Patent Publication Number: US-8110656-B2

Title: Compositions and methods of a phosphatidic acid binding protein

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with government support under MCB 0453858 awarded by The National Science Foundation. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to the field of phospholipid detection. In particular, certain embodiments provide the detection of phosphatidic acid. For example, certain proteins are capable of binding phosphatidic acid and can be used as a diagnostic and/or research tool to identify and quantitate phosphatidic acid. For example, a TGD2 protein may be fused with a fluorescent probe to monitor and measure phosphatidic acid in vitro as well as in vitro. 
     BACKGROUND 
     The biogenesis of the photosynthetic thylakoid membranes inside plant chloroplasts requires enzymes at the plastid envelope and the endoplasmic reticulum (ER). Extensive lipid trafficking is required for thylakoid lipid biosynthesis. Trigalactosyldiacylglycerol (TGD) proteins are believed to be permease-like components of an ABC transporter located in the chloroplast inner envelope membrane. 
     TGD proteins have been suggested to have a phosphatidic acid-binding protein with a predicted mycobacterial cell entry domain such that they may be tethered to the inner chloroplast envelope membrane facing the outer envelope membrane. However, these specific phosphatidic acid binding sites had not been identified, purified and/or isolated. 
     This lack of knowledge has hampered the development of specific diagnostic and detection methods designed to detect and quantify phosphatidic acid in plants. What is needed in the art is a reliable, quantitatively sensitive, and routine laboratory assay to detect plant phosphatidic acid for the purposes of botanical diagnostics and as a laboratory research tool. 
     SUMMARY OF THE INVENTION 
     The present invention is related to the field of phospholipid detection. In particular, certain embodiments provide the detection of phosphatidic acid. For example, certain proteins are capable of binding phosphatidic acid and can be used as a diagnostic and/or research tool to identify and quantitate phosphatidic acid. For example, a TGD2 protein may be fused with a fluorescent probe to monitor and measure phosphatidic acid in vitro as well as in vitro. 
     In one embodiment, the present invention contemplates a TDG2 protein comprising a phosphatidic acid binding domain, wherein said domain encompasses amino acid residues 201-225 (SEQ ID NO: 12), wherein at least one of said residues is a proline. In one embodiment, the protein lacks a transit peptide domain and a transmembrane domain. In one embodiment, the domain further comprises at least one accessory binding domain. In one embodiment, the accessory binding domain comprises amino acid residues 251-300 (SEQ ID NO: 103). In one embodiment, the accessory binding domain comprises amino acid residues 161-204 (SEQ ID NO: 104). In one embodiment, the accessory binding domain comprises amino acid residues 291-340 (SEQ ID NO: 105). In one embodiment, the domain comprises a phosphatidic acid binding motif. In one embodiment, an N-terminal β-strand and a C-terminal α-helix create the binding motif In one embodiment, the binding motif comprises a  221 Lysine. In one embodiment, the protein further comprises a label. 
     In one embodiment, the present invention contemplates a method, comprising: a) providing: i) a TDG2 protein comprising a phosphatidic acid binding domain, wherein said domain encompasses amino acid residues 201-225 (SEQ ID NO: 12), wherein at least one of said residues is a proline; ii) a sample suspected of containing phosphatidic acid capable of binding to said domain; b) contacting said sample with said protein under conditions such that said phosphatidic acid binds to said domain; c) determining an amount of said phosphatidic acid-domain binding. In one embodiment, the sample comprises a plant sample. In one embodiment, the method further comprises identifying a plant disease with said phosphatidic acid-domain binding amount. In one embodiment, the method further comprises identifying a plant wound with said phosphatidic acid-domain binding amount. In one embodiment, the method further comprises identifying a plant stress with said phosphatidic acid-domain binding amount. In one embodiment, the plant stress is selected from the group consisting of biotic stress, abiotic stress, pathogen infection, drought, salinity, and cold. 
     In one embodiment, the present invention contemplates a kit, comprising: a) a first container comprising a TDG2 protein comprising a phosphatidic acid binding domain, wherein said domain encompasses amino acid residues 201-225 (SEQ ID NO: 12), wherein at least one of said residues is a proline; b) a second container comprising a plurality of buffers and a plurality of reagents, wherein said protein is soluble; and c) a set of instructions for determining a phosphatidic acid. In one embodiment, the protein further comprises a label. In one embodiment, the phosphatidic acid is derived from a sample. In one embodiment, the protein further comprises at least one accessory binding protein. In one embodiment, the kit further comprises a test strip, capable of binding the TDG2 protein. 
     In one embodiment, the present invention contemplates a test strip comprising a phosphatidic acid binding protein and a test sample. In one embodiment, the test sample comprises a phospholipid. In one embodiment, the phospholipid comprises phosphatidylinositol. In one embodiment, the phosphatidylinositol comprises phosphatidic acid. In one embodiment, the test strip further comprises a phosphatidic acid binding protein/phosphatidic acid complex. In one embodiment, the test strip is Strip lot #JJ-032108-47. In one embodiment, the test strip is Strip lot #KB15011-47. 
     In one embodiment, the present invention contemplates a method comprising; a) providing i) a test strip comprising a phosphatidic acid binding protein; ii) a test sample, wherein the sample comprises a phospholipid; b) treating the phospholipid under conditions that release a phosphatidic acid; c) placing the phosphatidic acid on the test strip under conditions such that the phosphatidic acid is captured by the phosphatidic acid binding protein. In one embodiment, the method further comprises step (d) detecting said phosphatidic acid binding protein/phosphatidic acid complex. In one embodiment, the phospholipid comprises phosphatidylinositol. In one embodiment, the test strip is Strip lot #JJ-032108-47. In one embodiment, the test strip is Strip lot #KB15011-47. 
     Definitions 
     The term “phosphatidic acid” as used herein, refers to any one of several acids (RCOO) 2 C 3 H 5 OPO 3 H 2  that are formed from phosphatides by partial hydrolysis and that yield on hydrolysis two fatty-acid molecules RCOOH and one molecule each of glycerol and phosphoric acid 
     The term “affinity” as used herein, refers to any attractive force between substances or particles that causes them to enter into and remain in chemical combination. For example, an inhibitor compound that has a high affinity for a receptor will provide greater efficacy in preventing the receptor from interacting with its natural ligands, than an inhibitor with a low affinity. 
     The term “derived from” as used herein, refers to the source of a compound or sequence. In one respect, a compound or sequence may be derived from an organism or particular species. In another respect, a compound or sequence may be derived from a larger complex or sequence. 
     The term “protein” as used herein, refers to any of numerous naturally occurring extremely complex substances (as an enzyme or antibody) that consist of amino acid residues joined by peptide bonds, contain the elements carbon, hydrogen, nitrogen, oxygen, usually sulfur. In general, a protein comprises amino acids having an order of magnitude within the hundreds. 
     The term “peptide” as used herein, refers to any of various amides that are derived from two or more amino acids by combination of the amino group of one acid with the carboxyl group of another and are usually obtained by partial hydrolysis of proteins. In general, a peptide comprises amino acids having an order of magnitude with the tens. 
     The term, “purified” or “isolated”, as used herein, may refer to a peptide composition that has been subjected to treatment (i.e., for example, fractionation) to remove various other components, and which composition substantially retains its expressed biological activity. Where the term “substantially purified” is used, this designation will refer to a composition in which the protein or peptide forms the major component of the composition, such as constituting about 50%, about 60%, about 70%, about 80%, about 90%, about 95% or more of the composition (i.e., for example, weight/weight and/or weight/volume). The term “purified to homogeneity” is used to include compositions that have been purified to “apparent homogeneity” such that there is single protein species (i.e., for example, based upon SDS-PAGE or HPLC analysis). A purified composition is not intended to mean that some trace impurities may remain. 
     As used herein, the term “substantially purified” refers to molecules, either nucleic or amino acid sequences, that are removed from their natural environment, isolated or separated, and are at least 60% free, preferably 75% free, and more preferably 90% free from other components with which they are naturally associated. An “isolated polynucleotide” is therefore a substantially purified polynucleotide. 
     “Nucleic acid sequence” and “nucleotide sequence” as used herein refer to an oligonucleotide or polynucleotide, and fragments or portions thereof, and to DNA or RNA of genomic or synthetic origin which may be single- or double-stranded, and represent the sense or antisense strand. 
     The term “an isolated nucleic acid”, as used herein, refers to any nucleic acid molecule that has been removed from its natural state (e.g., removed from a cell and is, in a preferred embodiment, free of other genomic nucleic acid). 
     The terms “amino acid sequence” and “polypeptide sequence” as used herein, are interchangeable and to refer to a sequence of amino acids. 
     As used herein the term “portion” when in reference to a protein (as in “a portion of a given protein”) refers to fragments of that protein. The fragments may range in size from four amino acid residues to the entire amino acid sequence minus one amino acid. 
     The term “portion” when used in reference to a nucleotide sequence refers to fragments of that nucleotide sequence. The fragments may range in size from 5 nucleotide residues to the entire nucleotide sequence minus one nucleic acid residue. 
     The terms “specific binding” or “specifically binding” when used in reference to the interaction of a lipid (i.e., for example, PA) and a protein or peptide (i.e., for example, TGD2 protein and/or a truncated TGD2 peptide) means that the interaction is dependent upon the presence of a particular structure (i.e., for example, a tertiary amino acid structure) on a protein; in other words a lipid is recognizing and binding to a specific protein structure rather than to proteins in general. For example, if a lipid is specific for tertiary structure “A”, the presence of a protein containing tertiary structure A (or free, unlabelled A) in a reaction containing labeled “A”, the lipid will reduce the amount of labeled A bound to the lipid. 
     A “variant” of a protein is defined as an amino acid sequence which differs by one or more amino acids from a polypeptide sequence or any ortholog and/or homolog of the polypeptide sequence. The variant may have “conservative” changes, wherein a substituted amino acid has similar structural or chemical properties, e.g., replacement of leucine with isoleucine. More rarely, a variant may have “nonconservative” changes, e.g., replacement of a glycine with a tryptophan. Similar minor variations may also include amino acid deletions or insertions (i.e., additions), or both. Guidance in determining which and how many amino acid residues may be substituted, inserted or deleted without abolishing biological or immunological activity may be found using computer programs including, but not limited to, DNAStar® software. 
     A “variant” of a nucleotide is defined as a novel nucleotide sequence which differs from a reference oligonucleotide by having deletions, insertions and substitutions. These may be detected using a variety of methods (e.g., sequencing, hybridization assays etc.). Included within this definition are alterations to the genomic DNA sequence which encodes TGD2 (i.e., for example, SEQ ID NO:1), the inability of a selected fragment of SEQ ID NO:1 to hybridize under high stringency conditions to a sample of genomic DNA (e.g., using allele-specific oligonucleotide probes), and improper or unexpected hybridization, such as hybridization to a locus other than a wild type chromosomal locus (e.g., using fluorescent in situ hybridization (FISH)). 
     A “deletion” is defined as a change in either nucleotide or amino acid sequence in which one or more nucleotides or amino acid residues, respectively, are absent. 
     An “insertion” or “addition” is that change in a nucleotide or amino acid sequence which has resulted in the addition of one or more nucleotides or amino acid residues, respectively, as compared to, for example, the naturally occurring protein. 
     A “substitution” results from the replacement of one or more nucleotides or amino acids by different nucleotides or amino acids, respectively. 
     As used herein, the terms “complementary” or “complementarity” are used in reference to “polynucleotides” and “oligonucleotides” (which are interchangeable terms that refer to a sequence of nucleotides) related by the base-pairing rules. For example, the sequence “C-A-G-T,” is complementary to the sequence “G-T-C-A.” Complementarity can be “partial” or “total.” “Partial” complementarity is where one or more nucleic acid bases is not matched according to the base pairing rules. “Total” or “complete” complementarity between nucleic acids is where each and every nucleic acid base is matched with another base under the base pairing rules. The degree of complementarity between nucleic acid strands has significant effects on the efficiency and strength of hybridization between nucleic acid strands. This is of particular importance in amplification reactions, as well as detection methods which depend upon binding between nucleic acids. 
     The terms “homology” and “homologous” as used herein in reference to nucleotide sequences refer to a degree of complementarity with other nucleotide sequences. There may be partial homology or complete homology (i.e., identity). A nucleotide sequence which is partially complementary, i.e., “substantially homologous,” to a nucleic acid sequence is one that at least partially inhibits a completely complementary sequence from hybridizing to a target nucleic acid sequence. The inhibition of hybridization of the completely complementary sequence to the target sequence may be examined using a hybridization assay (Southern or Northern blot, solution hybridization and the like) under conditions of low stringency. A substantially homologous sequence or probe will compete for and inhibit the binding (i.e., the hybridization) of a completely homologous sequence to a target sequence under conditions of low stringency. This is not to say that conditions of low stringency are such that non-specific binding is permitted; low stringency conditions require that the binding of two sequences to one another be a specific (i.e., selective) interaction. The absence of non-specific binding may be tested by the use of a second target sequence which lacks even a partial degree of complementarity (e.g., less than about 30% identity); in the absence of non-specific binding the probe will not hybridize to the second non-complementary target. 
     The terms “homology” and “homologous” as used herein in reference to amino acid sequences refer to the degree of identity of the primary structure between two amino acid sequences. Such a degree of identity may be directed a portion of each amino acid sequence, or to the entire length of the amino acid sequence. Two or more amino acid sequences that are “substantially homologous” may have at least 50% identity, preferably at least 75% identity, more preferably at least 85% identity, most preferably at least 95%, or 100% identity. 
     As used herein, the term “primer” refers to an oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product which is complementary to a nucleic acid strand is induced, (i.e., in the presence of nucleotides and an inducing agent such as DNA polymerase and at a suitable temperature and pH). The primer is preferably single stranded for maximum efficiency in amplification, but may alternatively be double stranded. If double stranded, the primer is first treated to separate its strands before being used to prepare extension products. Preferably, the primer is an oligodeoxy-ribonucleotide. The primer must be sufficiently long to prime the synthesis of extension products in the presence of the inducing agent. The exact lengths of the primers will depend on many factors, including temperature, source of primer and the use of the method. 
