UCS15A, a non-kinase inhibitor of Src signal transduction

Src tyrosine kinase plays key roles in signal transduction following growth factor stimulation and integrin-mediated cell-substrate adhesion. Since src-signal transduction defects are implicated in a multitude of human diseases, we have sought to develop new ways to identify small molecule inhibitors using a yeast-based, activated-src over-expression system. In the present study, we describe the identification of a unique src-signal transduction inhibitor, UCS15A. UCS15A was found to inhibit the src specific tyrosine phosphorylation of numerous proteins in v-src-transformed cells. Two of these phosphoproteins were identified as bona-fide src substrates, cortactin and Sam68. UCS15A differed from conventional src-inhibitors in that it did not inhibit the tyrosine kinase activity of src. In addition, UCS15A appeared to differ from src-destabilizing agents such as herbimycin and radicicol that destabilize src by interfering with Hsp90. Our studies suggest that UCS15A exerted its src-inhibitory effects by a novel mechanism that involved disruption of protein-protein interactions mediated by src. One of the biological consequences of src-inhibition by UCS15A was its ability to inhibit the bone resorption activity of osteoclasts in vitro. These data suggest that UCS15A may inhibit the bone resorption activity of osteoclasts, not by inhibiting src tyrosine kinase activity, but by disrupting the interaction of proteins associated with src, thereby modulating downstream events in the src signal transduction pathway.

Molecular cloning of a dendritic cell-associated transmembrane protein, DC-HIL, that promotes RGD-dependent adhesion of endothelial cells through recognition of heparan sulfate proteoglycans

We isolated a novel molecule (DC-HIL) expressed abundantly by the XS52 dendritic cell (DC) line and epidermal Langerhans cells, but minimally by other cell lines. DC-HIL is a type I transmembrane protein that contains a heparin-binding motif and an integrin-recognition motif, RGD, in its extracellular domain (ECD). A soluble fusion protein (DC-HIL-Fc) of the ECD and an immunoglobulin Fc bound to the surface of an endothelial cell line (SVEC). This binding induced adhesion of SVEC to its immobilized form. Sulfated polysaccharides (e.g. heparin and fucoidan) inhibited binding of soluble DC-HIL-Fc and adhesion of SVEC. By contrast, an integrin inhibitor (RGDS tetramer) had no effect on binding to SVEC, but prevented adhesion of SVEC. This differential RGD requirement was confirmed by the finding that DC-HIL-Fc mutant lacking the RGD motif can bind to SVEC but is unable to induce adhesion of SVEC. Furthermore, DC-HIL appears to recognize directly these sulfated polysaccharides. These results suggest that DC-HIL binds to SVEC by recognizing heparan sulfate proteoglycans on endothelial cells, thereby inducing adhesion of SVEC in an RGD-dependent manner. We propose that DC-HIL serves as a DC-associated, heparan sulfate proteoglycan-dependent integrin ligand, which may be involved in transendothelial migration of DC.

Identification of a PEST-like motif in listeriolysin O required for phagosomal escape and for virulence in Listeria monocytogenes

The hly-encoded listeriolysin O (LLO) is a major virulence factor secreted by the intracellular pathogen Listeria monocytogenes, which plays a crucial role in the escape of bacteria from the phagosomal compartment. Here, we identify a putative PEST sequence close to the N-terminus of LLO and focus on the role of this motif in the biological activities of LLO. Two LLO variants were constructed: a deletion mutant protein, lacking the 19 residues comprising this sequence (residues 32-50), and a recombinant protein of wild-type size, in which all the P, E, S or T residues within this motif have been substituted. The two mutant proteins were fully haemolytic and were secreted in culture supernatants of L. monocytogenes in quantities comparable with that of the wild-type protein. Strikingly, both mutants failed to restore virulence to a hly-negative strain in vivo. In vitro assays showed that L. monocytogenes expressing the LLO deletion mutant was strongly impaired in its ability to escape from the phagosomal vacuole and, subsequently, to divide in the cytosol of infected cells. This work reveals for the first time that the N-terminal portion of LLO plays an important role in the development of the infectious process of L. monocytogenes.

