PDZ proteins organize synaptic signaling pathways.

The mechanism by which PSD-95 and its homologs mediate molecular clustering is not certain. Data from Drosophila using the CD8/Shaker K+ channel chimeric protein (described above) suggest that the Shaker channel is initially targeted uniformly to the membrane and then is actively clustered or selectively retained at the synapse by DLG. One model of clustering proposes that the first and second PDZ domains of PSD-95 each bind the tail of an NMDA receptor or Shaker channel subunit, while each channel, because it exists as a multimer, binds multiple molecules of PSD-95, resulting in the aggregation of the two proteins. Also, the GKAPs can bind to the GK domain of multiple PSD-95 molecules, similarly contributing to aggregation. However, this model cannot explain how the N terminus and a single PDZ domain of PSD-95 are sufficient to cluster Shaker K+ channels or NMDA receptors in heterologous cells (Hsueh et al. 1997xHsueh, Y.P, Kim, E, and Sheng, M. Neuron. 1997; 18: 803–814Abstract | Full Text | Full Text PDF | PubMed | Scopus (163)See all ReferencesHsueh et al. 1997). Alternatively, PSD-95 might itself oligomerize and thereby aggregate interacting proteins. In support of this model, two conserved N-terminal cysteine residues in PSD-95 were found essential for both protein multimerization and K+ channel clustering in transfected cells (Hsueh et al. 1997xHsueh, Y.P, Kim, E, and Sheng, M. Neuron. 1997; 18: 803–814Abstract | Full Text | Full Text PDF | PubMed | Scopus (163)See all ReferencesHsueh et al. 1997). In nonreducing conditions these cysteines can form disulfide bonds, allowing for the multimerization of PSD-95 molecules. However, these cysteines are also sites of palmitoylation (Topinka and Bredt 1998xTopinka, J.R and Bredt, D.S. Neuron. 1998; 20: 125–134Abstract | Full Text | Full Text PDF | PubMed | Scopus (187)See all ReferencesTopinka and Bredt 1998). Palmitate is a fatty acid that is enzymatically added to specific proteins in a reversible fashion and that dynamically regulates protein interactions with lipid bilayers. PSD-95 in brain and in transfected COS cells is palmitoylated, and mutations in either one of the N-terminal cysteines disrupt palmitoylation and the association of PSD-95 with the membrane and with Kv1.4. Therefore, the N-terminal cysteines are important in localizing PSD-95 to the membrane so it can associate with transmembrane proteins (Figure 1Figure 1). Questions therefore remain as to whether PSD-95 can multimerize in the reducing environment of cells, and whether multimerization is required for receptor clustering.Figure 1Postsynaptic Interactions of PSD-95(A) N-terminal palmitoylation (jagged line) targets PSD-95 to synaptic membranes and promotes association of PSD-95 with membrane proteins including the NMDA receptor.(B) PSD-95 then aggregates and organizes synaptic proteins through either active clustering or selective retention of interacting proteins. Through these mechanisms PSD-95 can both assemble signal transduction pathways and help determine synaptic structure.View Large Image | View Hi-Res Image | Download PowerPoint SlideStabilization of MAGUK-receptor clusters appears to be mediated by interactions with the cytoskeleton. In erythrocytes the MAGUK protein p55 binds protein 4.1, allowing p55 and an associated transmembrane protein, glycophorin C, to associate with the spectrin-actin lattice of the cytoskeleton. A similar interaction between neuronal MAGUKs and protein 4.1 family members at synapses could help link NMDA receptors and K+ channel clusters to the actin-rich cytoskeleton. The NMDA R1 subunit also interacts with the actin binding protein α-actinin, providing another cytoskeletal contact point. Also, a newly identified PSD-95 binding protein, CRIPT, may mediate an association between PSD-95 and the tubulin-based cytoskeleton (Niethammer et al. 1998xNiethammer, M, Valtschanoff, J.G, Kapoor, T.M, Allison, D.W, Weinberg, R.J, Craig, A.M, and Sheng, M. Neuron. 1998; 20: 693–707Abstract | Full Text | Full Text PDF | PubMed | Scopus (221)See all ReferencesNiethammer et al. 1998). CRIPT colocalizes with PSD-95 at synapses and coimmunoprecipitates with PSD-95 and tubulin from brain. When PSD-95 and CRIPT are coexpressed in heterologous cells, PSD-95 shifts from a diffuse localization to colocalize with CRIPT at the microtubule cytoskeleton. CRIPT therefore provides a possible link between PSD-95 and another intracellular structure.While our understanding of synaptic organization has been aided by the discovery of the MAGUKs, much work remains. The most decisive evidence for MAGUKs in clustering receptors, organizing signaling cascades, and influencing synaptic structure has come from studies of the Drosophila NMJ, and it is not clear how this relates to synapses in the mammalian nervous system. Also, it is unknown how MAGUKs are themselves targeted to synapses, and questions remain about how they recruit interacting proteins there. The study of PSD-95 and relatives promises to enhance our understanding of synaptic structure, function, and plasticity.