     DNA molecules are said to have “5′ ends” and “3′ ends” because mononucleotides are reacted to make oligonucleotides in a manner such that the 5′ phosphate of one mononucleotide pentose ring is attached to the 3′ oxygen of its neighbor in one direction via a phosphodiester linkage. Therefore, an end of an oligonucleotide is referred to as the “5′ end” if its 5′ phosphate is not linked to the 3′ oxygen of a mononucleotide pentose ring. An end of an oligonucleotide is referred to as the “3′ end” if its 3′ oxygen is not linked to a 5′ phosphate of another mononucleotide pentose ring. As used herein, a nucleic acid sequence, even if internal to a larger oligonucleotide, also may be said to have 5′ and 3′ ends. In either a linear or circular DNA molecule, discrete elements are referred to as being “upstream” or 5′ of the “downstream” or 3′ elements. This terminology reflects the fact that transcription proceeds in a 5′ to 3′ fashion along the DNA strand. The promoter and enhancer elements which direct transcription of a linked gene are generally located 5′ or upstream of the coding region. However, enhancer elements can exert their effect even when located 3′ of the promoter element and the coding region. Transcription termination and polyadenylation signals are located 3′ or downstream of the coding region. 
     Transcriptional control signals in eukaryotes comprise “promoter” and “enhancer” elements. Promoters and enhancers consist of short arrays of DNA sequences that interact specifically with cellular proteins involved in transcription. Maniatis, T. et al.,  Science  236:1237 (1987). Promoter and enhancer elements have been isolated from a variety of eukaryotic sources including genes in plant, yeast, insect and mammalian cells and viruses (analogous control elements, i.e., promoters, are also found in prokaryotes). The selection of a particular promoter and enhancer depends on what cell type is to be used to express the protein of interest. 
     As used herein, the terms “nucleic acid molecule encoding”, “DNA sequence encoding,” and “DNA encoding” refer to the order or sequence of deoxyribonucleotides along a strand of deoxyribonucleic acid. The order of these deoxyribonucleotides determines the order of amino acids along the polypeptide (protein) chain. The DNA sequence thus codes for the amino acid sequence. 
     The term “Southern blot” refers to the analysis of DNA on agarose or acrylamide gels to fractionate the DNA according to size, followed by transfer and immobilization of the DNA from the gel to a solid support, such as nitrocellulose or a nylon membrane. The immobilized DNA is then probed with a labeled oligodeoxyribonucleotide probe or DNA probe to detect DNA species complementary to the probe used. The DNA may be cleaved with restriction enzymes prior to electrophoresis. Following electrophoresis, the DNA may be partially depurinated and denatured prior to or during transfer to the solid support. Southern blots are a standard tool of molecular biologists. J. Sambrook et al. (1989) In:  Molecular Cloning: A Laboratory Manual , Cold Spring Harbor Press, NY, pp 9.31-9.58. 
     The term “Northern blot” as used herein refers to the analysis of RNA by electrophoresis of RNA on agarose gels to fractionate the RNA according to size followed by transfer of the RNA from the gel to a solid support, such as nitrocellulose or a nylon membrane. The immobilized RNA is then probed with a labeled oligodeoxyribonucleotide probe or DNA probe to detect RNA species complementary to the probe used. Northern blots are a standard tool of molecular biologists. J. Sambrook, J. et al. (1989) supra, pp 7.39-7.52. 
     The term “reverse Northern blot” as used herein refers to the analysis of DNA by electrophoresis of DNA on agarose gels to fractionate the DNA on the basis of size followed by transfer of the fractionated DNA from the gel to a solid support, such as nitrocellulose or a nylon membrane. The immobilized DNA is then probed with a labeled oligoribonucleotide probe or RNA probe to detect DNA species complementary to the ribo probe used. 
     As used herein the term “coding region” when used in reference to a structural gene refers to the nucleotide sequences which encode the amino acids found in the nascent polypeptide as a result of translation of a mRNA molecule. The coding region is bounded, in eukaryotes, on the 5′ side by the nucleotide triplet “ATG” which encodes the initiator methionine and on the 3′ side by one of the three triplets which specify stop codons (i.e., TAA, TAG, TGA). 
     The term “label” or “detectable label” are used herein, to refer to any composition detectable by spectroscopic, photochemical, biochemical, immunochemical, electrical, optical or chemical means. Such labels include biotin for staining with labeled streptavidin conjugate, magnetic beads (e.g., Dynabeads), fluorescent dyes (e.g., fluorescein, texas red, rhodamine, green fluorescent protein, and the like), radiolabels (e.g.,  3 H,  125 I,  35 S,  14 C, or  32 P), enzymes (e.g., horse radish peroxidase, alkaline phosphatase and others commonly used in an ELISA), and calorimetric labels such as colloidal gold or colored glass or plastic (e.g., polystyrene, polypropylene, latex, etc.) beads. Patents teaching the use of such labels include, but are not limited to, U.S. Pat. Nos. 3,817,837; 3,850,752; 3,939,350; 3,996,345; 4,277,437; 4,275,149; and 4,366,241 (all herein incorporated by reference). The labels contemplated in the present invention may be detected by many methods. For example, radiolabels may be detected using photographic film or scintillation counters, fluorescent markers may be detected using a photodetector to detect emitted light. Enzymatic labels are typically detected by providing the enzyme with a substrate and detecting, the reaction product produced by the action of the enzyme on the substrate, and calorimetric labels are detected by simply visualizing the colored label. 
     The term “binding” as used herein, refers to any interaction between an infection control composition and a surface. Such as surface is defined as a “binding surface”. Binding may be reversible or irreversible. Such binding may be, but is not limited to, non-covalent binding, covalent bonding, ionic bonding, Van de Waal forces or friction, and the like. An infection control composition is bound to a surface if it is impregnated, incorporated, coated, in suspension with, in solution with, mixed with, etc. 
     The term “sample” as used herein is used in its broadest sense and includes environmental and biological samples. Environmental samples include material from the environment such as soil and water. Biological samples may be animal, including, human, fluid (e.g., blood, plasma and serum), solid (e.g., stool), tissue, liquid foods (e.g., milk), and solid foods (e.g., vegetables). For example, a pulmonary sample may be collected by bronchoalveolar lavage (BAL) which comprises fluid and cells derived from lung tissues. A biological sample may comprise a cell, tissue extract, body fluid, chromosomes or extrachromosomal elements isolated from a cell, genomic DNA (in solution or bound to a solid support such as for Southern blot analysis), RNA (in solution or bound to a solid support such as for Northern blot analysis), cDNA (in solution or bound to a solid support) and the like. 
     The term, “test strip” as used herein, refers to any material capable of binding a protein, wherein the protein may capture a ligand without releasing from the material. For example, a test strip may comprises a glass slide coated with a polymer matrix, a silica material, absorbent fiber (i.e., for example, cloth or paper). 
     The term, “phosphatidic acid binding protein” as used herein, refers to any protein and/or enzyme that is capable of forming a complex with phosphatidic acid. 
     The term “test sample” or “sample” as used herein, refers to any material comprising phosphatidic acid that may be placed on a test strip, or may be treated for placement on a test strip such that the phosphatidic acid may be detected. 
     The term “complex” as used herein, refers to any stable interaction between two compounds such that a close association is formed. The complex may be stabilized by atomic interactions including, but not limited to, covalent bonding, non-covalent bonding, electrostatic interactions, hydrophobic interactions, or Van der Waals forces. 
     The term “capture” as used herein, refers to any compound having a stereospecific affinity for a second compound. For example, an antibody may capture a ligand wherein the antibody has been raised by an antigen to the ligand. Alternatively, a protein or enzyme may have a tertiary structure such that a ligand finds multiple points of interaction such that a stable complex is formed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  illustrates various embodiments and relationships of TGD2 amino acid sequences. Gene bank accession numbers for disclosed sequences:  Arabidopsis thaliana , NP — 566659.1 (SEQ ID NO: 5);  Vitis vinifera , CAN71395.1 (SEQ ID NO: 6);  Oryza sativa , EAY77419.1 (SEQ ID NO: 7);  Physcomitrella patens , XP — 001778862.1 (SEQ ID NO: 8);  Ostreococcus tauri , CAL53419.1 (SEQ ID NO: 9);  Chlamydomonas reinhardtii , XP — 001699315.1 (SEQ ID NO: 10);  Prochlorococcus marinus  str. NATL2A, YP — 292846.1 (SEQ ID NO: 115);  Prochlorococcus marinus  str. MIT 9301, YP — 001090537.1 (SEQ ID NO: 116);  Synechococcus  sp. WH 5701, ZP — 01083418.1 (SEQ ID NO: 117);  Synechococcus  sp. CC9902, YP — 376253.1 (SEQ ID NO: 118);  Synechococcus  sp. JA-2-3B′a(2-13), YP — 477327.1 (SEQ ID NO: 119);  Anabaena variabilis , YP — 323182.1 (SEQ ID NO: 120);  Nodularia spumigena , ZP — 01630545.1 (SEQ ID NO: 121);  Crocosphaera watsonii , ZP — 00516249.1 (SEQ ID NO: 122);  Cyanothece  sp. PCC 8801, ZP — 02940544.1 (SEQ ID NO: 123);  Microcystis aeruginosa , CAO90615.1 (SEQ ID NO: 124);  Acaryochloris marina , YP — 001516641.1 (SEQ ID NO: 125);  Thermosynechococcus elongatus , NP — 683197.1 (SEQ ID NO: 126). 
         FIG. 1A : Alignments of the TGD2 sequence with various orthologs in plants and green algae. Predicted TGD2 secondary structure is shown on the top. Open boxes mark conserved residues, and black boxes indicate identical residues. 
         FIG. 1B : An ‘unrooted tree’ diagram showing the apparent relatedness of predicted TGD2 orthologs in plants, green algae and Cyanobacteria. Boot strapping values &gt;950 are marked by +, those between 500 and 950 are marked with a solid circle, and those under 500 are marked by open square. 
         FIG. 2  presents exemplary data showing binding of DsRed-TGD2C WT fusion protein to PA as a function of weight percent of PA in PA/PC mixture. 
         FIG. 2A : Analysis by liposome-association assay. A mixture of dioleoyl-PA and dioleoyl-PC was used where the weight percent of PA was varied from 0-100% (wt/wt), maintaining the total lipid invariant at 250 μg protein was used. P, protein recovered in the absence of lipids. 
         FIG. 2B : Association of DsRed-WT TGD2C to PA/PC liposomes as determined by scanning densitometry (left), and the values are plotted as a function of PA concentration in the liposomes (right). The data were fit to the modified Hill equation for receptor-ligand binding. A Hill number of 5.8 was obtained, suggestive of positive cooperativity. 
         FIG. 3  presents one embodiment of a phosphatidic acid (PA) binding domain on TGD2C by deletion and truncation mutagenesis. 
         FIG. 3A : A schematic of TGD2 domains indicating a predicted transit peptide domain (TP), a transmembrane domain (TMD) and a conservative mammalian cell entry (MCE) domain. Upper number represent linear order of amino acid residues. 
         FIGS. 3B and 3C : Deletion and truncation mutants generated on TGD2C and C-terminally fused to the DsRed open reading frame the same manner as WT TGD2C. Black ball represents DsRed protein, grey bars represent deletion fragment. Liposome-association assays were performed to assess binding of various mutants to PC, PA/PC or PA liposomes. PA-specific binding data were summarized on the right. +++++, ++++, +++, ++, +, indicate a qualitative assessment of PA-specific binding in decreasing intensity, and − indicate no binding. 
         FIG. 4  presents exemplary data showing the binding of a TGD2 minimal domain to PA. 
         FIG. 4A : Truncation mutants generated to localize a PA binding domain. PA binding activity were assessed by liposome-association assay. 
         FIG. 4B : Verification of PA binding to a minimal domain (TGD2C T8 (201-225) (SEQ ID NO: 12)) as compared to wild type (TGD2C WT (119-381) (SEQ ID NO: 107)) by protein-lipid overlay assay conducted with commercial phospholipid-containing membrane strip. LPA, lysophosphatidic acid; LPC, lysophosphatidylcholine; Ptdlns, phosphatidylinositol; Ptdlns(3)P, phosphatidylinositol 3-phosphate; Ptdlns(4)P, phosphatidylinositol 4-phosphate; Ptdlns(5)P, phosphatidylinositol 5-phosphate; PE, phosphatidylethanolamine; PC, phosphatidylcholine; SIP, sphingosine 1-phosphate; Ptdlns(3,4)P2, phosphatidylinositol 3,4-bisphosphate; Ptdlns(3,5)P2, phosphatidylinositol 3,5-bisphosphate; Ptdlns(4,5)P2, phosphatidylinositol 4,5-bisphosphate; Ptdlns(3,4,5)P3, phosphatidylinositol 3,4,5-bisphosphate; PA, phosphatidic acid; PS, phosphatidylserine. 
         FIG. 4C : PA binding of point mutations on the minimal domain shown by liposome association assay with 100% PA liposomes. Point mutations are indicated by arrows. 
         FIG. 5  presents exemplary data showing loss of positive cooperativity by a minimal binding domain. 
         FIG. 5A : PA binding for DsRed-TGD2C WT (DR-WT). 
         FIG. 5B : PA binding for DsRed-TGD2C minimal domain (DR-25). 
         FIG. 5C : Quantification of relative binding of PA for DR-WT and R-25. 
         FIG. 5D : PA binding for DsRed-TGD2C minimal domain (DR-25). 
         FIG. 5E : PA binding for DsRed-TGD2C with deletion of minimal domain (DR-Δ25). 
         FIG. 5F : Quantification of relative binding of PA for DR-25 and DR-Δ25. 
         FIG. 6  illustrates additional embodiments of PA binding sites. 
         FIG. 6A : A schematic of TGD2 domains indicating a predicted transit peptide domain (TP), a transmembrane domain (TMD), a conservative mammalian cell entry (MCE) domain, and a PA binding minimal domain (MBD). 
         FIG. 6B : Deletion and truncation mutants were generated on TGD2C and C-terminally fused to the DsRed open reading frame. Liposome-association assays were performed to assess binding of various mutants to PA liposomes (chromatographic plate, bottom). 
         FIG. 7  presents exemplary data showing the binding of DsRed-TGD2C WT fusion protein to PA. 
         FIG. 7A  schematically illustrates a TGD2 protein that is N-terminally truncated lacking a TMD and is C-terminally fused to the  Discosoma  sp. red fluorescent protein (DsRed, DR) open reading frame. 
         FIG. 7B  presents exemplary data from the expressed fusion protein using an protein-lipid overlay assay with a commercially available phospholipid-containing membrane strip. LPA, lysophosphatidic acid; LPC, lysophosphatidylcholine; Ptdlns, phosphatidylinositol; Ptdlns(3)P, phosphatidylinositol 3-phosphate; Ptdlns(4)P, phosphatidylinositol 4-phosphate; Ptdlns(5)P, phosphatidylinositol 5-phosphate; PE, phosphatidylethanolamine; PC, phosphatidylcholine; SIP, sphingosine 1-phosphate; Ptdlns(3,4)P2, phosphatidylinositol 3,4-bisphosphate; Ptdlns(3,5)P2, phosphatidylinositol 3,5-bisphosphate; Ptdlns(4,5)P2, phosphatidylinositol 4,5-bisphosphate; Ptdlns(3,4,5)P3, phosphatidylinositol 3,4,5-bisphosphate; PA, phosphatidic acid; PS, phosphatidylserine TGD2 
         FIG. 8  presents exemplary data showing specific phosphidate binding to a recombinant TGD2C-His protein. Upper bars show the relative overlapping of a 6×His MCE binding fragment to a TGD2 protein. The 6×His TGD2 protein variant is N-terminally truncated lacking the TMD to exclude lipid binding to this region of the protein. 
         FIG. 8A : Membrane binding assay with commercial phospholipid-containing membrane. 
         FIG. 8B : Membrane binding assay with a plant lipid-containing membrane. 
         FIG. 8C : Liposome binding assay. Liposomes consisted of phosphatidylcholine only (PC, first lane) or PC (60% wt/wt, second through fourth lanes) mixed with different molecular species of PA (40% wt/wt). PA molecular species tested were dioleoyl-PA (18:1), sn1-oleoyl, sn2-palmitoyl PA (18:1/16:0), and dipalmitoyl-PA (16:0). DGDG, prokaryotic digalactosyldiacylglycerol; DGDGe, eukaryotic digalactosyldiacylglycerol; L-PA, lysophosphatidic acid; L-PC, lysophosphatidylcholine; MGDG, prokaryotic monogalactosyldiacylglycerol; MGDGe, eukaryotic monogalactosyldiacylglycerol; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PI, phosphatidylinositol; PIP(3), phosphatidylinositol 3-phosphate; PIP(4), phosphatidylinositol 4-phosphate; PIP(5), phosphatidylinositol 5-phosphate; PIP2(3,4), phosphatidylinositol 3,4-bisphosphate; PIP2(3,5), phosphatidylinositol 3,5-bisphosphate; PIP2(4,5), phosphatidylinositol 4,5-bisphosphate; PIP3(3,4,5), phosphatidylinositol 3,4,5-bisphosphate; PS, phosphatidylserine; SIP, sphingosine 1-phosphate; SQDG, sulfoquinovosyldiacylglycerol; TGDG, trigalactosyldiacylglycerol. 