Themes and variations on ubiquitylation

Ubiquitylation--the conjugation of proteins with a small protein called ubiquitin--touches upon all aspects of eukaryotic biology, and its defective regulation is manifest in diseases that range from developmental abnormalities and autoimmunity to neurodegenerative diseases and cancer. A few years ago, we could only have dreamt of the complex arsenal of enzymes dedicated to ubiquitylation. Why has nature come up with so many ways of doing what seems to be such a simple job?

Functions of WW domains in the nucleus

The WW domain is a protein module found in a wide range of signaling proteins. It is one of the smallest protein modules that folds as a monomer without disulfide bridges or cofactors. WW domains bind proteins containing short linear peptide motifs that are proline-rich or contain at least one proline. Although the WW domain was initially considered a 'cytoplasmic module', the proteins containing WW domains have also been localized in the cell nucleus. Moreover, these proteins have been documented to participate in co-activation of transcription and modulation of RNA polymerase II activity. The carboxy-terminal domain (CTD) of RNA polymerase II acts as an assembly platform for distinct WW domain-containing proteins that affect the function of the RNA polymerase II. The formation of complexes between CTD and WW domain-containing proteins is regulated by phosphorylation of the CTD. Since the CTD sequence is highly repetitive and a target of several post-translational modifications and conformational changes, it presents a unique structure capable of enormous molecular diversity. The WW domain has been implicated in several human diseases including Alzheimer's disease. The WW domain-containing iso-prolyl isomerase named Pin1, a protein known to be essential for cell cycle progression, was shown to be active in restoration of the microtubule-binding activity of Tau, a protein of neurofibrillar tangles found in the brains of Alzheimer's patients. It is the WW domain of Pin1 that interacts directly with Tau protein. In addition, the WW domain-containing adapter protein FE65 was shown to regulate processing of Alzheimer's amyloid precursor protein. It is expected that by understanding the details of the WW domain-mediated protein-protein interactions, we will be able to illuminate numerous signaling pathways which control certain aspects of transcription and cell cycle.

Metabolism 2000: the emperor needs new clothes

Metabolism is one of the corner stones of nutritional science. As biology enters the post-genomic era and with functional genomics beginning to takeoff, we anticipate that the study of metabolism will play an increasingly important role in helping to link advances made via the reductionist paradigm, that has been so successful in molecular and cellular biology, with those emerging from observational studies in animals and human subjects. A reconstructive metabolically-focused approach offers a timely paradigm for enhancing the elegance of nutritional science. Here we give particular attention to the use of tracers as phenotyping tools and discuss the application of our metaprobe concepts with respect to some novel features of metabolism, including 'underground metabolism', 'metabolic hijacking', 'catalytic promiscuity' and 'moonlighting proteins'. The opportunities for enhancing the study of metabolism by new and emerging technologies, and the importance of the interdisciplinary research enterprise are also touched upon. We conclude that: (1) the metaprobe concepts and approach, discussed herein, potentially yield a quantitative physiological (metabolic) phenotype against which to elaborate partial or focused genotypes; (2) physiological (metabolic) phenotypes which have a whole-body or kinetically-discernible inter-organ tissue-directed metabolic signature are an ideal target for this directed tracer-based definition of the 'functional' genotype; (3) metabolism, probed with tracer tool kits suitable for measuring rates of turnover, change and conversion, becomes in the current sociology of the 'Net', like AOL, Yahoo. Alta Vista, Lycos or Ask Jeeves, the portal for an exploration of the metabolic characteristics of the 'Genomics Internet'.

