, Homer 2a, Homer 2b and Homer 3. 7. The method of claim 1, wherein said Homer protein is Homer la. 8. The method of claim 1, wherein said cellular response is an increase or decrease in calcium mobilization. 9. The method of claim 1, wherein said compound is selected from peptides, peptidomimetics, polypeptides, pharmaceuticals, chemical compounds, biological agents, antibodies, neurotropic agents, combinatorial compound libraries and anti-epileptic agents. 10. The method of claim 1, wherein said cell is selected from the group consisting of a neuronal cell, a glial cell, a cardiac cell, a bronchial cell, a uterine cell, a testicular cell, a liver cell, a renal cell, an intestinal cell, a thymus cell, a spleen cell, a placental cell, a skeletal muscle cell and a smooth muscle cell. 11. A method for identifying a compound that modulates a cellular response mediated by an intracellular protein comprising: (a) incubating the compound and a cell expressing an intracellular protein and a Homer protein under conditions sufficient to permit the compound to interact with the cell; (b) exposing the cell to conditions that activate said intracellular protein; and (c) comparing a cellular response in a cell incubated with said compound with the cellular response of a cell not incubated with said compound, thereby identifying a compound that modulates a cellular response. 12. The method of claim 11, wherein said intracellular protein is selected from the group consisting of an inositol trisphosphate receptor, a ryanodine receptor, 142, 130, hlnaD, and ACK-2. 13. The method of claim 11 wherein said Homer protein is selected from the group consisting of Homer la, Homer lb, Homer lc, Homer 2a, Homer 2b and Homer 3. 14. The method of claim 11 , wherein said Homer protein is Homer 1 a. 15. The method of claim 11 , wherein said cellular response is an increase or decrease in calcium mobilization. 16. The method of claim 11 , wherein the compound is selected from peptides, peptidomimetics, polypeptides, pharmaceuticals, chemical compounds, biological agents, antibodies, neurotropic agents and anti-epileptic agents. 17. A method for identifying a compound that modulates receptor- activated calcium mobilization comprising: (a) incubating the compound and a cell expressing a Homer protein under conditions sufficient to permit the compound to interact with the cell; (b) exposing the cell to conditions sufficient to activate calcium mobilization; and (c) comparing the cellular response in a cell incubated with said compound with the cellular response of a cell not incubated with said compound, thereby identifying a compound that modulates receptor- activated calcium mobilization. 18. The method of claim 17, wherein said Homer protein is selected from Homer la, Homer lb, Homer ic, Homer 2a, Homer 2b, Homer 3. 19. The method of claim 17, wherein said Homer protein is Homer 1 a. 20. The method of claim 17, wherein said cell response is an increase in calcium mobilization. 21. The method of claim 17, wherein said cell response is a decrease in calcium mobilization. 22. The method of claim 17, wherein the compound is selected from peptides, peptidomimetics, polypeptides, pharmaceuticals, chemical compounds, biological agents, antibodies, neurotropic agents and anti-epileptic agents. 23. A method of modulating receptor-mediated calcium mobilization comprising exposing a cell to a compound in a sufficient amount to modulate the calcium mobilization that normally occurs when a cell is exposed to an amount of ligand sufficient to activate an intracellular signaling pathway. 24. The method of claim 23, wherein the compound is selected from peptides, peptidomimetics, polypeptides, pharmaceuticals, chemical compounds, biological agents, antibodies, neurotropic agents and anti-epileptic agents. 25. The method of claim 23, wherein the ligand is an agonist or antagonist of metabotropic glutamate receptors. 26. The method of claim 23, wherein the intracellular signaling pathway is an inositol trisphosphate signaling pathway. 