         FIG. 9  demonstrates one embodiment of alignment comparisons showing that the TGD2 minimal PA binding domain is adjacent to the MCE domain. 
         FIG. 10  presents exemplary data showing a lipid phenotype of the tgd2-1 mutant as compared with the tgd1-1 mutant and the Col-2 wild type. Fatty acids are indicated with number of carbons:number of double bonds. DGDG, digalactosyldiacylglycerol; MGDG, monogalactosyldiacylglycerol; O, origin; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PI, phosphatidylinositol; PIG, pigments; SQDG, sulfoquinovosyldiacylglycerol; TAG, triacylglycerol; TGDG, trigalactosyldiacylglycerol. 
         FIG. 10A : Thin-layer chromatogram of polar lipids. Lipids were visualized by α-naphthol staining. 
         FIG. 10B : Thin-layer chromatogram of neutral lipids. Lipids were visualized by exposure to iodine vapor. 
         FIG. 10C : Polar lipid composition (relative mol %) determined by quantification of fatty acid methylesters derived from individual lipids. 
         FIG. 10D : Fatty acid composition of the two galactolipids MGDG and DGDG. 
         FIG. 11  presents exemplary data showing an identification of a TGD2 locus. 
         FIG. 11A : Map position of the tgd2-1 mutation on chromosome 3 and structure of the TGD2 gene (At3g20320). Markers used for mapping and the respective number of recombinations are indicated. The TGD2 gene is indicated by a black box and expanded on the lowest line. The coding region of At3g20320 is shown as a shaded box. The darker shading indicates the predicted TMD. A region encoding an MCE domain is shown hashed. Introns are indicated by a line. Noncoding regions of the gene deduced from the cDNA are shown as open boxes. 
         FIG. 11B : Growth of different plants on soil (8 weeks old) with a genotype as indicated below the panel. Mutants were homozygous at all indicated loci. Three plants from independent transformation events expressing the TGD2 cDNA are indicated by “(c).” 
         FIG. 11C : Genotyping at the DGD1 locus. Point mutation-specific dCAPS markers were used, and ethidium bromide stained DNA diagnostic DNA fragments are shown with their respective lengths in base pairs. 
         FIG. 11D : Genotyping at the TGD2 locus. Point mutation-specific dCAPS markers were used, and ethidium bromidestained DNA diagnostic DNA fragments are shown with their respective lengths in base pairs. 
         FIG. 11E : Lipid phenotype of the six different plant lines. A section of thin-layer chromatogram stained for glycolipids is shown. DGDG, digalactosyldiacylglycerol; TGDG, trigalactosyldiacylglycerol. 
         FIG. 12  presents exemplary data showing an expression of the tgd2-1 mutant cDNA in the Col-2 wild type. The untransformed wild type (Col-2) and the untransformed tgd1-1 and tgd2-1 mutants are included for comparison. Three independent transformants are shown. 
         FIG. 12A : Semiquantitative RT-PCR of mRNA levels derived from the TGD2 wild-type gene (top), the TGD2 wild-type gene and the tgd2-1 transgene (middle), and the ubiquitin (UBQ10) control (bottom). Negative images of ethidium bromide-stained gels are shown. 
         FIG. 12B : Polar lipid phenotype of the indicated plants. A section of the thin-layer chromatogram stained for glycolipids is shown. DGDG, digalactosyldiacylglycerol; SQDG, sulfoquinovosyldiacylglycerol; TGDG, trigalactosyldiacylglycerol. 
         FIG. 13  presents exemplary data showing a subcellular localization and topology of TGD2 after transient expression in tobacco leaves. 
         FIG. 13A : Localization of full-length TGD2 protein fused to GFP (TGD2-GFP). The insertion of the respective protein into the membrane is schematically shown on the left. GFP, green fluorescence specific for GFP; Ch1, red fluorescence of chloroplasts; the overlay of the two images is shown on the right. Confocal images are shown. (Scale bars: 10 μm) 
         FIG. 13B : Topology of the TGD2 protein. The wild-type TGD2 protein, the tgd2-1 mutant protein, and the GFP fusion were transiently produced in tobacco leaves, and isolated chloroplasts were analyzed. The TGD2 and tgd2-1 proteins were detected by using a TGD2-specific antibody. The GFP fusion was detected by using a GFP-specific antibody. Samples were untreated with protease (−) or treated with thermolysin (+, Th) or with trypsin (+, Tr). Immunoblots are shown. 
         FIG. 14  presents one embodiment of a test strip that identifies a phosphatidic acid. Phosphatidylinositol 4,5 bis phosphate was chromatographed and compared to standard chromatograms of phosphatidic acid (PA) and phosphatidylserine (PS). Slides #1=Strip lot #JJ-032108-47 (#1 Left slide pair); Slides #2=Strip lot #KB15011-47 (#2 Left slide pair). 
         FIG. 15  presents exemplary data identifying a PA-binding minimal domain on TGD2C by deletion and truncation mutagenesis. Identification of a PA binding minimal domain on TGD2C by deletion and truncation mutagenesis. 
         FIG. 15A : Primary structure of TGD2 indicating a predicted transit peptide (TP), transmembrane domain (TMD) and a conservative mammalian cell entry (MCE) domain. 
         FIGS. 15B &amp; 15C : A series of deletion and truncation mutants were generated on TGD2C and C-terminally fused to dsRed protein the same manner as WT TGD2C. Black ball represents dsRed protein, black bars represent deletion fragment. Liposome-association assays were performed to assess binding of various mutants to PC, PA/PC or PA liposomes. PA-specific binding data were summarized on the right. +++++, ++++, +++, ++, +, indicate a qualitative assessment of PA-specific binding in decreasing intensity, and − indicate no binding. 
         FIG. 16  presents an illustration showing the similarity between TGD proteins and bacterial ABC transporters. 
         FIG. 17  presents TGD2 ortholog sequences and phylogenetic organization in plants and Cyanobacteria. 
         FIG. 17A : A partial sequence alignment of TGD2 orthologs showing the region of minimal PA binding domain. Conserved residues are highlighted in red, similar residues are boxed in yellow. 
         FIG. 17B : An unrooted phylogenetic tree showing the relatedness of predicted TGD2 orthologs in plants and Cyanobacteria. Boot strapping values&gt;950 are marked by +, those between 500 and 950 are marked with a solid circle, and those under 500 are marked by open square. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is related to the field of phospholipid detection. In particular, certain embodiments provide the detection of phosphatidic acid. For example, certain proteins are capable of binding phosphatidic acid and can be used as a diagnostic and/or research tool to identify and quantitate phosphatidic acid. For example, a TGD2 protein may be fused with a fluorescent probe to monitor and measure phosphatidic acid in vitro as well as in vitro. 
     The TGD2 protein of  Arabidopsis  is proposed to be the substrate binding component of a lipid transfer complex in the inner chloroplast envelope membrane. Loss of function of this protein or other components of this complex may disrupt the endoplasmic reticulum (ER)-pathway of thylakoid lipid biosynthesis. 
     In one embodiment, the present invention contemplates a method comprising fusing an open reading frame encoding the TGD2C truncated protein wherein the transit peptide and transmembrane domain are removed. In one embodiment, the protein is attached to the C-terminal of the  Discosoma  sp. red (DsRed) fluorescent protein open reading frame. In one embodiment, the fusion protein is in operable combination with a T7 promoter. 
     In one embodiment, the present invention contemplates a method comprising expressing a labeled TGD2C truncated fusion protein. In one embodiment, the label is a fluorescent label. In one embodiment, the fluorescent label comprises a  Discosoma  s. red fluorescent protein (DsRed). Although it is not necessary to understand the mechanism of an invention, it is believed that the DsRed-TGD2C fusion protein specifically binds phosphatidic acid (PA). The data presented herein, demonstrates that the binding of DsRed-TGD2C to PA displays positive cooperativity with a Hill number of 5.8 and the apparent K d  of 39.81 mol % PA (wt/wt). Further data presented herein, utilized deletion and truncation mutagenesis to identify a 25 amino acid TGD2C segment as a specific PA minimal binding domain. 
     The task of studying lipid-protein interactions is difficult due to the hydrophobicity property of the interacting molecules. Moreover, there are few reliable quantitative techniques available to assess specific binding kinetics and each method has its own limitations. Therefore, the present invention overcame these limitations by utilizing: (1) a protein-lipid overlay assay for rapid detection and qualitative assessment of binding; and (2) a liposome-association assay combined with densitometry quantification to evaluate relative binding between proteins. Together, these methods allow us to identify a specific binding domain and evaluate it semi-quantitatively. 
     I. Plant Lipid Biosynthesis 
     As plant leaves expand, the demand on the lipid biosynthetic machinery is high because leaf cells contain one of the most extensive membrane systems found in Nature, for example, a chloroplast photosynthetic thylakoid membrane. Chloroplast thylakoid lipids include, but are not limited to, nonphosphorous galactolipids. 
     Galactolipid biosynthesis involves the formation of phosphatidic acid (PA) in the plastid and at the endoplasmic reticulum (ER) in many plants, including  Arabidopsis . Browse et al., (1991) Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:467-506; and Roughan et al., (1982) Annu. Rev. Plant Physiol. 33:97-132. Fatty acids derived from de novo synthesis in the plastid are assembled into PA in the plastid or at the ER. In  Arabidopsis , diacylglycerols derived from the plastid pathway or the ER pathway are present in galactolipids in approximately equal proportion. Browse et al., (1986) Biochem. J. 235:25-31. The  Arabidopsis  lipid galactosyltransferases MGD1 and DGD1, which successively galactosylate diacylglycerol, are associated with the inner and the outer chloroplast envelope membranes, respectively. Benning et al., (2005) J. Biol. Chem. 280:2397-2400. The topology of the galactolipid biosynthetic machinery and the involvement of the ER pathway require extensive subcellular lipid trafficking, most of which is mechanistically not understood. 
     To date, two mutants of  Arabidopsis  have been described that affect lipid trafficking from the ER to the plastid. The act1(ats1) mutant is deficient in the plastidic glycerol 3-phosphate acyltransferase, and most of the galactolipids in this mutant are derived from the ER pathway. Kunst et al., (1988) Proc. Natl. Acad. Sci. USA 85:4143-4147. In contrast, galactolipids in the tgd1 mutant are primarily derived from the plastid pathway. Xu et al., (2003) EMBO J. 22:2370-2379. This mutant presents a complex lipid phenotype comprising: i) the accumulation of oligogalactolipids (i.e., for example, trigalactosyldiacylglycerol) and triacylglycerols in the leaves; ii) a 5-fold increase in PA content; and iii) an increase of 16-carbon fatty acids in the galactolipids. Xu et al., (2005) Plant Cell 17:3094-3110. 
     Such observations are believed indicative of a change in molecular species toward those formed de novo in the plastid. Xu et al., (2003)  EMBO J.  22:2370-2379; and Xu et al., (2005)  Plant Cell  17:3094-3110. These observations comprised pulse-chase labeling of leaves that were consistent with a disruption of the transfer of lipid molecular species from the ER to the plastid in the tgd1 mutant. Isolated tgd1 chloroplasts showed a decreased rate of conversion of labeled PA into galactolipids. The TGD1 protein resembles the permease component of bacterial ABC transporters and was shown to be an integral component of the inner chloroplast envelope membrane. Such data supports a proposed that TGD1 is a component of a PA transporter in the inner chloroplast envelope and may play a role in the biosynthesis of ER-derived molecular species of galactolipids. Stronger alleles of tgd1 led to increased embryo arrest and seed abortion, suggesting that the affected biological process is essential. 
     In one embodiment, the present invention contemplates a composition comprising a trigalactosyldiacylglycerol 2 (tgd2) mutant of  Arabidopsis . In one embodiment, the composition comprises a TGD2 gene. In one embodiment, the composition comprises a TGD2 protein. 
     Pulse-chase labeling of leaves also indicates a disruption of the transfer of lipid molecular species from the ER to the plastid in the tgd1 mutant. For example, isolated tgd1 chloroplasts show a decreased rate of conversion of labeled PA into galactolipids. The TGD1 protein resembles the permease component of bacterial ABC transporters and was shown to be an integral component of the inner chloroplast envelope membrane. Such observations lead to the proposal that TGD1 is a component of a PA transporter in the inner chloroplast envelope and that may be involved in biosynthesis of ER-derived molecular species of galactolipids. A second  Arabidopsis  TGD, trigalactosyldiacylglycerol 2 (tgd2), has been identified and characterized. 
     Protein importation into chloroplasts is believed to involve an interaction of protein complexes spanning the inner and outer chloroplast envelope membranes. Gutensohn et al., (2006) J. Plant Physiol. 163:333-347; and Jarvis et al., (2004) Curr. Biol. 14:R1064-R1077. Currently, knowledge about lipid importation into the plastid is extremely limited. Like protein importation into the plastid, ER-derived lipid importation during chloroplast biogenesis is extensive and presumably requires transporters mediating the transfer of lipids between and through the involved membranes. 
     As discussed above, TGD1 and TGD2 proteins may comprise components of a lipid transporter of the inner chloroplast envelope membrane. Although the analysis of the tgd1-1 mutant to date is far more extensive, it is apparent that the tgd2-1 mutation causes identical biochemical and physiological phenotypes: i) the accumulation of oligogalactolipids and triacylglycerols; ii) the increase of 16-carbon fatty acids in plastid lipids indicative of reduced presence of ER-derived molecular species; and iii) the increase in growth in the dgd1 background. Until the presently disclosed invention, a difference in phenotypes between TGD1 and TGD2 had not been identified, thereby suggesting that the products of the two genes are involved in the same biological process, thylakoid lipid biosynthesis from ER-derived precursors. 
     Currently available molecular analysis supported this interpretation because: i) TGD1 and TGD2 proteins are localized in the inner chloroplast envelope membrane; and ii) expression of green fluorescent protein (GFP) fusions for both proteins cause punctate fluorescence patterns in the periphery of plastids. Moreover, the  Arabidopsis  TGD1 and TGD2 proteins were reported as similar permeases and substrate-binding proteins of bacterial ABC transporters, respectively. Their corresponding bacterial orthologs are found in clusters, which is usually interpreted as meaning that the function of the gene products are in the same pathway or process. Overbeek et al., (2005) Nucleic Acids Res. 33:5691-5702. 
     Nonetheless, past research was unable to identify unambiguous evidence for any direct similarities in TGD1 and TGD2 function. Two findings suggest that TGD2 is active in a protein-lipid complex in  Arabidopsis  because: i) ectopic expression of the tgd2-1 mutant cDNA gives rise to the mutant phenotype, i.e., a dominant-negative mutation; and ii) the wild-type TGD2 protein is protected in isolated chloroplasts against trypsin whereas the TGD2 fusion protein is not. Both results can be interpreted as the association of the TGD2 protein with other proteins and/or specific lipid domains inaccessible to proteolytic activity. 