Domains of the Rsp5 ubiquitin-protein ligase required for receptor-mediated and fluid-phase endocytosis

Yeast Rsp5p and its mammalian homologue, Nedd4, are hect domain ubiquitin-protein ligases (E3s) required for the ubiquitin-dependent endocytosis of plasma membrane proteins. Because ubiquitination is sufficient to induce internalization, E3-mediated ubiquitination is a key regulatory event in plasma membrane protein endocytosis. Rsp5p is an essential, multidomain protein containing an amino-terminal C2 domain, three WW protein-protein interaction domains, and a carboxy-terminal hect domain that carries E3 activity. In this study, we demonstrate that Rsp5p is peripherally associated with membranes and provide evidence that Rsp5p functions as part of a multimeric protein complex. We define the function of Rsp5p and its domains in the ubiquitin-dependent internalization of the yeast alpha-factor receptor, Ste2p. Temperature-sensitive rsp5 mutants were unable to ubiquitinate or to internalize Ste2p at the nonpermissive temperature. Deletion of the entire C2 domain had no effect on alpha-factor internalization; however, point mutations in any of the three WW domains impaired both receptor ubiquitination and internalization. These observations indicate that the WW domains play a role in the important regulatory event of selecting phosphorylated proteins as endocytic cargo. In addition, mutations in the C2 and WW1 domains had more severe defects on transport of fluid-phase markers to the vacuole than on receptor internalization, suggesting that Rsp5p functions at multiple steps in the endocytic pathway.

Actopaxin, a new focal adhesion protein that binds paxillin LD motifs and actin and regulates cell adhesion

Paxillin is a focal adhesion adapter protein involved in the integration of growth factor- and adhesion-mediated signal transduction pathways. Paxillin LD motifs have been demonstrated to bind to several proteins associated with remodeling of the actin cytoskeleton including the focal adhesion kinase, vinculin, and a complex of proteins comprising p95PKL, PIX, and PAK (Turner, C.E., M. C. Brown, J.A. Perrotta, M.C. Riedy, S.N. Nikolopoulos, A.R. McDonald, S. Bagrodia, S. Thomas, and P.S. Leventhal. 1999. J. Cell Biol. 145:851-863). In this study, we report the cloning and initial characterization of a new paxillin LD motif-binding protein, actopaxin. Analysis of the deduced amino acid sequence of actopaxin reveals a 42-kD protein with two calponin homology domains and a paxillin-binding subdomain (PBS). Western blotting identifies actopaxin as a widely expressed protein. Actopaxin binds directly to both F-actin and paxillin LD1 and LD4 motifs. It exhibits robust focal adhesion localization in several cultured cell types but is not found along the length of the associated actin-rich stress fibers. Similar to paxillin, it is absent from actin-rich cell-cell adherens junctions. Also, actopaxin colocalizes with paxillin to rudimentary focal complexes at the leading edge of migrating cells. An actopaxin PBS mutant incapable of binding paxillin in vitro cannot target to focal adhesions when expressed in fibroblasts. In addition, ectopic expression of the PBS mutant and/or the COOH terminus of actopaxin in HeLa cells resulted in substantial reduction in adhesion to collagen. Together, these results suggest an important role for actopaxin in integrin-dependent remodeling of the actin cytoskeleton during cell motility and cell adhesion.

The splicing factor, Prp40, binds the phosphorylated carboxyl-terminal domain of RNA polymerase II

We showed previously that the WW domain of the prolyl isomerase, Ess1, can bind the phosphorylated carboxyl-terminal domain (phospho-CTD) of the largest subunit of RNA Polymerase II. Analysis of phospho-CTD binding by four other WW domain-containing Saccharomyces cerevisiae proteins indicates the splicing factor, Prp40, and the RNA polymerase II ubiquitin ligase, Rsp5, can also bind the phospho-CTD. The identification of Prp40 as a phospho-CTD binding protein represents the first demonstration of direct interaction between a documented splicing factor and the phospho-CTD. Domain dissection studies reveal that phospho-CTD binding occurs at multiple locations in Prp40, including sites in both the WW and FF domain regions. Because the conserved repeats of the CTD make it an ideal ligand for multi-site binding events, the implications of multi-site binding are discussed. Our data suggest a mechanism by which the phospho-CTD of elongating RNA polymerase II facilitates commitment complex formation by juxtaposing the 5' and 3' splice sites.