27. A method of identifying a compound that inhibits Homer protein activity comprising: (a) designing a potential inhibitor for Homer protein activity that will form non-covalent bonds with amino acids in a Homer binding site based upon the crystal structure co-ordinates of Homer protein binding domain; (b) synthesizing the inhibitor; and (c) determining whether the inhibitor inhibits Homer protein activity. 28. The method of claim 27, wherein the crystal structure coordinates of the Homer protein binding domain are obtained from a Homer protein crystal having orthorhombic space group symmetry P2 1 2 1 2 1 with a = 33.79, b = 51.40, and c = 66.30 Angstroms. 29. The method of claim 28, wherein the coordinates of the Homer protein binding domain are obtained by means of computational analysis. 30. The method of claim 27, wherein the potential inhibitor is designed to form hydrogen bonds with tryptophan 24 , phenylalanine 74 , threonine 66 , threonine , glutamine , alamne , threonine , and valine of the Homer binding domain. 31. A method for identifying a compound that affects the formation of cell surface receptors into clusters, comprising: (a) incubating the compound and a cell expressing a Homer protein and a Shank protein under conditions sufficient to allow the compound to interact with the cell; (b) determining the effect of the compound on the formation of cell-surface receptors into clusters; and (c) comparing the formation of cell-surface receptors into clusters of said cell contacted with said compound with the formation of cell-surface receptors into clusters in a cell not contacted with said compounds, thereby identifying a compound that affects the formation of cell-surface receptors into clusters. 32. The method of claim 31 , wherein the cell-surface receptor is a NMDA receptor. 33. The method of claim 31 , wherein the cell-surface receptor is a metabotropic glutamate receptor. I l l 34. The method of claim 33, wherein the metabotropic glutamate receptor is a group I metabotropic glutamate receptor. 35. The method of claim 34, wherein the metabotropic glutamate receptor is metabotropic glutamate receptor lα. 36. The method of claim 34, wherein the metabotropic glutamate receptor is metabotropic glutamate receptor 5. 37. The method of claim 31 , wherein said Homer protein is selected from the group consisting of Homer la, Homer lb, Homer lc, Homer 2a, Homer 2b and Homer 3. 38. The method of claim 31 , wherein the compound is selected from peptides, peptidomimetics, polypeptides, pharmaceuticals, chemical compounds, biological agents, antibodies, neurotropic agents and anti-epileptic agents. 39. The method of claim 31 , wherein said Shank protein is selected from the group consisting of Shank la, Shank lb and Shank 3, and cortactin binding protein. 40. The method of claim 31 , wherein the effect is inhibition of the recruitment of cell-surface receptors into clusters. 41. The method of claim 31 , wherein the effect is stimulation of the formation of cell-surface receptors into clusters. 42. The method of claim 31 , wherein said cell is selected from the group consisting of a neuronal cell, a glial cell, a cardiac cell, a bronchial cell, a uterine cell, a testicular cell, a liver cell, a renal cell, an intestinal cell, a thymus cell, a spleen cell, a placental cell, a skeletal muscle cell and a smooth muscle cell. 43. The method of claim 31 , wherein said compound is selected from peptides, peptidomimetics, polypeptides, pharmaceuticals, chemical compounds, biological agents, antibodies, neurotropic agents and anti-epileptic agents. 44. A method of treating a disorder associated with glutamate receptors comprising administering to a subject in need thereof a therapeutically effective amount of a compound that modulates Homer protein activity. 45. The method of claim 44, wherein said Homer protein is selected from the group consisting of Homer la, Homer lb, Homer lc, Homer 2a, Homer 2b, and Homer 3. 