     Previous investigation of the tgd1-1 mutant indicated the accumulation of PA, and the reduced incorporation of PA into glycolipids of isolated plastids, led to the suggestion that the TGD1 protein is a component of a PA transporter. Xu et al., (2005)  Plant Cell  17:3094-3110. Consistent with the proposed interaction of TGD1 and TGD2 in a PA-transporting complex, the recombinant TGD2 protein lacking the membrane-spanning domain was found to specifically bind PA. See,  FIG. 8 . An alternative interpretation would be that TGD2 binds PA as an effector molecule modulating the activity of TGD1. Further, TGD2 could remove a PA molecule from the outer envelope membrane and make it available to TGD1 for import into the plastid and conversion by the plastidic PA phosphatase. Because TGD2 appears to be tethered with its membrane-spanning domain to the inner envelope membrane, the PA binding domain might reach out to the inside of the outer envelope membrane either locally fusing the two membranes or extracting an ER-derived PA. Although, to date, there is no direct evidence for this hypothesis, one intriguing observation in support is derived from mycobacterial orthologs of TGD2 required for cell entry of the bacterium. Chitale et al., (2001)  Cell. Microbiol.  3:247-254. Recombinant bacterial orthologs can mediate the uptake of latex beads into mammalian cells, a process requiring an interaction of the protein on the bacterial surface with the mammalian cell membrane. The MCE domains present in the MCE proteins or bacterial substrate binding proteins associated with ABC transporters have been delineated based on sequence. The finding that TGD2 specifically binds PA, possibly through its MCE domain, might also be relevant to the possibility that these bacterial proteins interact with membrane lipids. 
     II. Phosphatidic Acid and Plant Diseases 
     Phosphatidic acid (PA) has only recently been identified as a putative signaling molecule in both plants and animals. Nonetheless, PA already appears to be equivalent to the classic second messengers Ca 2+  and/or cAMP. In plants, PA&#39;s formation may be triggered in response to various biotic and abiotic stress factors, including pathogen infection, drought, salinity, wounding and cold. In general, PA signal production is fast (i.e., for example, in minutes) and transient. Recently, reports indicated that PA formation in stress responses may be a result of phospholipases C and D activity. Moreover, some protein targets of PA have been identified. Testerink et al., “Phosphatidic acid: a multifunctional stress signaling lipid in plants” Trends Plant Sci. 2005 August; 10(8):368-375. 
     Phospholipid-derived molecules maybe involved as second messengers in plant defense signaling. Recent research has begun to reveal PA signals produced by the enzymes phospholipase C, phospholipase D and phospholipase A2 in relationship to their putative downstream targets. These include, but are not limited to, the activation of a MAP kinase cascade and triggering of an oxidative burst by phosphatidic acid; the regulation of ion channels and proton pumps by lysophospholipids and free fatty acids; and the conversion of free fatty acids into bioactive octadecanoids such as jasmonic acid. Laxalt et al., “Phospholipid signalling in plant defence” Curr Opin Plant Biol. 2002 August; 5(4):332-338. 
     PA may also be a positive regulator of RPM1- or RPS2-mediated disease resistance signalling, and that an observed biphasic PA production may be a conserved feature of signalling induced by the coiled-coil nucleotide binding domain leucine-rich repeat class of resistance proteins. Bacterial pathogens are believed to deliver type III effector proteins into plant cells during an infection. On susceptible host plants, type III effectors contribute to virulence, but on resistant hosts they betray the pathogen to the plant&#39;s immune system and are functionally termed avirulence (Avr) proteins. Recognition induces a complex suite of cellular and molecular events comprising the plant&#39;s inducible defence response. As recognition of type III effector proteins occurs inside host cells, defence responses can be elicited by in planta expression of bacterial type III effectors. Andersson et al., “Phospholipase-dependent signalling during the AvrRpm1- and AvrRpt2-induced disease resistance responses in  Arabidopsis thaliana ” Plant J. 2006 September; 47(6):947-59. Recognition of either of two type III effectors, AvrRpm1 or AvrRpt2 from  Pseudomonas syringae , induced a biphasic accumulation of phosphatidic acid (PA). The first wave of PA accumulation correlated with disappearance of monophosphatidylinosotol (PIP) and is thus tentatively attributed to activation of a PIP specific phospholipase C (PLC) in concert with diacylglycerol kinase (DAGK) activity. Subsequent activation of phospholipase D (PLD) produced large amounts of PA from structural phospholipids. This later wave of PA accumulation was several orders of magnitude higher than the PLC-dependent first wave. Inhibition of phospholipases blocked the response, and feeding PA directly to leaf tissue caused cell death and defence-gene activation. Inhibitor studies ordered these events relative to other known signalling events during the plant defense response. Influx of extracellular Ca 2+  occurred downstream of PIP-degradation, but upstream of PLD activation. Production of reactive oxygen species occurred downstream of the phospholipases. 
     The involvement of phospholipase C/diacylglycerol kinase (PLC/DGK)-mediated signalling in oxidative burst and hypersensitive cell death was studied in rice suspension-cultured cells treated with benzothiadiazole (BTH) and infected by  Xanthomonas oryza  pv.  oryza  (Xoo), believed to be a causative factor of rice leaf blight disease. Treatment of rice suspension cells with BTH resulted in a significant oxidative burst, as indicated by accumulation of superoxide anion and H 2 O 2 , and hypersensitive cell death, as determined by Evans blue staining. A peak in oxidative burst was detected 3-4 h after BTH treatment and hypersensitive cell death was observed 8 h after treatment. In addition, significant oxidative burst and hypersensitive cell death were detected in BTH-treated suspension cells, but not in untreated control cells, after Xoo infection. Scavengers and antioxidants of active oxygen species, e.g., superoxide dismutase, catalase, N-acetylcysteine, and flavone, reduced significantly the BTH-induced oxidative burst and hypersensitive cell death, indicating that oxidative burst is required for BTH-induced hypersensitive cell death. Expression of the PLC/DGK pathway genes, a diacylglycerol kinase gene, OsDAGK1, and a phosphoinositide-specific phospholipase C gene, OsPI-PLC1, and a defense-related EREBP transcriptional factor gene, OsBIERF3, was activated in rice cells after BTH treatment and in the BTH-treated cells after Xoo infection. Treatment of rice cells with phosphatidic acid, a phospholipid signalling molecule, resulted in the production of oxidative burst and hypersensitive cell death. However, neomycin, a PLC inhibitor, inhibited partially but not completely the production of oxidative burst, hypersensitive cell death, and expression of OsBIERF3 and OsDAGK1 induced by BTH in rice cells. These results suggest that PLC/DGK-mediated signalling plays an important role in BTH-induced oxidative burst, hypersensitive response, and activation of defense response in rice. Chen et al., “Phospholipase C/diacylglycerol kinase-mediated signalling is required for benzothiadiazole-induced oxidative burst and hypersensitive cell death in rice suspension-cultured cells” Protoplasma. 2007; 230(1-2):13-21. 
     Phospholipase D (PLD) has been implicated in multiple plant stress responses. Its gene transcription and activity increase upon exposure to various stresses, and manipulation of PLD protein levels leads to altered stress tolerance. The plant PLD family is relatively large and heterogeneous, and different PLD isoforms are involved in separate stress responses. PLD and its product, phosphatidic acid, exert their effects by functioning in signal transduction cascades and by influencing the biophysical state of lipid membranes. Bargmann et al., “The role of phospholipase D in plant stress responses” Curr Opin Plant Biol. 2006 October; 9(5):515-22. 
     Metabolomic approaches were used to elucidate some key metabolite changes occurring during interactions of  Magnaporthe grisea —a causative factor of rice blast disease—with an alternate host,  Brachypodium distachyon . Fourier-transform infrared (FT-IR) spectroscopy provided a high-throughput metabolic fingerprint of  M. grisea  interacting with the  B. distachyon  accessions ABR1 (susceptible) and ABR5 (resistant). Principal component-discriminant function analysis (PC-DFA) allowed the differentiation between developing disease symptoms and host resistance. Examination of PC-DFA loading plots indicated that fatty acids were one chemical group that discriminated between responses by ABR1 and ABR5 to  M. grisea . To identify these, non-polar extracts of  M. grisea -challenged  B. distachyon  were directly infused into an electrospray ionization mass spectrometer (ESI-MS). PC-DFA indicated that  M. grisea -challenged ABR1 and ABR5 were differentially clustered away from healthy material. Subtraction spectra and PC-DFA loadings plots revealed discriminatory analytes (m/z) between each interaction and seven metabolites were subsequently identified as phospholipids (PLs) by ESI-MS-MS. Phosphatidyl glycerol (PG) PLs were suppressed during both resistant and susceptible responses. By contrast, different phosphatidic acid PLs either increased or were reduced during resistance or during disease development. This suggests considerable and differential PL processing of membrane lipids during each interaction which may be associated with the elaboration/suppression of defence mechanisms or developing disease symptoms. Allwood et al., “Metabolomic approaches reveal that phosphatidic and phosphatidyl glycerol phospholipids are major discriminatory non-polar metabolites in responses by  Brachypodium distachyon  to challenge by  Magnaporthe grisea ” Plant J. 2006 May; 46(3):351-68. 
     Multiple forms of phospholipase D (PLD) were activated in response to wounding, and the expressions of PLDα, PLDβ, and PLDγ differed in wounded  Arabidopsis  leaves. Antisense abrogation of PLDα decreased post-wounding phosphatidic acid induction, jasmonic acid (JA), and a JA-regulated gene for vegetative storage protein. Examination of the genes involved in the initial steps of oxylipin synthesis revealed that abrogation of the PLDα attenuated the wound-induced expression of lipoxygenase 2 (LOX2) but had no effect on allene oxide synthase (AOS) or hydroperoxide lyase in wounded leaves. The systemic induction of LOX2, AOS, and vegetative storage protein was lower in the PLDα-suppressed plants than in wild-type plants, with AOS exhibiting a distinct pattern. These results indicate that activation of PLD mediates wound induction of JA and that LOX2 is probably a downstream target through which PLD promotes the production of JA. Wang et al., “Involvement of phospholipase D in wound-induced accumulation of jasmonic acid in  arabidopsis ” Plant Cell. 2000 November; 12(11):2237-2246. 
     III. Phosphatidic Acid as a Signaling Lipid 
     Over the years, several signaling lipids have been identified in plants (1, 2). Among those are various important sphingolipids, glycerol lipids and fatty acid metabolites (3-6). Phosphatidic acid (PA), was found to be one representative signaling lipid. PA may represent a lipid second messenger that transiently accumulates in plants within minutes after a pathogen attack and/or a variety of stress conditions (i.e., for example, osmotic and temperature stress) (7-9). PA may be generated via two distinct pathways: i) by phosphalipase D (PLD), which is believed to hydrolyze structural phospholipids to generate PA; or ii) by sequential action of phospholipase C (PLC) and diacylglycerol (DAG) kinase (DGK), wherein PLC can hydrolyze phosphatidylinositol-4,5-bisphosphate [Ptdlns(4,5)P2, PIP2] into inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] and DAG, which may be immediately converted to PA by DGK (10). 
     A. Phosphatidic Acid Targets 
     Despite ongoing efforts, the identification of PA targets has remained elusive. A few cellular targets of PA have been described but no clear lipid binding motif has been found. Although it is not necessary to understand the mechanism of an invention, it is believed that predicting biochemical interactions with PA may be difficult because since the putative targets may not share sequence similarity. For example, in mammalian cells, protein kinases Raf-1 (11; 12), protein phosphatases SHP-1(13) and PP1(14), and protein kinase Cε (15) have been reported as PA targets. In yeast, the SNARE protein Spo20p (16) and the inositol-regulated transcriptional repressor Opi1p(17) are putative PA targets. 
     Similarly, a limited number of PA targets have so far been identified in plants, for example, ABI1 (ABA insensitive 1) (18) and PDK1 (phosphoinositide-dependent kinase 1) (19). In one embodiment, the present invention contemplates that PA may be a positive regulator of the ABA signaling pathway. Although it is not necessary to understand the mechanism of an invention, it is believed that ABI1 may be a protein phosphatase 2C that negatively regulates ABA signaling, whereupon the ABA response, PA becomes induced and binds to ABI1, thereby reducing its phosphatase activity and resulting in translocation to the plasma membrane. Alternatively,  Arabidopsis  PDK1 is believed to be a protein kinase that binds both PA and phosphoinositides, whose activation is limited to PA and not by polyphosphoinositides (19, 20). Additional PA targets were isolated using a PA-affinity matrix, consisting of a PA analogue covalently linked to Affi-Gel 10, which is incubated with suspension-cultured tomato or  Arabidopsis  cell lysates (21). Mass spectrometry has shown that phosphoenolpyruvate carboxylase (PEPC) preferentially binds to PA over other phospholipids (21). 
     B. TGD2 as a PA Carrier 
     PA is also believed to act as a substrate that may be directly transported across the membranes by phospholipids and thus play a role in membrane biogenesis. For instance, it is believed that TGD2 comprises a PA target involved in lipid trafficking between the ER and chloroplast. On study has suggested that TGD2C (i.e., the C-terminal 6×-His tag-fused protein of TGD2 having both the N-terminal transit peptide and transmembrane domain removed) interacts selectively with PA (22). Further, an  Arabidopsis  TGD2 protein is proposed to be the substrate binding component of a lipid transfer complex in the inner chloroplast envelope membrane. Supporting this mechanism is the observation that the lipid transfer complex also comprises TGD1, a permease, wherein loss of function of TGD1 results in accumulation of PA in  Arabidopsis  plants (23, 24). Loss of function of other lipid transfer complex components also disrupt the endoplasmic reticulum (ER)-pathway of thylakoid lipid biosynthesis. Thus, TGD proteins, in general, play an active role in PA transport between the ER and the chloroplast, and possibly thylakoid lipid biosynthesis pathway as well. 
     In some embodiments, the present invention overcomes the known problems in the art in identifying PA-binding domains of TGD2 proteins because they do not share sequence homology to any other known PA-binding domains including, but not limited to, PX (25), pleckstrin homology (20) and some C2 domains (26). In one embodiment, the present invention contemplates a method for characterizing PA-binding domains in TGD2. In one embodiment, the TGD2 PA-binding domain is characterized using a protein-lipid overlay. In one embodiment, the TGD2 PA-binding domain is characterized using a liposome-association assay. In one embodiment, the TGD2 PA-binding domain is characterized using a mutagenesis. 
     1. TGD2 Orthologs 
     In one embodiment, a TGD2 gene encodes a 381 amino acid protein with a calculated molecular mass of 41.6 kDa (i.e., for example, Accession Number At3g20320; SEQ ID NO: 1). TGD2 proteins may contain a conserved mycobacterial cell entry domain (MCE, amino acids 127-204; SEQ ID NO: 2) expressed as a surface protein of some pathogenic mycobacteria. MCE proteins are believed to be virulence factors proposed to facilitate the bacterial entry into mammalian host cells (32). 
     In one embodiment, the present invention contemplates an MCE domain comprising a TGD2 PA-binding site and/or complex. For example, a TGD2 transmembrane domain (amino acids 96-118; SEQ ID NO: 3) and a TGD2 chloroplast targeting peptide (amino acids 1-45; SEQ ID NO: 4) were predicted. See,  FIG. 3A . Orthologs to these sequences were found in plants, green algae and Cyanobacteria. (29); See,  FIG. 1B . Further, a multiple sequence alignment of TGD2 to these orthologs demonstrates their relatedness. See,  FIG. 1A . 
     2. PA Binding to a dsRed-TGD2C Wild Type Fusion Protein 
     TGD2C-His has been hypothesized to specifically bind to PA, possibly through its predicted mammalian cell entry (MCE) domain. (22) The present invention found that a DsRed fusion protein system provided a fusion protein having improved solubility in order to perform quantitative binding assays to validate this hypothesis. 