LAT, the linker for activation of T cells: a bridge between T cell-specific and general signaling pathways

A key event in the regulation of the adaptive immune response is the binding of major histocompatibility complex-bound foreign peptides to T cell antigen receptors (TCRs) that are present on the cell surface of T lymphocytes. Recognition of the presence of cognate antigen in the host animal induces a series of biochemical changes within the T cell; these changes, in the context of additional signals from other surface receptors, ultimately result in massive proliferation of receptor-engaged T cells and the acquisition of effector and memory functions. Early studies established the importance of the activation of the enzymes phospholipase C-gamma1 (PLC-gamma1) and phosphatidylinositol 3-kinase (PI3K), as well as the small molecular weight heterotrimeric guanine nucleotide binding protein (G protein) Ras, in this process. These biochemical events are dependent on the activity of several protein tyrosine kinases that become activated immediately upon TCR engagement. An unresolved question in the field has been which molecules and what sequence of events tie together the early tyrosine phosphorylation events with the activation of these downstream signaling molecules. A likely candidate for linking the proximal and distal portions of the TCR signaling pathway is the recently described protein, LAT. LAT is a 36-kD transmembrane protein that becomes rapidly tyrosine-phosphorylated after TCR engagement. Phosphorylation of LAT creates binding sites for the Src homology 2 (SH2) domains of other proteins, including PLC-gamma1, Grb2, Gads, Grap, 3BP2, and Shb, and indirectly binds SOS, c-Cbl, Vav, SLP-76, and Itk. LAT is localized to the glycolipid-enriched membrane (GEM) subdomains of the plasma membrane by virtue of palmitoylation of two cysteine residues positioned near the endofacial side of the plasma membrane. Notably, in the absence of LAT, TCR engagement does not lead to activation of distal signaling events. This review examines the circumstances surrounding the discovery of LAT and our current understanding of its properties, and discusses current models for how LAT may be functioning to support the transduction of TCR-initiated, T cell-specific signaling events to the distal, general signaling machinery.

A PPxY motif within the VP40 protein of Ebola virus interacts physically and functionally with a ubiquitin ligase: implications for filovirus budding

VP40, the putative matrix protein of both Ebola and Marburg viruses, possesses a conserved proline-rich motif (PY motif) at its N terminus. We demonstrate that the VP40 protein can mediate its own release from mammalian cells, and that the PY motif is important for this self-exocytosis (budding) function. In addition, we used Western-ligand blotting to demonstrate that the PY motif of VP40 can mediate interactions with specific cellular proteins that have type I WW-domains, including the mammalian ubiquitin ligase, Nedd4. Single point mutations that disrupted the PY motif of VP40 abolished the PY/WW-domain interactions. Significantly, the full-length VP40 protein was shown to interact both physically and functionally with full-length Rsp5, a ubiquitin ligase of yeast and homolog of Nedd4. The VP40 protein was multiubiquitinated by Rsp5 in a PY-dependent manner in an in vitro ubiquitination assay. These data demonstrate that the VP40 protein of Ebola virus possesses a PY motif that is functionally similar to those described previously for Gag and M proteins of specific retroviruses and rhabdoviruses, respectively. Last, these studies imply that VP40 likely plays an important role in filovirus budding, and that budding of retroviruses, rhabdoviruses, and filoviruses may proceed via analogous mechanisms.

Caveolin-3 directly interacts with the C-terminal tail of beta -dystroglycan. Identification of a central WW-like domain within caveolin family members

