Identification of a novel polyproline recognition site in the cytoskeletal associated protein, proline serine threonine phosphatase interacting protein

Deciphering the BAR code of membrane modulators

The BAR domain is the eponymous domain of the “BAR-domain protein superfamily”, a large and diverse set of mostly multi-domain proteins that play eminent roles at the membrane cytoskeleton interface. BAR domain homodimers are the functional units that peripherally associate with lipid membranes and are involved in membrane sculpting activities. Differences in their intrinsic curvatures and lipid-binding properties account for a large variety in membrane modulating properties. Membrane activities of BAR domains are further modified and regulated by intramolecular or inter-subunit domains, by intermolecular protein interactions, and by posttranslational modifications. Rather than providing detailed cell biological information on single members of this superfamily, this review focuses on biochemical, biophysical, and structural aspects and on recent findings that paradigmatically promote our understanding of processes driven and modulated by BAR domains.

Linking up at the BAR: Oligomerization and F-BAR protein function

As cells grow, move, and divide, they must reorganize and rearrange their membranes and cytoskeleton. The F-BAR protein family links cellular membranes with actin cytoskeletal rearrangements in processes including endocytosis, cytokinesis, and cell motility. Here we review emerging information on mechanisms of F-BAR domain oligomerization and membrane binding, and how these activities are coordinated with additional domains to accomplish scaffolding and signaling functions.

Proline-serine-threonine phosphatase interacting protein 1 inhibition of T-cell receptor signaling depends on its SH3 domain

Proline-serine-threonine phosphatase interacting protein 1 (PSTPIP1) is an adaptor protein associated with the cytoskeleton that is mainly expressed in hematopoietic cells. Mutations in PSTPIP1 cause the rare autoinflammatory disease called pyogenic arthritis, pyoderma gangrenosum, and acne. We carried out this study to further our knowledge on PSTPIP1 function in T cells, particularly in relation to the phosphatase lymphoid phosphatase (LYP), which is involved in several autoimmune diseases. LYP-PSTPIP1 binding occurs through the C-terminal homology domain of LYP and the F-BAR domain of PSTPIP1. PSTPIP1 inhibits T-cell activation upon T-cell receptor (TCR) and CD28 engagement, regardless of CD2 costimulation. This function of PSTPIP1 depends on the presence of an intact SH3 domain rather than on the F-BAR domain, indicating that ligands of the F-BAR domain, such as the PEST phosphatases LYP and PTP-PEST, are not critical for its negative regulatory role in TCR signaling. Additionally, PSTPIP1 mutations that cause the pyogenic arthritis, pyoderma gangrenosum and acne syndrome do not affect PSTPIP1 function in T-cell activation through the TCR.

Role of protein-tyrosine phosphatases in regulation of osteoclastic activity

Osteoclasts, the primary cell type mediating bone resorption, are multinucleated, giant cells derived from hematopoietic cells of monocyte-macrophage lineage. Osteoclast activity is, in a large part, regulated by protein-tyrosine phosphorylation. While information about functional roles of several protein-tyrosine kinases (PTK), including c-Src, in osteoclastic resorption has been accumulated, little is known about the roles of protein-tyrosine phosphatases (PTPs) in regulation of osteoclast activity. Recent evidence implicates important regulatory roles for four PTPs (SHP-1, cyt-PTP-epsilon, PTP-PEST, and PTPoc) in osteoclasts. Cyt-PTP-epsilon, PTP-PEST, and PTP-oc are positive regulators of osteoclast activity, while SHP-1 is a negative regulator. Of these PTPs in osteoclasts, only PTP-oc is a positive regulator of c-Src PTK through dephosphorylation of the inhibitory phosphotyrosine-527 residue. Although some information about mechanisms of action of these PTPs to regulate osteoclast activity is reviewed in this article, much additional work is required to provide more comprehensive details about their functions in osteoclasts.