     For example, a commercial membrane strip pre-spotted with different phospholipids was used in a protein-lipid overlay assay with a DsRed-TGD2C WT fusion protein performed in accordance with Example II. The results suggested that, like TGD2C-His, a DsRed-TGD2C WT protein also shows specificity for PA over other lipids.  FIG. 15 , right. To verify that the binding was not due to non-specific PA interactions with DsRed, DsRed protein itself was also assayed for binding. No binding to any lipid for DsRed control was detected, indicating the specificity of this PA binding due to TGD2C protein moiety.  FIG. 15 , left. 
     This result was further supported by lipsome association assay in accordance with Example III. In this assay, purified recombinant proteins were incubated with liposomes for 30 min at 30° C. before centrifugation at 20,000 g for 10 min to pellet the liposomes. Proteins bound to the liposomes were found associated with the lipid pellet, whereas non-binding proteins remained in the supernatant. In an effort to determine the optimal concentration of PA required for high specificity binding, a PA/PC liposome mixture containing varying weight fractions of PA was prepared and incubated with DsRed-TGD2CWT or DsRed alone. The DsRed-TGD2CWT fusion proteins were found to bind PC/PA liposome mixtures, as most of the proteins remained in the pellet/bound fraction.  FIG. 2A , left panel, bottom. On the contrary, DsRed alone is almost exclusively present in the supernatant as a free form.  FIG. 2A , right panel, top. 
     At the tested protein concentration (1 μg total protein), a significant increase in binding occurred when the liposomes contained between approximately 30-40% PA. These blot were scanned, each individual band was quantified, and the resulted data was plotted and fit to the Hill equation for receptor-ligand binding.  FIG. 2B . The data revealed that liposomes made with 100% PA bound the greatest amount of the protein. Moreover, from the Sigmoidal fit, the half maximal binding affinity (K d ) of DsRed-TGD2C WT for PA was estimated to be 39.8 mol % PA (wt/wt), which is comparable to the results obtained for RafC-PA association (20 mol % PA) (11). From the binding plot, a Hill number of 5.8 was obtained, suggestive of positive cooperativity.  FIG. 2B . Again, this value is similar to that obtained for RafC-PA interaction (Hill number between 3.3 and 6.2) (11). The results may reflect that there is a cooperative sequestering of a domain of PA surrounding the C-terminal part of the TGD2 protein. 
     3. Identification of TGD2C PA Binding Regions 
     It has been reported that various reported PA-binding regions share no significant homology in primary structures (10). Consequently, attempting to identify any TGD2 PA binding domain was not intuitively obvious. In one embodiment, the present invention contemplates a method to identify TGD2 PA-binding regions by using a liposome-association assay. In one embodiment, the liposome association assay comprised incubating liposomes with purified mutant proteins. In one embodiment, the mutant proteins comprised amino acid sequences generated using a TGD2C nucleic acid template. In one embodiment, the TGD2C nucleic acid template generated deletion or truncation nucleic acid sequence mutants encoding a mutant TDG2C protein. In one embodiment, the nucleic acid sequence mutants were fused to a C-terminal end of a DsRed nucleic acid open reading frame. Although it is not necessary to understand the mechanism of an invention, it is believed that because the liposome association assay relies on a nonquantitative assessment of binding to identify regions of lipid interaction within the protein, maximizing the binding of TGD2 proteins was highly desired. The present data show that liposomes made with 100% PA bind the greatest amount of the TGD2 protein.  FIG. 2 . Hence, the binding reactions reported herein included liposomes comprised of 100% PA to achieve the highest lipid binding specificity. As a specificity control, liposomes comprised of 100% phosphatidylcholine (PC) and/or 50% PC+50% PA were included for comparison. Insolubility problems due to the deletion of large portions of the protein (i.e., for example, possibly exposing hydrophobic domain) were solved by using the DsRed protein as a solubilizing and stabling partner. As a result, all the generated mutant proteins disclosed herein were obtained at a satisfactory amount and purity. PA binding data for these representative TGD2 protein mutants are presented.  FIG. 3 . 
     These data show binding characteristics of representative truncated TGD2 mutants ranging in length from between approximately 130 to 180 amino acids.  FIG. 3A . DRWT (119-381) (SEQ ID NO: 107) and four mutants displayed significant binding to PA, while having no interaction with the PC control lipid. DsRed itself does not display binding to either PC or PA, confirming the specificity of PA binding by TGD2.  FIG. 3B . Although it is not necessary to understand the mechanism of an invention, it is believed that these data suggest that the PA-specific binding domain might reside in the TGD2 region comprising 221-250 amino acid residues (SEQ ID NO: 40), since this region overlaps between the tested mutants. 
     Two internal deletion mutants within the 221-250 amino acid residues (SEQ ID NO: 40) were then generated and tested for PA binding. Surprisingly, the deletion of entire 221-250 region (SEQ ID NO: 40) did not seem to affect PA binding, while the deletion of a smaller 221-225 region (SEQ ID NO: 108) decreases binding activity dramatically.  FIG. 3C . 
     This data was completely counterintuitive and required considerable analysis before proceeding with further evaluation. Although it is not necessary to understand the mechanism of an invention, it is believed that protein folding effects may mediate this observation, wherein a deletion could potentially disrupt or reconstitute the protein structure and thus affect protein function depending on the realistic location of the function domain. 
     In one embodiment, the present invention contemplates a PA binding domain that is in or close to a TGD2 region comprising amino acid residues 221-250 (SEQ ID NO: 40). Observations that a fifth mutant (i.e., comprising, amino acid residues 119-300 (SEQ ID NO: 28)) also shows strong binding to PA provide corroborating data.  FIG. 3C . 
     These initial deletion studies indicate that a region between residues 201 and 225 (SEQ ID NO: 12) may be sufficient for PA specific binding, even when fused with DsRed. Furthermore, it was observed that this short fusion segment has much less overall binding, suggesting the presence of a minimal PA binding domain (infra). 
     4. Minimal TGD2 PA Binding Domain 
     In one embodiment, the present invention contemplates a minimum TGD2 PA binding domain. In one embodiment, the binding domain was identified by fragmenting a TGD2 region comprising amino acid residues 119-250 (SEQ ID NO: 11). In one embodiment, the fragments were fused to DsRed, and assayed using liposome association. 
     In brief, amino acids were removed from the N-terminal of TGD2C down to the middle of a TGD2C region comprising amino acid residues 221-250 (SEQ ID NO: 40) (i.e., for example, amino acid residue 225). See,  FIG. 4A . The data indicated that a 25 amino acid sequence (i.e., for example, amino acid residues 201-225; SEQ ID NO: 12) is sufficient to mediate specific binding to PA.  FIG. 4A . A TGD2C region comprising amino acid residues 221-250 (SEQ ID NO: 40) was also tested; however, no interaction to PA was detected. These data indicate that this TGD2C region may play a lesser role in PA binding, and partially explains why deletion of this region does not appreciably affect PA binding (supra). A protein-lipid overlay in accordance with Example II verified PA binding by the 25 amino acid sequence (SEQ ID NO: 12).  FIG. 4B . Similar to DR-WT, this mutant itself binds PA on the membrane strip, with apparent lower affinity. 
     Some reports have identified that TGD2 PA-binding regions involve basic amino acids and/or tryptophan residues (10). In particular, one recent study shows that electrostatic interactions of PA with basic amino acids (i.e., for example, lysine and/or arginine) combined with hydrogen bond interactions, may form a basis for specific binding of PA to PA targets (33). Based on sequence similarity of TGD2 to its closely related homologs in plants and green algae, several charged and/or conserved amino acids were picked as potential interesting residues in the 25-mer minimal domain for possibly mediating interactions with PA. An alanine screen was then performed to evaluate these residues within a minimal PA binding region of TGD2 (i.e., for example, SEQ ID NO:12). Point mutations were generated in the 25-mer minimal domain and fused with DsRed to test PA binding by liposome-association assay using 100% PA liposomes. The data demonstrate that, all point mutations have little or no effect on PA-liposome binding except K221A.  FIG. 4C . This lysine-to-alanine mutation significantly reduced the amount of interaction with PA-liposomes. No detectable PC-liposome binding was observed for any of the constructs (data not shown). 
     In one embodiment, the present invention contemplates a TGD2 PA binding domain comprising amino acid residues 201-225 (SEQ ID NO: 12). In one embodiment, the binding domain is adjacent to a MCE domain. Although it is not necessary to understand the mechanism of an invention, it is believed that mutation of  221 Lys to  221 Ala significantly diminishes PA binding. Further, upon generation of a point mutant (K221A) within a minimal domain, PA binding was diminished, thereby identifying  221 Lys as an amino acid residue involved in PA binding. This discovery is consistent with previous hypotheses that basic amino acids and/or tryptophan might be involved in lipid PA binding (10; 33). 
     Surprisingly, a PA binding TGD2 minimal domain is sufficient, but not necessary, to mediate interactions between TGD2 and PA liposomes. For example, a TGD2 fragment wherein a minimal domain was deleted still retains residual binding activity, albeit with significantly lower affinity. Moreover, a TGD2 protein having a minimal domain deletion still displays positive cooperativity to PA binding.  FIGS. 5E and 5F . These data suggested the presence of accessory PA binding domains or segments that also play a role in cooperating PA binding by the minimal domain. 
     5. Accessory TGD2 PA Binding Components 
     The above data showing that DR-WT protein displayed positive cooperativity upon PA binding suggested that a 25 amino acid sequence comprising a minimal PA binding domain may not be acting independently. Although it is not necessary to understand the mechanism of an invention, it is believed that the minimal binding domain may comprise accessory biochemical properties involved in PA binding. Liposome-association assay was performed with mixed PA/PC liposomes using DR-WT as a quantification control. The data show DR-25 binding to PA loses positive cooperativity, while DR-WT binding to PA still obeys the Hill equation, with a modified K d  of 37.66 mol % and a Hill number of 2.8.  FIGS. 5A-5C . 
     A 25-mer deletion mutant (designated as DR-Δ25) was generated that retained some residual PA binding activity. But moreover, the binding of this deletion mutant to PA also displayed positive cooperativity. An increased Kd of 53.47 mol % and a Hill number of 7.3 were identified from the fitting curve.  FIGS. 5E and 5F . In contrast, the data show that DR-25 is not cooperative.  FIGS. 5D and 5F . Apparently, a 25-mer minimal domain, alone, is sufficient to facilitate PA binding, but might also involve accessory components. In one embodiment, the present invention contemplates PA binding accessory components capable of modulating PA binding of TGD2 protein. 
     This hypothesis is consistent with observations that some deletions of the TDG2 region comprising amino acid residues 221-250 (SEQ ID NO: 40) do not affect PA binding, while some deletions of the TDG2 region comprising amino acid residues 221-225 (SEQ ID NO: 108) significantly decrease PA binding activity. Although it is not necessary to understand the mechanism of an invention, it is believed that these observations also suggest that there are accessory PA binding components flanking the TGD2 region comprising amino acid residues 201-225 (SEQ ID NO: 12), wherein different deletions differentially affect protein folding and, ultimately, functionality. This semi-quantitative analysis demonstrated that PA binding by a minimal domain lost positive cooperativity, which was also a property of wild type TGD2C protein. 
     In one embodiment, the present invention contemplates a plurality of TGD2 accessory PA binding segments. For example, TGD2 mutants were generated with truncated sequences from either the C-terminus (i.e., for example, amino acid residue 381) or within the middle of TGD2 (i.e., for example, amino acid residue 204) and fused to an DsRed open reading frame. ( FIG. 6B ). These mutated TGD2 proteins were tested for PA binding by using the liposome association assay using 100% PA liposomes. The data show that, at least four mutants were found to have various PA binding activity.  FIG. 6B . 
     In one embodiment, a TGD2 accessory PA binding site comprises amino acid residues 251-300 (SEQ ID NO: 103). In one embodiment, a TGD2 accessory PA binding site comprises amino acid residues 161-204 (SEQ ID NO: 104). In one embodiment, a TGD2 accessory PA binding site comprises amino acid residues 291-340 (SEQ ID NO: 105). 
     6. A TDG2 PA Binding Motif 
     In one embodiment, the present invention contemplates a TGD2 minimal PA binding region comprising a PA binding motif. In one embodiment, a PA binding motif further comprises at least three other regions in proximity with, or adjacent to, a TGD2 minimal PA binding domain. 
     In one embodiment, the TGD2 minimal PA binding domain comprises amino acid residues 201-225 (SEQ ID NO: 12), wherein at least one amino acid residue is a proline. In one embodiment, at least two amino acids are prolines. In one embodiment, at least three amino acids are prolines. In one embodiment, at least four amino acids are proline. In one embodiment, at least five amino acids are prolines. In one embodiment, at least six amino acids are prolines. 
     Although it is not necessary to understand the mechanism of an invention, it is believed that proline residues within the TGD2 region comprising amino acid residues 201-225 (SEQ ID NO: 12) may induce folding alongside an N-terminal β-strand and a C-terminal α-helix to form a PA binding site. 
     This proline-induced folding hypothesis is supported by a secondary structure prediction showing that residues 201-225 (SEQ ID NO: 12) is a loop-strand fold lacks helical or β-strand structure.  FIG. 1A . Therefore, a full PA binding domain on TGD2 likely comprises amino acid residues comprising a minimal PA binding domain (i.e., for example, TGD2 amino acid residues 201-225 (SEQ ID NO: 12)) as well as amino acid residues present in both sides of a minimal PA binding domain (i.e., for example, amino acid residues 161-204 (SEQ ID NO: 104) and/or amino acid residues 251-300 (SEQ ID NO: 103)). Such a combination of a minimal PA binding domain with at least one accessory PA binding domain is believed to generate a complete PA binding domain having a complicated tertiary binding structure. 
     Because the TGD2 protein resembles substrate binding proteins of bacterial ABC transporters, and because the tgd2-1 phenotype was consistent with a defect in lipid transfer into the chloroplast, the TGD2 protein was tested for the specific binding of different lipids. To distinguish lipid binding to the TMD from lipid binding to a possible substrate site in the C-terminal domain, an N-terminally truncated version, TGD2-dTMD-His, was produced in  Escherichia  tag was used for purification and detection of TGD2-dTMD-His by an anti-His tag antibody. A commercial membrane with different phospholipids and membranes with plant-specific lipids were used. Of all of the lipids tested, including diacylglycerol (data not shown), only PA bound to TGD2-dTMD-His. See,  FIG. 8A . 
     By employing an independent approach, the TGD2-dTMD-His protein bound phosphatidylcholine liposomes containing different molecular species of PA. See,  FIG. 8B . Liposomes consisting of phosphatidylcholine alone did not bind. Binding was independent of the molecular species of PA at least at the semiquantitative immunoblot level. The results suggested that TGD2 contains a PA-specific binding domain in the C-terminal part of the protein. 
     7. Expression of TGD2 Fusion Proteins 
     Proteins containing different fragments of  Arabidopsis  TGD2 were C-terminally fused to DsRed protein (i.e., for example, a  Discosoma  sp. reef coral protein) and expressed in  E. coli  BL-21 (DE3) strain using a DsRed-plw01-His vector in accordance with Example I. The quality of the expressed fusion protein was routinely monitored by SDS-PAGE followed by Coomassie Brilliant Blue staining. Typically, the purity of the DsRed-TGD2 fusion proteins was greater than 90%. A variety of DsRed-TGD2 mutated fusion proteins have been evaluated. Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 PCR primers used to create dsRed-TGD2 mutated fusion proteins. 
               