46. The method of claim 44, wherein said Homer protein is Homer la. 47. The method of claim 44, wherein said disorder is selected from epilepsy, glutamate toxicity, disorders of memory, disorders of learning, stroke, schizophrenia, Alzheimer's disease, tissue degeneration and disorders of brain development. 48. A method of treating a disorder associated with Homer protein activity comprising administering to a subject in need thereof a therapeutically effective amount of a compound that modulates Homer protein activity. 49. The method of claim 48, wherein said disorder is a cardiac disorder, a disorder of musculature, a vasculature disorder, a neurological disorder, a psychiatric disorder, a renal disorder, a uterine disorder or a disorder of bronchial tissue. 50. An isolated nucleic acid encoding Homer protein lb, wherein said nucleic acid has the nucleotide sequence set forth in SEQ. I.D NO: 3. 51. An isolated Homer protein lb, wherein said protein has substantially the same amino acid sequence as set forth in SEQ ID NO:4. 52. An isolated nucleic acid encoding Homer protein lc, wherein said nucleic acid has the nucleotide sequence set forth in SEQ ID NO: 5. 53. An isolated Homer protein 1 c, wherein said protein has substantially the same amino acid sequence as set forth in SEQ ID NO:6. 54. An isolated nucleic acid encoding Homer protein 2a, wherein said nucleic acid has the nucleotide sequence set forth in SEQ ID No: 7. 55. An isolated Homer protein 2a, wherein said protein has substantially the same amino acid sequence as set forth in SEQ ID NO: 8. 56. An isolated nucleic acid encoding Homer protein 2b, wherein said nucleic acid has the nucleotide sequence set forth in SEQ ID NO: 9. 57. An isolated Homer protein 2b, wherein said protein has substantially the same amino acid sequence as set forth in SEQ ID NO: 10. 58. An isolated nucleic acid encoding Homer protein 3, wherein said nucleic acid has the nucleotide sequence set forth in SEQ ID NO: 11. 59. An isolated Homer protein 3, wherein said protein has substantially the same amino acid sequence as set forth in SEQ ID NO: 12. 60. An isolated peptide, wherein said peptide is set forth in SEQ ID NO: 13, wherein R can be arginine, or zero, or one other amino acid residue. 61. An isolated peptide, wherein said peptide is set forth in SEQ ID NO: 14. 62. An isolated nucleic acid encoding a Homer Interacting Protein, wherein said nucleic acid has the nucleotide sequence set forth in SEQ ID No: 15 or SEQ. ID No: 17. 63. An isolated Homer Interacting Protein, wherein said protein has substantially the same amino acid sequence as set forth in SEQ ID NO: 16 or 18. 64. An isolated nucleic acid encoding Homer Interacting Protein, wherein said nucleic acid is encoded by SEQ ID NO: 19. 65. An isolated Homer Interacting Protein, wherein said protein has substantially the same amino acid sequence as set forth in SEQ ID NO: 20. 66. A substantially purified polypeptide containing a proline-rich region that binds to a polypeptide of the Homer family. 67. The polypeptide of claim 66, wherein said polypeptide is a cell-surface receptor. 68. The polypeptide of claim 66, wherein said polypeptide is an intracellular receptor. 69. A transgenic non-human animal having a transgene that expresses Homer protein la chromosomally integrated into the germ cells of the animal. 70. The transgenic animal of claim 69, wherein the animal is murine. 71. A method for affecting the natural aging process comprising administering to a subject in need thereof, an effective amount of a compound that modulates Homer protein activity.

Inventors: BENEKEN JUTTA, LANAHAN ANTHONY A, LEAHY DANIEL, TU JIAN CHENG, WORLEY PAUL F, XIAO BO, BENEKEN, JUTTA, LANAHAN, ANTHONY, A., LEAHY, DANIEL, TU, JIAN, CHENG, WORLEY, PAUL, F., XIAO, BO
Applicants: BENEKEN JUTTA, LANAHAN ANTHONY A, LEAHY DANIEL, TU JIAN CHENG, UNIV JOHNS HOPKINS MED, WORLEY PAUL F, XIAO BO, BENEKEN, JUTTA, LANAHAN, ANTHONY, A., LEAHY, DANIEL, THE JOHNS HOPKINS UNIVERSITY SCHOOL OF MEDICINE, TU, JIAN, CHENG, WORLEY, PAUL, F., XIAO, BO
Application year: 1999
Main classification: C12Q
IPC codes: A01K67, A61K38, A61K45, A61P9, A61P13, A61P15, A61P21, A61P25, A61P39, C07K14, C12N15, C12Q1, G01N33