Caveolin-3, the most recently recognized member of the caveolin gene family, is muscle-specific and is found in both cardiac and skeletal muscle, as well as smooth muscle cells. Several independent lines of evidence indicate that caveolin-3 is localized to the sarcolemma, where it associates with the dystrophin-glycoprotein complex. However, it remains unknown which component of the dystrophin complex interacts with caveolin-3. Here, we demonstrate that caveolin-3 directly interacts with beta-dystroglycan, an integral membrane component of the dystrophin complex. Our results indicate that caveolin-3 co-localizes, co-fractionates, and co-immunoprecipitates with a fusion protein containing the cytoplasmic tail of beta-dystroglycan. In addition, we show that a novel WW-like domain within caveolin-3 directly recognizes the extreme C terminus of beta-dystroglycan that contains a PPXY motif. As the WW domain of dystrophin recognizes the same site within beta-dystroglycan, we also demonstrate that caveolin-3 can effectively block the interaction of dystrophin with beta-dystroglycan. In this regard, interaction of caveolin-3 with beta-dystroglycan may competitively regulate the recruitment of dystrophin to the sarcolemma. We discuss the possible implications of our findings in the context of Duchenne muscular dystrophy.

Identification of an alternatively spliced form of the Tat interactive protein (Tip60), Tip60(beta)

Tip60 was originally isolated as a Tat interactive protein. It was subsequently shown that Tip60 had histone acetyltransferase (HAT) activity. In studies to understand gene-expression regulation that might involve HAT activity, we PCR-amplified Tip60 from a human heart marathon-ready cDNA library. As a result, we identified an alternatively spliced form of Tip60, Tip60beta (we refer to the previously cloned Tip60 as Tip60alpha). Tip60beta cDNA is slightly smaller than Tip60alpha, and sequencing indicates that there is a deletion of 156 bp in the coding region of the gene. The predicted Tip60beta protein therefore lacks 52 amino acids when compared with Tip60alpha. The Tip60alpha gene is encoded by 14 exons, and Tip60beta is an alternatively spliced form resulting from the exclusion of exon 5 during the splicing process. Exon 5 encodes a proline-rich region that is known to be important for protein-protein interaction. Tip60beta is expressed in a variety of human tissues and cell lines, and the protein is present in both the nucleus and cytoplasm in contrast to Tip60alpha, which is entirely nuclear. The results suggest that Tip60beta may have functions additional to those of Tip60alpha in cells and tissues.

cAMP-dependent protein kinase phosphorylation of EVL, a Mena/VASP relative, regulates its interaction with actin and SH3 domains

Proteins of the Ena/VASP family are implicated in processes that require dynamic actin remodeling such as axon guidance and platelet activation. In this work, we explored some of the pathways that likely regulate actin dynamics in part via EVL (Ena/VASP-like protein). Two isoforms, EVL and EVL-I, were highly expressed in hematopoietic cells of thymus and spleen. In CD3-activated T-cells, EVL was found in F-actin-rich patches and at the distal tips of the microspikes that formed on the activated side of the T-cells. Like the other family members, EVL localized to focal adhesions and the leading edge of lamellipodia when expressed in fibroblasts. EVL was a substrate for the cAMP-dependent protein kinase, and this phosphorylation regulated several of the interactions between EVL and its ligands. Unlike VASP, EVL nucleated actin polymerization under physiological conditions, whereas phosphorylation of both EVL and VASP decreased their nucleating activity. EVL bound directly to the Abl, Lyn, and nSrc SH3 domains; the FE65 WW domain; and profilin, likely via its proline-rich core. Binding of Abl and nSrc SH3 domains, but not profilin or other SH3 domains, was abolished by cAMP-dependent protein kinase phosphorylation of EVL. We show strong cooperative binding of two profilin dimers on the polyproline sequence of EVL. Additionally, profilin competed with the SH3 domains for binding to partially overlapping binding sites. These data suggest that the function of EVL could be modulated in a complex manner by its interactions with multiple ligands and through phosphorylation by cyclic nucleotide dependent kinases.

Lymphocytes with a complex: adapter proteins in antigen receptor signaling

Adapters can be defined as proteins that mediate intermolecular interactions within a signal transduction pathway and that lack both intrinsic enzymatic and transcriptional activity. Their essential role in lymphocyte signaling was revealed by recent analyses of mice and cell lines deficient in LAT, SLP-76 and BLNK. These and other adapters nucleate signaling complexes and facilitate coupling of antigen receptor triggering to functional responses in lymphocytes.