            
           
           
               
               
               
               
            
               
                 dsRed-TGD2 
                   
                   
                   
               
               
                 protein 
                 mutation 
                 5′ primer 
                 3′ primer 
               
               
                   
               
               
                 TGD2C WT 
                 WT 
                 5′-CCG GAG CTC GGT TTT CAA ATG CCG TC-3′ 
                 5′-CGG CTC GAG TAG TAG CCT GCT TAG 
               
               
                 (SEQ ID NO: 13) 
                   
                 (SEQ ID NO: 14) 
                 GG-3′ 
               
               
                 (119-391) 
                   
                   
                 (SEQ ID NO: 15) 
               
               
                   
               
               
                 TGD2C T1 
                 119-250 
                 5′-CCG GAG CTC GGT TTT CAA ATG CGG TC-3′ 
                 5′-GCG CTC GAG AAT ACG AGT GAA AAT 
               
               
                 (SEQ ID NO: 11) 
                   
                 (SEQ ID NO: 14) 
                 TCC-3′ 
               
               
                 (119-250) 
                   
                   
                 (SEQ ID NO: 18) 
               
               
                   
               
               
                 TGD2C T2 
                 171-300 
                 5′-CCG GAG CTC GCT GAG ATA GAA GAT G-3′ 
                 5′-CGA CTC GAG GCT ATC ACG AAA CTC 
               
               
                 (SEQ ID NO: 19) 
                   
                 (SEQ ID NO: 20) 
                 AG-3′ 
               
               
                 (171-300) 
                   
                   
                 (SEQ ID NO: 21) 
               
               
                   
               
               
                 TGD2C T3 
                 221-350 
                 5′-CAG GAG CTC AAG GAA GGT CTG ATC G-3′ 
                 5′-CGG CTC GAG GAC GTT CTT CAA AGT 
               
               
                 (SEQ ID NO: 22) 
                   
                 (SEQ ID NO: 23) 
                 AT-3′ 
               
               
                 (221-350) 
                   
                   
                 (SEQ ID NO: 24) 
               
               
                   
               
               
                 TGD2C T4 
                 201-381 
                 5′-CCG GAG CTC ATT ATG CCT AGG AAT CCG-3′ 
                 5′-CGG CTC GAG TAG TAG CCT GCT TAG 
               
               
                 (SEQ ID NO: 25) 
                   
                 (SEQ ID NO: 26) 
                 GG-3′ 
               
               
                 (201-381) 
                   
                   
                 (SEQ ID NO: 27) 
               
               
                   
               
               
                 TGD2C T5 
                 119-300 
                 5′-CCG GAG CTC GGT TTT CAA ATG CGG TC-3′ 
                 5′-CGA CTC GAG GCT ATC ACG AAA CTC 
               
               
                 (SEQ ID NO: 28) 
                   
                 (SEQ ID NO: 14) 
                 AG-3′ 
               
               
                 (119-300) 
                   
                   
                 (SEQ ID NO: 21) 
               
               
                   
               
               
                 TGD2C T6 
                 119-225 
                 5′-CCG GAG CTC GGT TTT CAA ATG CGG TC-3′ 
                 5′-CGG CTC GAG GAT CAG ACC TTC CTT 
               
               
                 (SEQ ID NO: 31) 
                   
                 (SEQ ID NO: 14) 
                 AC-3′ 
               
               
                 (119-225) 
                   
                   
                 (SEQ ID NO: 33) 
               
               
                   
               
               
                 TGD2C T7 
                 171-225 
                 5′-CCG GAG CTC GCT GAG ATA GAA GAT G-3′ 
                 5′-CGG CTC GAG GAT CAG ACC TTC CTT 
               
               
                 (SEQ ID NO: 34 
                   
                 (SEQ ID NO: 35) 
                 AC-3′ 
               
               
                 (171-225) 
                   
                   
                 (SEQ ID NO: 33) 
               
               
                   
               
               
                 TGD2C T8 
                 201-225 
                 5′-CCG GAG CTC ATT ATG CCT AGG AAT CCG-3′ 
                 5′-CGG CTC GAG GAT CAG ACC TTC CTT 
               
               
                 (SEQ ID NO: 12 
                   
                 (SEQ ID NO: 26) 
                 AC-3′ 
               
               
                 (201-225) 
                   
                   
                 (SEQ ID NO: 33) 
               
               
                   
               
               
                 TGD2C T9 
                 221-250 
                 5′-CAG GAG CTC AAG GAA GGT CTG ATC G-3′ 
                 5′-GCG CTC GAG AAT ACG AGT GAA AAT 
               
               
                 (SEQ ID NO: 40) 
                   
                 (SEQ ID NO: 23) 
                 TCC-3′ 
               
               
                 (221-250) 
                   
                   
                 (SEQ ID NO: 18) 
               
               
                   
               
               
                 TGD2C D1 
                 221-250 
                 5′-CTG CAT CCT GAA TGT GGT GGA CGC GAA 
                 5′-GGC CTC AAC TTC GCG TCC ACC ACA 
               
               
                 (SEQ ID NO: 43) 
                 deleted 
                 GTT GAG GCC-3′ 
                 TTC AGG ATG CAG-3′ 
               
               
                 (Δ221-250)  
                 (SEQ ID NO: 44) 
                 (SEQ ID NO: 45) 
                   
               
               
                   
               
               
                 TGD2C D2 
                 221-225 
                 5′-CTG CAT CCT GAA TGT GGT GTT TGT GAT 
                 5′-TGT CTG CCT ATC ACA AAC ACC ACA 
               
               
                 (SEQ ID NO: 46) 
                 deleted 
                 AGG CAG ACA-3′ 
                 TTC AGG ATG CAG-3′ 
               
               
                 (Δ221-225) 
                 (SEQ ID NO: 47) 
                 (SEQ ID NO: 48) 
               
               
                   
               
            
           
         
       
     
     One previous study suggested that the C-terminus of TGD2 protein lacking a transit peptide domain and transmembrane domain (TGD2C) could bind to PA when fused with 6×His tag. (22) However, a major drawback of using this reported His-tag-fused-TGD2C protein is bad solubility, which brings significant technical difficulties when attempting mutagenesis and other in vitro studies. In fact, most reports in the lipid binding field use GST-fusion techniques to create a better solubilized protein. However, GST-TGD2 fusion proteins also resulted in unsatisfactory results. While expression and purification of the GST-TGD2 fusion protein was possible, GST alone resulted in non-specific PA binding to the tested lipid substrates, leading to controversial conclusions (data not shown). 
     Among several other expression systems tested, DsRed-fusion provided an optimized assay system and is described herein. The DsRed-monomer is an engineered mutant of the red fluorescent protein from  Discosoma  sp. reef coral, and has specific advantages of being extremely stable and highly soluble. These properties allow expression of soluble DsRed-TGD2 fusion proteins in order to monitor ‘real time’ fluorescence during recombinant protein production and purification. 
     The data presented herein utilizes the same C-terminus of TGD2 protein as reported in the GST fusions, but were fused to DsRed instead. As discussed above, these DsRed-TGD2 fusion proteins demonstrated specific PA binding using protein-lipid overlay assay. See,  FIG. 7 . Furthermore, a minimal PA binding domain in TGD2 was identified that is sufficient to mediate the interaction between the protein and lipid. These data: i) demonstrate that TGD2 specifically binds PA and is a possible substrate for transportation by the proposed TGD123 complex; and (2) define a specific TGD2 PA binding domain that does not show any sequence or structure homology with known PA targets. 
     8. TGD2 Crystallography 
     As discussed above, it is generally known that PA-binding regions reveal no significant homology in primary protein structure (i.e., linear amino acid sequence). (10). None of the previously reported PA targets were predicted by common amino acid sequences. 
     Attempts to identify other PA binding proteins using a TGD2-minimal PA binding domain sequence (i.e., for example, amino acid residues 201-225 (SEQ ID NO: 12)) yielded no results when searching a non-redundant protein database.  FIG. 1A . Hence, homology modeling of TGD2 failed to find other possible PA binding sites in order to generate a working model. Therefore, further analysis will focus on crystallization PA with the full-length TGD2 in an effort to circumvent these difficulties. 
     IV. Isolation of a tgd2-1 Mutant 
     The tgd2-1 mutant was initially identified during a suppressor screen in the dgd1 mutant background using a chemically induced mutant population. Xu et al., (2003)  EMBO J.  22:2370-2379. The dgd1 mutant was reported to be deficient in DGD1, the protein believed responsible for the bulk of digalactolipid biosynthesis. Do{umlaut over ( )}rmann et al., (1999)  Science  284:2181-2184. Presence of the tgd2-1 mutation in the dgd1 background partially alleviated the digalactolipid deficiency and caused the accumulation of a lipid co-chromatographing with trigalactosyldiacylglycerol diagnostic for all tgd mutants. Crossing the double-homozygous dgd1/tgd1-1 and dgd1/tgd2 mutants gave rise to uniform plants in the F1 generation with a homozygous dgd1-like phenotype, suggesting that tgd1-1 and tgd2-1 are not allelic. The tgd2-1/dgd1 homozygous double mutant was crossed to  Arabidopsis  wild-type, ecotype Columbia-2 (Col-2). The F1 plants showed a wild-type lipid phenotype confirming that the tgd2-1 mutant allele is recessive. After selfing and lipid analysis, F2 plants homozygous at the tgd2-1 locus were genotyped at the DGD1 locus by using a derived cut amplified polymorphic sequence (dCAPS) marker to test for loss of the dgd1 mutation. A homozygous tgd2-1 mutant line was back-crossed with wild type (Col-2) three times to reduce the chance of secondary mutations. Unless indicated otherwise, further analysis was done with this tgd2-1 mutant in the wild-type background. 
     Compared to the wild type, tgd2-1 plants were consistently smaller and slightly pale, as was observed for the tgd1-1 mutants. Xu et al., (2005)  Plant Cell  17:3094-3110. Chlorophyll contents were reduced to a similar extent in the tgd1-1 and tgd2-1 mutants [chlorophyll (Ch1) per gram of fresh weight (FW) ±SD, n=4: wild type, 1,136±138 μg Ch1 ·g −1  FW; tgd1-1, 553±115 μg Ch1 ·g −1  FW; tgd2-1, 656±145 μg Ch1 ·g −1  FW]. Leaf lipid extracts of the wild type and the tgd1-1 and tgd2-1 mutants were compared by TLC. In the tgd2-1 sample a lipid staining positive for sugar and cochromatographing with authentic trigalactolipid of tgd1-1 is present. See,  FIG. 10A . A lipid co-chromatographing with authentic triacylglycerol accumulating in tgd1-1 leaves was present in the tgd2-1 sample as well. See,  FIG. 10B . Quantitative analysis of the polar lipids indicated similar changes in the two mutants with relative amounts of the monogalactolipid and digalactolipid reduced and relative amounts of phosphatidylcholine increased. See,  FIG. 10C . In addition, trigalactolipid was present to a similar extent in both mutants (tgd1-1, 2.7±1.4 mol %; tgd2-1, 1.6±0.4 mol %; n=4; data are ±SD) but was not detectable in the wild type. Analyzing the fatty acid composition of the two galactolipids indicated a reduction of 18-carbon fatty acids and an increase in 16-carbon fatty acids to the same extent in both mutants. See,  FIG. 10D . These overall fatty acid compositions for the tgd2-1 mutant imply a change in molecular species distribution in the two galactolipids consistent with a reduction of molecular species derived from the ER pathway. In addition, similar to the tgd1-1 mutant carrying a weak chemically-induced mutant allele, the tgd2-1 mutant produced a fraction (˜43%, 281 of 651 in a representative sample) of aborted seeds. 
     In a mapping population of 93 homozygous tgd2-1 F2 mutant plants (186 chromosomes) from a cross between the homozygous tgd2-1 mutant in the dgd1(Col-2) background and a plant from the ecotype Landsberg erecta the tgd2-1 mutant locus was mapped close to cut amplified polymorphic sequence (CAPS) marker ARLIM15.1 (arabidopsis.org) at ˜30 cM on chromosome 3. See,  FIG. 11A . In an enlarged F2 mapping population from the same cross (3,506 chromosomes) the tgd2-1 mutant locus was mapped to an ˜45-kb fragment flanked by CAPS marker MQC12-3 and dCAPS marker MQC12-4. See,  FIG. 11A . 
     This region falls onto the  Arabidopsis  bacterial artificial chromosome clone MQC12 (Gen-Bank accession no. AB024036; SEQ ID NO: 129) and encompasses 14 predicted or confirmed genes (At3g20270-At3g20390). Notably, the translation product of At3g20320 (SEQ ID NO: 1) was similar to the ttg2C protein (GenBank accession no. AAD17959 (SEQ ID NO: 128); 25.0% identity over &gt;100 aa) of  Pseudomonas putida . This protein is predicted to be the substrate-binding protein of an ABC transporter, and its ORF is flanked by one encoding the ABC transporter permease ttg2B (GenBank accession no. AAD17958; SEQ ID NO: 127). Most notably, the  Arabidopsis  TGD1 protein is similar to ttg2B (29.6% identity over &gt;100 aa) of  P. putida . The predicted bacterial ABC transporter encoded by the ttg2 operon in  P. putida  has been genetically implicated in toluene resistance. Kim et al., (1998)  J. Bacteriol.  180:3692-3696. The At3g20320 cDNA sequence obtained by RT-PCR from the  Arabidopsis  tgd2-1 mutant contained a G-to-A mutation (See,  FIG. 2A ) corresponding to position 7,088,870 of the assembled chromosome 3 sequence (GenBank accession no. NC — 003074) and leading to a glycine-to-arginine change in the amino acid sequence. See,  FIG. 11A . This mutation was confirmed by designing a tgd2-1 allele-specific dCAPS marker that was later used for genotyping. See,  FIG. 11D . 
     The TGD2 ORF of 1,146 by encodes a protein of 41.6 kDa. In addition to the similarity to bacterial substrate binding proteins, the TGD2 protein contains a MCE domain (amino acids 99-216 (SEQ ID NO: 109)). See,  FIG. 11A , bottom. This domain is found in surface proteins of pathogenic mycobacteria. These proteins may comprise virulence factors proposed to facilitate the bacterial entry into mammalian host cells. Chitale et. al., (2001)  Cell. Microbiol.  3:247-254. The mutation in tgd2-1 affects amino acid 234 just outside this MCE domain. A transmembranespanning domain (TMD) in TGD2 (amino acids 96-118) (SEQ ID NO: 3) was predicted by using SOSUI software. Hirokawa et al., (1998)  Bioinformatics  14:378-379. A chloroplast targeting peptide of 45 N-terminal amino acids was predicted (score 0.545) by using CHLOROP with default settings. Emanuelsson et al., (1999)  Protein Sci.  8:978-984. 
     V. TGD2 cDNA Expression 
     The tgd2-1 mutation in the dgd1 mutant background led to increased growth compared with the homozygous dgd1 mutant. This phenotype was reversed by expression of the wild-type TGD2 cDNA under the control of the 35S-CMV (cauliflower mosaic virus) promoter in the tgd2-1/dgd1 homozygous double mutant. See,  FIG. 11B . The genotypes were confirmed by using mutant allele-specific dCAPS markers. See,  FIGS. 11C and 11D . In both transgenic lines two bands were present, a first band corresponding to a wild-type cDNA and a second band corresponding the a tgd2-1 genomic mutant locus. See,  FIG. 11D . Reversion of the digalactolipid and the trigalactolipid phenotype of the tgd2-1/dgd1 double mutant to the homozygous dgd1 phenotype was observed as well. See,  FIG. 11E . This complementation analysis confirmed the identity of the TGD2 gene as At3g20320. 
     The similarity of tgd1-1 and tgd2-1 mutant phenotypes and the organization of predicted bacterial orthologs of these two  Arabidopsis  genes in operons suggested that TGD1 and TGD2 act together in the same cellular process possibly as part of a larger lipid transfer complex. Expression of the tgd2-1 mutant cDNA under the control of the 35S-CMV promoter in the wild type led to the accumulation of a lipid cochromatographing with the trigalactolipid accumulating in the tgd1-1 and tgd2-1 mutants. See,  FIG. 12B . 
     Semiquantitative RT-PCR confirmed that this effect was not due to cosuppression of the genomic wild-type TGD2 gene and the tgd2-1 cDNA expression construct, because RNA derived from both genes was abundant in the transgenic lines. See,  FIG. 12A . One interpretation of this dominant negative effect is that the tgd2-1-encoded mutant protein is impaired in its activity but can still become part of its native protein complex, thereby disrupting overall function of the process involving the complex. In addition, this result provided independent corroboration for the identity of TGD2 with At3g20320. 
     VI. TGD2 Intracellular Localization 
     To determine the subcellular localization of the TGD2 protein, a construct encoding a full-length C-terminal fusion between the TGD2 protein and a GFP was transiently expressed in tobacco the periphery of chloroplasts. See,  FIG. 13A . It should be noted that the equivalent experiment for the TGD1-GFP fusion construct showed a similar punctate fluorescence pattern at the chloroplast surface. Xu et al., (2005)  Plant Cell  17:3094-3110. 
     To further explore the association of the TGD2 protein with one of the two chloroplast envelope membranes and to determine its topology, chloroplasts were isolated from tobacco leaves expressing a wild-type TGD2 cDNA or the tgd2-1 mutant cDNA. See,  FIG. 13B . The TGD2 wild type and the tgd2-1 mutant proteins were detected with a polyclonal antibody against TGD2. The chloroplasts were either untreated or treated with thermolysin, a protease unable to penetrate the outer envelope membrane, or trypsin, a protease able to penetrate the outer envelope but not the inner envelope membrane. Interestingly, the wild-type TGD2 protein was resistant to both proteases, whereas the mutant protein tgd2-1 was resistant to thermolysin but not trypsin. See,  FIG. 13B , top and middle. 
     When the full-length wild-type TGD2 protein C-terminally fused to GFP was tested, the GFP tag detected by a GFP-specific antibody was resistant to thermolysin but not to trypsin. See,  FIG. 13B , bottom. With the exception of the TGD2 wild-type protein, the result suggests that the TGD2 protein is associated with the inner envelope membrane with the C terminus facing the intermembrane space. The wild-type TGD2 is trypsin-resistant either because it is inside the plastid or, more likely, because it is in a complex or a membrane domain inaccessible to trypsin. 
     VII. Kits 
     In one embodiment, the present invention contemplates a kit, comprising: a) a first container comprising a test strip comprising a phosphatidic acid binding protein; b) a second container comprising a plurality of buffers and a plurality of reagents, wherein said protein is soluble; and c) a set of instructions for determining a phosphatidic acid. In one embodiment, the protein further comprises a label. In one embodiment, the phosphatidic acid is derived from a sample. In one embodiment, the protein further comprises at least one accessory binding protein. 
     In another embodiment, the present invention contemplates kits for the practice of the methods of this invention. The kits preferably include one or more containers containing a phosphatidic acid determination method of this invention. 
     The kit can optionally include a a TDG2 protein comprising a phosphatidic acid binding domain, wherein said domain encompasses amino acid residues 201-225 (SEQ ID NO: 12), wherein at least one of said residues is a proline. The kit can optionally include a plurality of buffers as described herein. The kit can optionally include a plurality of reagents as described herein. The kit can optionally include enzymes as described herein. The kit can optionally include enzymes capable of performing PCR (i.e., for example, DNA polymerase, Taq polymerase and/or restriction enzymes). The kit can optionally include a pharmaceutically acceptable excipient and/or a delivery vehicle (e.g., a liposome). The reagents may be provided suspended in the excipient and/or delivery vehicle or may be provided as a separate component which can be later combined with the excipient and/or delivery vehicle. 
     The kits may also optionally include appropriate systems (e.g. opaque containers) or stabilizers (e.g. antioxidants) to prevent degradation of the reagents by light or other adverse conditions. 
     The kits may optionally include instructional materials containing directions (i.e., protocols) providing for the use of the reagents in the determination of phosphatidic acid for one of many plant disorders. In particular a plant disease, wounding and/or stress can include any one or more of the disorders described herein. While the instructional materials typically comprise written or printed materials they are not limited to such. Any medium capable of storing such instructions and communicating them to an end user is contemplated by this invention. Such media include, but are not limited to electronic storage media (e.g., magnetic discs, tapes, cartridges, chips), optical media (e.g., CD ROM), and the like. Such media may include addresses to internet sites that provide such instructional materials. 
     VIII. Phosphatidic Acid-Binding Protein 
     The TGD2 protein of  Arabidopsis  is proposed to be the substrate binding component of a lipid transfer complex in the inner chloroplast envelope. Loss of function of this protein or other components of this complex disrupts the ER-pathway of thylakoid lipid biosynthesis. Previous studies demonstrated that the C-terminal 6×-His tag-fused protein of TGD2 (TGD2C, with removal of the N-terminal transit peptide and transmembrane domain) interacts selectively with phosphatidic acid (PtdOH). Awai et al., (2006) “A phosphatidic acid-binding protein of the chloroplast inner envelope membrane involved in lipid trafficking”  Proc Natl Acad Sci USA  103: 10817-10822) To improve expression and solubilization of this protein, we fused the open reading frame encoding the TGD2C truncated protein C-terminally to the  Discosoma  sp. red fluorescent protein (DsRed) open reading frame and expressed the fused open reading frame under the control of the T7 promoter. Like its predecessor, the DsRed-TGD2C fusion protein was shown to specifically bind PtdOH. By deletion and truncation mutagenesis, the PtdOH binding site within TGD2C was further narrowed down to a 25-amino-acid segment. Our data suggest that this segment is necessary and sufficient for PtdOH binding. Crystallization of the DsRed-fusion protein will provide a stereochemical analysis. 
     Various TGD2 fusion proteins may be made by polymerase chain reaction (PCR) using primers identified in Table 1: 
                     TABLE 1                  PCR primers used for generation of dsRed-TGD2 fusion proteins                             dsRed fusion protein   mutation   5′ primer   3′ primer               TGD2C WT   WT   5′-CCG GAG CTC GGT TTT CAA ATG CGG TC-3′   5′-CGG CTC GAG TAG TAG CCT GCT       (SEQ ID NO: 13)       (SEQ ID NO: 14)   TAG GG-3′ (SEQ ID NO: 15)       (119-381)                           TGD2C T1   119-250   5′-CCG GAG CTC GGT TTT CAA ATG CGG TC-3′   5′-GCG CTC GAG AAT ACG AGT GAA       (SEQ ID NO: 11)       (SEQ ID NO: 14)   AAT TCC-3′ (SEQ ID NO: 18)       (119-250)                           TGD2C T2   171-300   5′-CCG GAG CTC GCT GAG ATA GAA GAT G-3′   5′-CGA CTC GAG GCT ATC ACG AAA       (SEQ ID NO: 19)       (SEQ ID NO: 20)   CTC AG-3′ (SEQ ID NO: 21)       (171-300)                           TGD2C T3   221-350   5′-CAG GAG CTC AAG GAA GGT CTG ATC G-3′   5′-CGG CTC GAG GAC GTT CTT CAA       (SEQ ID NO: 22)       (SEQ ID NO: 23)   AGT AT-3′ (SEQ ID NO: 24)       (221-350)                           TGD2C T4   201-381   5′-CCG GAG CTC ATT ATG CCT AGG AAT CCG-3′   5′-CGG CTC GAG TAG TAG CCT GCT       (SEQ ID NO: 25)       (SEQ ID NO: 26)   TAG GG-3′ (SEQ ID NO: 27)       (201-381)                           TGD2C T5   119-300   5′-CCG GAG CTC GGT TTT CAA ATG CGG TC-3′   5′-CGA CTC GAG GCT ATC ACG AAA       (SEQ ID NO: 28)       (SEQ ID NO: 17)   CTC AG-3′ (SEQ ID NO: 21)       (119-300)                           TGD2C T6   119-225   5′-CCG GAG CTC GGT TTT CAA ATG CGG TC-3′   5′-CGG CTC GAG GAT CAG ACC TTC       (SEQ ID NO: 31)       (SEQ ID NO: 17)   CTT AC-3′ (SEQ ID NO: 33)       (119-225)                           TGD2C T7   171-225   5′-CCG GAG CTC GCT GAG ATA GAA GAT G-3′   5′-CGG CTC GAG GAT CAG ACC TTC       (SEQ ID NO: 34)       (SEQ ID NO: 35)   CTT AC-3′ (SEQ ID NO: 33)       (171-225)                           TGD2C T8   201-225   5′-CCG GAG CTC ATT ATG CCT AGG AAT CCG-3′   5′-CGG CTC GAG GAT CAG ACC TTC       (SEQ ID NO: 12)       (SEQ ID NO: 26)   CTT AC-3′ (SEQ ID NO: 33)       (201-225)                           TGD2C T9   221-250   5′-CAG GAG CTC AAG GAA GGT CTG ATC G-3′   5′-GCG CTC GAG AAT ACG AGT GAA       (SEQ ID NO: 40)       (SEQ ID NO: 23)   AAT TCC-3′ (SEQ ID NO: 18)       (221-250)                           TGD2C D1   221-250   5′-CTG CAT CCT GAA TGT GGT GGA CGC GAA   5′-GGC CTC AAC TTC GCG TCC ACC       (SEQ ID NO: 43)   deleted   GTT GAG GCC-3′   ACA TTC AGG ATG CAG-3′       (Δ221-250)       (SEQ ID NO: 44)   (SEQ ID NO: 45)               TGD2C D2   221-225   5′-CTG CAT CCT GAA TGT GGT GTT TGT GAT   5′-TGT CTG CCT ATC ACA AAC ACC       (SEQ ID NO: 46)   deleted   AGG CAG ACA-3′   ACA TTC AGG ATG CAG-3′       (Δ221-225)       (SEQ ID NO: 47)   (SEQ ID NO: 48)                    
The TGD2 protein is N-terminally truncated lacking the TMD and C-terminally fused to the  Discosoma  sp. red fluorescent protein (DsRed, dR) open reading frame. Fusion protein was expressed and protein-lipid overlay assay was conducted with commercial phospholipid-containing membrane strip. LPA, lysophosphatidic acid; LPC, lysophosphatidylcholine; Ptdlns, phosphatidylinositol; Ptdlns(3)P, phosphatidylinositol 3-phosphate; Ptdlns(4)P, phosphatidylinositol 4-phosphate; Ptdlns(5)P, phosphatidylinositol 5-phosphate; PE, phosphatidylethanolamine; PC, phosphatidylcholine; S1P, sphingosine 1-phosphate; Ptdlns(3,4)P2, phosphatidylinositol 3,4-bisphosphate; Ptdlns(3,5)P2, phosphatidylinositol 3,5-bisphosphate; Ptdlns(4,5)P2, phosphatidylinositol 4,5-bisphosphate; Ptdlns(3,4,5)P3, phosphatidylinositol 3,4,5-bisphosphate; PA, phosphatidic acid; PS, phosphatidylserine. See  FIG. 7 .
 