The mouse fidgetin gene defines a new role for AAA family proteins in mammalian development

The mouse mutation fidget arose spontaneously in a heterogeneous albino stock. This mutant mouse is characterized by a side-to-side head-shaking and circling behaviour, due to reduced or absent semicircular canals. Fidget mice also have small eyes, associated with cell-cycle delay and insufficient growth of the retinal neural epithelium, and lower penetrance skeletal abnormalities, including pelvic girdle dysgenesis, skull bone fusions and polydactyly. By positional cloning, we found the gene mutated in fidget mice, fidgetin (Fign), which encodes a new member of the 'meiotic' or subfamily-7 (SF7; ref. 7) group of ATPases associated with diverse cellular activities (AAA proteins). We also discovered two closely related mammalian genes. AAA proteins are molecular chaperones that facilitate a variety of functions, including membrane fusion, proteolysis, peroxisome biogenesis, endosome sorting and meiotic spindle formation, but functions for the SF7 AAA proteins are largely unknown. Fidgetin is the first mutant AAA protein found in a mammalian developmental mutant, thus defining a new role for these proteins in embryonic development.

Convergent evolution with combinatorial peptides

Once the sequence of a genome is in hand, understanding the function of its encoded proteins becomes a task of paramount importance. Much like the biochemists who first outlined different biochemical pathways, many genomic scientists are engaged in determining which proteins interact with which proteins, thereby establishing a protein interaction network. While these interactions have evolved in regard to their specificity, affinity and cellular function over billions of years, it is possible in the laboratory to isolate peptides from combinatorial libraries that bind to the same proteins with similar specificity, affinity and primary structures, which resemble those of the natural interacting proteins. We have termed this phenomenon 'convergent evolution'. In this review, we highlight various examples of convergent evolution that have been uncovered in experiments dissecting protein-protein interactions with combinatorial peptides. Thus, a fruitful approach for mapping protein-protein interactions is to isolate peptide ligands to a target protein and identify candidate interacting proteins in a sequenced genome by computer analysis.

Domain repertoires as a tool to derive protein recognition rules

Several approaches, some of which are described in this issue, have been proposed to assemble a complete protein interaction map. These are often based on high throughput methods that explore the ability of each gene product to bind any other element of the proteome of the organism. Here we propose that a large number of interactions can be inferred by revealing the rules underlying recognition specificity of a small number (a few hundreds) of families of protein recognition modules. This can be achieved through the construction and characterization of domain repertoires. A domain repertoire is assembled in a combinatorial fashion by allowing each amino acid position in the binding site of a given protein recognition domain to vary to include all the residues allowed at that position in the domain family. The repertoire is then searched by phage display techniques with any target of interest and from the primary structure of the binding site of the selected domains one derives rules that are used to infer the formation of complexes between natural proteins in the cell.

A HECT domain ubiquitin ligase closely related to the mammalian protein WWP1 is essential for Caenorhabditis elegans embryogenesis

The highly conserved ubiquitin/proteasome pathway controls the degradation of many critical regulatory proteins. Proteins are posttranslationally conjugated to ubiquitin through a concerted set of reactions involving activating (E1), conjugating (E2), and ligase (E3) enzymes. Ubiquitination targets proteins for proteolysis via the proteasome and may regulate protein function independent of proteolysis. We describe the cloning and functional analysis of new members of the HECT domain family of E3 ubiquitin ligases. Murine Wwp1 encoded a broadly expressed protein containing a C2 domain, four WW domains, and a catalytic HECT domain. A Caenorhabditis elegans gene was cloned encoding a HECT domain protein (CeWWP1), which was highly homologous to murine and human WWP1. Disruption of CeWwp1 via RNA interference yielded an embryonic lethal phenotype, despite the presence of at least six additional C. elegans genes encoding HECT domain proteins. The embryonic lethality was characterized by grossly abnormal morphogenesis during late embryogenesis, despite normal proliferation early in embryogenesis. CeWWP1 must therefore have unique and nonredundant functions critical for embryogenesis.