     Gene bank accession numbers for representative TGD2 ortholog sequences include, but are not limited to:  Arabidopsis thaliana , NP — 566659.1 (SEQ ID NO: 5);  Vitis vinifera , CAN71395.1 (SEQ ID NO: 6);  Oryza sativa , EAY77419.1 (SEQ ID NO: 7);  Physcomitrella patens , XP — 001778862.1 (SEQ ID NO: 8);  Ostreococcus tauri , CAL53419.1 (SEQ ID NO: 9);  Chlamydomonas reinhardtii , XP — 001699315.1 (SEQ ID NO: 10);  Prochlorococcus marinus str. NATL2A, YP — 292846.1 (SEQ ID NO: 115);  Prochlorococcus marinus  str. MIT 9301, YP — 001090537.1 (SEQ ID NO: 116);  Synechococcus  sp. WH 5701, ZP — 01083418.1 (SEQ ID NO: 117);  Synechococcus  sp. CC9902, YP — 376253.1 (SEQ ID NO: 118);  Synechococcus  sp. JA-2-3B′a(2-13), YP — 477327.1 (SEQ ID NO: 119);  Anabaena variabilis , YP — 323182.1 (SEQ ID NO: 120);  Nodularia spumigena , ZP — 01630545.1 (SEQ ID NO: 121);  Crocosphaera watsonii , ZP — 00516249.1 (SEQ ID NO: 122);  Cyanothece  sp. PCC 8801, ZP — 02940544.1 (SEQ ID NO: 123);  Microcystis aeruginosa , CAO90615.1 (SEQ ID NO: 124);  Acaryochloris marina , YP — 001516641.1 (SEQ ID NO: 125);  Thermosynechococcus elongatus , NP — 683197.1 (SEQ ID NO: 126). See,  FIG. 17 . 
     EXPERIMENTAL 
     Example I 
     Expression and Purification of DsRed-TGD2 Fusion Proteins 
     All the TGD2 truncated proteins used in this example were obtained from DNA generated by PCR using a TGD2-dTMD-pQE31 (also known as TGD2C-pQE31) plasmid template (22). Following digestion with NcoI and XhoI, the fragment was ligated into DsRed-plw01-His (a gift from Dr. Michael Garavito, Michigan State University, East Lansing, Mich.). Internal deletion mutants and/or point mutants were generated by site-directed mutagenesis approach on TGD2CDsRed-plw01 via PCR, with the primers and mutation sites listed in Table 1 (supra). 
     All fusion proteins were expressed in the  Escherichia coli  strain, BL21 (DE3) (Novagen, Madison, Wis.). An overnight pre-culture of LB medium (5 mL) was used to start a 200 mL culture in LB medium. The protein was induced with 50 μM IPTG (isopropyl-β-D-thiogalactopyranoside) at OD600 0.6-0.8, 16° C. and growth was continued overnight. Cultures were cooled to 4° C., washed twice and resuspended in lysis buffer (50 mM Tris-HCl, pH7.5, 300 mM NaCl, 10 mM imidazole). The suspensions were lysed by sonication, followed by centrifugation at 18,000 g. 
     The resultant supernatant was applied to Ni-NTA agarose column (Qiagen, Valencia, Calif.). Non-specific binding proteins were washed off the column by lysis buffer containing 20 mM imidazole. The His-tagged protein was then eluted with lysis buffer containing 250 mM imidazole. 
     Samples were concentrated and dialyzed into assay buffer (10 mM KH 2 PO 4 , pH ˜7.4), using Amicon centrifugal filter devices (Millipore, Billerica, Mass.). Protein concentration was determined according to Bradford (27) using bovine serum albumin as a standard. The fusion proteins were analyzed for purity by SDS-PAGE (28) and stored at 4° C. for a few weeks without significant loss of activity. 
     Phylogenetic Analysis of TGD2-full-length TGD2 amino acid sequences were BLASTed against non-redundant protein database (29) and the resulted sequences with high similarities and identities were aligned using Clustalx® software (version 1.81). Generation of the bootstrapped phylogenetic tree was performed using the PHYLIP software package as previously described (30). 
     Example II 
     Protein-lipid Overlay Assay 
     Membrane lipid strips were purchased from Echelon Biosciences (Salt Lake City, Utah). The strips were first blocked with 3% bovine serum albumin in TBST (10 mM Tris-HCl, pH 8.0, 150 mM NaCl, and 0.25% Tween-20) for two hours and incubated in 0.5 μg/mL DsRed-TGD2 fusion protein solution in the blocking buffer at 4° C. overnight. The strips were washed 10 min for 3 times with TBST the next day and soaked in 3% bovine serum albumin in TBST with a Penta-His mouse monoclonal antibody (Sigma-Aldrich, St. Louis, Mo.) at 1:2,000 dilution at 4° C. overnight. The strips were washed twice with TBST and soaked in 3% bovine serum albumin in TBST with horseradish peroxidase-conjugated anti mouse antibody (Bio-Rad, Hercules, Calif.) at 1:20,000 dilution for an hour at room temperature. Following washing with TBST for 1 hour, the protein was detected by using the chemiluminescent detection system (Sigma-Aldrich). 
     Example III 
     Liposome-Association Assay 
     The liposome association assay was performed as previously reported. (31). Briefly, lipids (dioleoyl-phosphatidylcholine, DOPC or dioleoyl-PA, DOPA) were incubated in TBS (50 mM Tris-HCl, pH 7; 0.1 M NaCl) at 37° C. for an hour followed by vigorous vortexing for 5 min. The liposomes were precipitated at 20,000 g and washed twice with ice-cold TBS. 
     Liposomes (200 μg) were mixed with purified DsRed-TGD2 fusion protein and TBS to make a final 100 μL solution. The mixture was incubated at 30° C. for 30 min and washed twice with ice-cold TBS by centrifugation at 20,000 g at 4° C. The liposome pellet mixed with sample buffer was analyzed by SDS-PAGE (28). Immuno-detection of the His-tagged protein was accomplished using the above mentioned Penta-His antibody at 1:15,000 and the anti mouse antibody at 1:75,000 dilution. 
     The protein band was visualized by chemiluminescent detection kit from Sigma. The autoradiography film was scanned, distinct protein bands were quantified using computer software Multi Gauge V3.0 (Fujifilm USA, Valhalla, N.Y.) and resulted data were plotted and analyzed by OriginPro8 (Origin lab corporation, Northampton, Mass.). 
     Example IV 
     Plant Material 
       Arabidopsis thaliana  plants were of the ecotypes Columbia-2 (Col-2) or Landsberg erecta (Ler). The tgd1-1 and dgd1 mutants were previously isolated. Xu et al., (2003)  EMBO J.  22:2370-2379; and Do{umlaut over ( )}rmann et al., (1995)  Plant Cell  7:1801-1810. Standard growth conditions were used for surface-sterilized seeds on agar-solidified MS medium supplemented with 1% (wt/vol) sucrose or for plants grown on soil. Murashige et al., (1962)  Physiol. Plant.  15, 473-497; and Xu et al., (2002)  Plant Physiol.  129:594-604. 
     Example V 
     Lipid Analysis 
     Lipids were extracted, and fatty acid methylesters were prepared and quantified by gas chromatography as previously Mallinckrodt, Baker, N.J.) by using a solvent system of acetone/toluene/water (90/30/7, vol/vol). Neutral lipids were separated on untreated TLC plates and developed with petroleum ether/ether/acetic acid (70/30/1, vol/vol). Polar lipids were analyzed on activated ammonium sulfate-impregnated silica gel TLC plates (Si250PA; Mallinckrodt, Baker, N.J.) by using a solvent system of acetone/toluene/water (90/30/7, vol/vol). Neutral lipids were separated on untreated TLC plates and developed with petroleum ether/ether/acetic acid (70/30/1, vol/vol). Lipids were visualized by brief exposure to iodine vapor or staining with α-naphthol to detect glycolipids. Benning et al., (1995)  Arch. Biochem. Biophys.  317:103-111. 
     Example VI 
     Markers for Genetic Mapping and Genotyping 
     For fine mapping, 10 CAPS markers (Konieczny et al., (1993)  Plant J.  4, 403-410) and 1 dCAPS (MQC12-4) marker (Neff et al., (1998)  Plant J.  14:387-392) were generated, taking advantage of the Monsanto Polymorphism and Ler Sequence Collection (arabidopsis.org/Cereon/index.jsp). Primers and restriction enzymes were as follows: 
                        MYF24:           5′-GACAGCCCACAAATTGATGG-3′   (SEQ ID NO: 49)       and           5′-ACCAACGCTCAATGCCTAC-3′   (SEQ ID NO: 50)       cut with HinfI.                   MLD14:           5′-GGGGTCCTTAAAATAGAGAC-3′   (SEQ ID NO: 51)       and           5′-GGCCTTTTGAGTTGGGAAAAG-3′   (SEQ ID NO: 52)       cut with HindIII.                   MIL23:           5′-GGGGGTGATATCTATCGTAG-3′   (SEQ ID NO: 53)       and           5′-GCACCCTGGATATTCTTTCG-3′   (SEQ ID NO: 54)       cut with HinfI.                   MPN9:           5′-CGGTCATATGCTGGCTGAAG-3′   (SEQ ID NO: 55)       and           5′-GACAGCACACAAGTTCCAGG-3′   (SEQ ID NO: 56)       cut with AluI.                   MPN9-2:           5′-GTGCTATGGTTCAGGAGTTC-3′   (SEQ ID NO: 57)       and           5′-CTTACCAGCCATGACGATTC-3′   (SEQ ID NO: 58)       cut with AccI.                   MAL21:           5′-GAGAAGAAACACCGATTCCG-3′   (SEQ ID NO: 59)       and           5′-GTTGTGATACGAATGGTGGC-3′   (SEQ ID NO: 60)       cut with RsaI.                   K10D20:           5′-GGACCTGCCTTTCCCATATC-3′   (SEQ ID NO: 61)       and           5′-GCCCAAGCCTCAAGATGTTG-3′   (SEQ ID NO: 62)       cut with HindIII.                   MSA6:           5′-GGAAGAGGGAGGTTTTGTTC-3′   (SEQ ID NO: 63)       and           5′-CCAATTCGTCTCCTTTTCACC-3′   (SEQ ID NO: 64)       cut with SpeI.                   MQC12-2:           5′-GTGAGACCAACAGTGTCAAC-3′   (SEQ ID NO: 65)       and           5′-CCACAATACACCACCACTTG-3′   (SEQ ID NO: 66)       cut with HinfI.                   MQC12-3:           5′-CCTCCGTCTCATACATCTAC-3′   (SEQ ID NO: 67)       and           5′-CCAATTCGGTTTCATCCAATCCTCT-3′   (SEQ ID NO: 68)       cut with BfaI.                   MQC12-4:           5′-CATATGCATTGATGATAACTGAAATCGA-3′   (SEQ ID NO: 69)       and           5′-CTTCTAGATCTCCTCCTTTC-3′   (SEQ ID NO: 70)       cut with EcoRI.                
For genotyping of the tgd2-1 mutant, a dCAPS marker was generated:
 
                        5′-TGATCGTTTGTGATAGGCAGCCTATAAA   (SEQ ID NO: 71)       A-3′           and                   5′-CCTTGCTTCCTCAATAACCG-3′,   (SEQ ID NO: 72)       cut with EcoNI.                
The dgd1 dCAPS marker was made as previously described. Xu et al., (2003)  EMBO J.  22:2370-2379.
 
     Example VII 
     Complementation and Dominant-Negative Mutation Analysis 
     The ORFs for TGD2 and tgd2-1 were isolated by RT-PCR from mRNA preparations by using RNeasy and Omniscript kits (Qiagen, Valencia, Calif.) and standard PCR conditions. The following primers were used: 
                        5′-GTCGACATGATTGGGAATCCAGTAATTCAA   (SEQ ID NO: 73)       G-3′           and                   5′-GTCGACTCATAGTAGCCTGCTTAGGG-3′.   (SEQ ID NO: 74)            
The fragments were ligated into pGEM-T Easy (Promega) and sequenced at the Michigan State University Genomics and Technology Facility. The resulting plasmids were digested with SalI and inserted into pCAMBIAmcs1300 followed by transformation into  Agrobacterium . Plants were transformed by the floral-dip method (22) and screened by resistance to hygromycin (25 μg/ml) on agarsolidified MS medium. Clough et al., (1998)  Plant J.  16:735-743. For semiquantitative PCR of TGD2 and tgd2 transcripts the following primers were used: TGD2-specific:
 
                                5′-CGGCTTGCTCAAGGAAGTTG-3′   (SEQ ID NO: 75)           and                       5′-CCAGTCTAAAATCTACAGGCTG-3′;   (SEQ ID NO: 76)            
TGD2 and tgd2-1:
 
                        5′-TGATCGTTTGTGATAGGCAGCCTATAAA   (SEQ ID NO: 77)       A-3′           and                   5′-CCTTGCTTCCTCAATAACCG-3′;   (SEQ ID NO: 78)            
UBQ10:
 
                        5′-TCAATTCTCTCTACCGTGATCAAGATGC   (SEQ ID NO: 79)       A-3′           and                   5′-GTGTCAGAACTCTCCACCTCAAGAGTA-3′.   (SEQ ID NO: 80)            
Isolation of RNA and reverse transcription were done as described above. Amplification conditions were as follows: 94° C. for 3 min followed by 25 cycles at 94° C. for 0.5 min, 55° C. for 0.5 min, and 72° C. for 0.5 min followed by 3 min at 72° C.
 
     Example VIII 
     TGD2GFP Fusion and In Vivo Chloroplast Import Assay 
     The sequence encoding the full-length TGD2 protein was amplified from the pCAMBIAmcs1300 plasmid derivative mentioned above by PCR using the following primers: forward, 5′-GTCGACATGATTGGGAATCCAGTAATTCAAG-3′ (SEQ ID NO: 81); reverse, 5′-GTCGACTAGTAGCCTGCTTAGGGATTTG-3′ (SEQ ID NO: 82). The fragment was inserted into the pGEM-T Easy vector, sequenced and digested with SalI, and inserted into pCAMBIAmcsGFP. In vivo analysis of the GFP-tagged protein was done by confocal fluorescence microscopy. 
     In vivo chloroplast import analysis was performed using transient expression of the constructs in tobacco leaves. Xu et al., (2005)  Plant Cell  17:3094-3110. For immunodetection of the TGD2 or tgd2-1 proteins, a polyclonal antibody was raised in rabbits (Cocalico Biologicals, Reamstown, Pa.) against the truncated TGD2 protein used also for the lipid binding assay. The anti-serum was purified with a Melon Gel IgG Purification Kit (Pierce). For TGD2 immunodetection, the purified anti-TGD2 antibody was used at a 1:2,000 dilution. For GFP immunodetection, a rabbit anti-GFP antibody (Molecular Probes) was used at a 1:3,000 dilution. The antibodies were detected with an anti-rabbit horseradish peroxidase-coupled antibody (Bio-Rad) at a dilution of 1:60,000 followed by development with Chemiluminescent Peroxidase Substrate (Sigma). 
     Example IX 
     Recombinant TGD2 Protein Production and Purification 
     The sequence encoding N-terminally truncated TGD2-dTMD protein (from Gly-119 to stop codon) lacking the targeting peptide and the TMD was PCR-amplified by using primers: 
                        5′-GTCGACGGTTTTCAAATGCGGTCGAAG-3′   (SEQ ID NO: 83)       and                   5′-GTCGACTCATAGTAGCCTGCTTAGGG-3′.   (SEQ ID NO: 84)            
This fragment was inserted into pPICT2 plasmid and sequenced. Kawaguchi et al., (2001) J. Bone Miner. Res. 16, 260-269. After digestion with SalI, the insert was ligated into pQE31 (Qiagen). An overnight preculture of LB medium (1 ml) was used to start a 500-ml culture in M9 medium. Duffieux et al., (2000)  Eur. J. Biochem.  267:5306-5312. The protein was induced with 0.1 mM isopropyl-β-D-thiogalactopyranoside at an OD 600  of 0.4 at 22° C., and growth was continued overnight. Cultures were cooled to 4° C., washed twice, and resuspended in lysis buffer (50 mM Tris.HCl, pH 7.5/600 mM NaCl/20 mM imidazole). The suspensions were lysed by sonication followed by brief centrifugation at 1,500×g to eliminate cell debris. The supernatants were centrifuged at 20,000×g and applied to a Ni-NTA agarose column (Qiagen). The His-tagged protein was eluted with lysis buffer containing 250 mM imidazole. Samples were dialyzed in the lysis buffer lacking imidazole. Protein concentration was determined by using BSA as a standard. Bradford, M. M. (1976)  Anal. Biochem.  72, 248-254.
 
     Example X 
     Lipid Binding Assays 
     Commercially available membrane strips prespotted with lipids were purchased. (Echelon Biosciences, Salt Lake City, Utah). Prokaryotic phosphatidylcholine and PA were also purchased (Avanti Polar Lipids). Prokaryotic monogalactolipid, digalactolipid, sulfolipid, and phosphatidylglycerol were purified from  Synechocystis  PCC6803 by TLC of lipid extracts. Eukaryotic monogalactolipid and digalactolipid was isolated from pea leaves. 
     Approximately 5 μg of lipids were spotted onto a Hybond-C membrane (Amersham Pharmacia Biosciences). The membranes were first blocked with 3% BSA in TBST (10 mM Tris.HCl, pH 8.0/150 mM NaCl/0.1% Tween 20) for 1 h and incubated in 0.5 μg/ml TGD2 protein solution in the blocking buffer at 4° C. overnight. The blots were washed five times with TBST and soaked in 3% BSA in TBST with a Penta-His mouse monoclonal antibody (Qiagen) at a 1:1,000 dilution at room temperature overnight. The membranes were washed twice with TBST and soaked in 3% BSA in TBST with alkaline phosphatase-conjugated anti-mouse antibody (Jackson ImmunoResearch) at a 1:5,000 dilution for 1 hour at room temperature. After washing with TBST twice, the protein was detected by using the Immun-Star AP detection system (Bio-Rad). 
     The liposome binding assay was performed as previously reported. Sano et al., (1998)  J. Biol. Chem.  273:4783-4789. Lipids (i.e., for example, phosphatidylcholine or a mixture of phosphatidylcholine and PA at 6:4 wt/wt) were incubated in TBS (50 mM Tris/HCl, pH 7/0.1M NaCl) at 37° C. for 1 hour followed by vigorous vortexing for 5 min. The liposomes were precipitated at 20,000×g and washed twice with ice-cold TBS. 
     Liposomes (200 μg) were mixed with purified TGD2 protein lacking the TMD (10 μg/ml) and TBS to make 100 μl of solution. The mixture was incubated at 30° C. for 30 min and washed twice with ice-cold TBS by centrifugation at 20,000×g at 4° C. The liposome pellet mixed with sample buffer was analyzed by SDS/PAGE. Laemmli, U. K. (1970) Nature  227, 680-685. Immunodetection of the His-tagged protein was accomplished by using the above-mentioned Penta-His antibody at 1:6,000 and the anti-mouse antibody at 1:10,000 dilution. The BCIP/NBT Kit from Bio-Rad was used for color detection. 
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