Rsp5 WW domains interact directly with the carboxyl-terminal domain of RNA polymerase II

RSP5 is an essential gene in Saccharomyces cerevisiae and was recently shown to form a physical and functional complex with RNA polymerase II (RNA pol II). The amino-terminal half of Rsp5 consists of four domains: a C2 domain, which binds membrane phospholipids; and three WW domains, which are protein interaction modules that bind proline-rich ligands. The carboxyl-terminal half of Rsp5 contains a HECT (homologous to E6-AP carboxyl terminus) domain that catalytically ligates ubiquitin to proteins and functionally classifies Rsp5 as an E3 ubiquitin-protein ligase. The C2 and WW domains are presumed to act as membrane localization and substrate recognition modules, respectively. We report that the second (and possibly third) Rsp5 WW domain mediates binding to the carboxyl-terminal domain (CTD) of the RNA pol II large subunit. The CTD comprises a heptamer (YSPTSPS) repeated 26 times and a PXY core that is critical for interaction with a specific group of WW domains. An analysis of synthetic peptides revealed a minimal CTD sequence that is sufficient to bind to the second Rsp5 WW domain (Rsp5 WW2) in vitro and in yeast two-hybrid assays. Furthermore, we found that specific "imperfect" CTD repeats can form a complex with Rsp5 WW2. In addition, we have shown that phosphorylation of this minimal CTD sequence on serine, threonine and tyrosine residues acts as a negative regulator of the Rsp5 WW2-CTD interaction. In view of the recent data pertaining to phosphorylation-driven interactions between the RNA pol II CTD and the WW domain of Ess1/Pin1, we suggest that CTD dephosphorylation may be a prerequisite for targeted RNA pol II degradation.

WW- and SH3-domain interactions with Epstein-Barr virus LMP2A

Epstein-Barr virus (EBV) is a human herpesvirus which establishes a lifelong latent infection in B lymphocytes. Latent membrane protein 2A (LMP2A) is expressed in both humans with EBV latent infection and EBV immortalized cell lines grown in culture. Previous studies have shown that the amino terminal domain of LMP2A, which contains eight tyrosines, associates with a variety of cellular proteins via SH2-phosphotyrosine interactions. Also contained within the LMP2A amino terminal domain are five proline-rich regions, three of which possess the PxxP core consensus sequence required for interacting with SH3 domains and two of which possess the PPxY core consensus sequence (PY motif) required for interacting with class I type WW domains. In the current study, the ability of LMP2A to interact with either modular SH3 or WW domains was investigated. The results of these studies indicate that the two LMP2A PY motifs interact strongly with representative class I WW domains, but not with representative class II WW domains. In contrast, no interactions were detected between LMP2A and any of the five different SH3 domains tested. These data demonstrate that a subset of the conserved proline-rich motifs within the amino terminus of LMP2A can potentially mediate interactions with cellular proteins and may play a role in EBV-mediated latency and/or transformation.

A novel pro-Arg motif recognized by WW domains

WW domains mediate protein-protein interactions through binding to short proline-rich sequences. Two distinct sequence motifs, PPXY and PPLP, are recognized by different classes of WW domains, and another class binds to phospho-Ser-Pro sequences. We now describe a novel Pro-Arg sequence motif recognized by a different class of WW domains using data from oriented peptide library screening, expression cloning, and in vitro binding experiments. The prototype member of this group is the WW domain of formin-binding protein 30 (FBP30), a p53-regulated molecule whose WW domains bind to Pro-Arg-rich cellular proteins. This new Pro-Arg sequence motif re-classifies the organization of WW domains based on ligand specificity, and the Pro-Arg class now includes the WW domains of FBP21 and FE65. A structural model is presented which rationalizes the distinct motifs selected by the WW domains of YAP, Pin1, and FBP30. The Pro-Arg motif identified for WW domains often overlaps with SH3 domain motifs within protein sequences, suggesting that the same extended proline-rich sequence could form discrete SH3 or WW domain complexes to transduce distinct cellular signals.