Phosphorylation of a Wiscott-Aldrich Syndrome Protein-associated Signal Complex Is Critical in Osteoclast Bone Resorption

The activities of different kinases have been correlated to the phosphorylation of Wiscott-Aldrich syndrome protein (WASP) by studies in multiple cell systems. The purpose of this study was to elucidate the regulatory mechanisms involved in WASP phosphorylation and the resulting sealing ring formation in osteoclasts. The phosphorylation state of WASP and WASP-interacting proteins was determined in osteoclasts treated with osteopontin or expressing either constitutively active or kinase-defective Src by adenovirus-mediated delivery. In vitro kinase analysis of WASP immunoprecipitates exhibited phosphorylation of c-Src, PYK2, WASP, protein-tyrosine phosphatase (PTP)-PEST, and Pro-Ser-Thr phosphatase-interacting protein (PSTPIP). Phosphorylation of these proteins was increased in osteopontin-treated and constitutively active Src-expressing osteoclasts. Pulldown analysis with glutathione S-transferase-fused proline-rich regions of PTP-PEST revealed coprecipitation of WASP, PYK2, c-Src, and PSTPIP proteins with the N-terminal region (amino acids 294-497) of PTP-PEST. Similarly, interaction of the same signaling proteins, as well as PTP-PEST, was observed with glutathione S-transferase-fused proline-rich regions of WASP. Furthermore, osteopontin stimulation or constitutively active Src expression resulted in serine phosphorylation and inhibition of WASP-associated PTP-PEST. The inhibition of PTP-PEST was accompanied by an increase in tyrosine phosphorylation of WASP and other associated signaling proteins. Experiments with an inhibitor to phosphatase and small interference RNA to PTP-PEST confirmed the involvement of PTP-PEST in sealing ring formation and bone resorption. WASP, which is identified in the sealing ring of resorbing osteoclasts, also demonstrates colocalization with c-Src, PYK2, PSTPIP, and PTP-PEST in immunostaining analyses. Our findings suggest that both tyrosine kinase(s) and the tyrosine phosphatase PTP-PEST coordinate the formation of the sealing ring and thus the bone-resorbing function of osteoclasts.

CD2BP1 modulates CD2-dependent T cell activation via linkage to protein tyrosine phosphatase (PTP)-PEST

Human CD2 regulates T cell activation and adhesion via mechanisms yet to be fully understood. This study focuses on CD2BP1, a CD2 cytoplasmic tail-binding protein preferentially expressed in hematopoetic cells. Structural and functional analyses suggest that CD2BP1 acts as a scaffold protein, participating in regulation of the actin cytoskeleton. In this study, using a murine Ag-specific primary T cell transduction system to assess CD69, IL-2, and IFN-gamma expression, we provide evidence that CD2BP1 directly and negatively impacts T cell activation via isolated CD2 triggering or TCR stimulation dependent on coordinate CD2 engagement. Disruption of protein tyrosine phosphatase-PEST and/or CD2BP1 association with the CD2 signalsome rescues T cells from the inhibitory effect of CD2 crosslinking. The overexpression of CD2BP1 selectively attenuates phospholipase Cgamma1, ERK1/2, and p38 phosphorylation without abrogating CD2-independent TCR stimulation. This study provides new insight on the regulation of T cell activation and may have implications for autoimmune processes known to be associated with CD2BP1 mutations.

Mutation of mouse Mayp/Pstpip2 causes a macrophage autoinflammatory disease

Macrophage actin-associated tyrosine phosphorylated protein (MAYP)/PSTPIP2, a PCH protein, is involved in the regulation of macrophage motility. Mutations in a closely related gene, PSTPIP1/CD2BP1, cause a dominantly inherited autoinflammatory disorder known as PAPA syndrome. A mutant mouse obtained by chemical mutagenesis exhibited an autoinflammatory disorder characterized by macrophage infiltration and inflammation, leading to osteolysis and necrosis in paws and necrosis of ears. Positional cloning of this recessive mutation, termed Lupo, identified a T to A nucleotide exchange leading to an amino acid substitution (I282N) in the sequence of MAYP. Mayp(Lp/Lp) disease was transferable by bone marrow transplantation and developed in the absence of lymphocytes. Consistent with the involvement of macrophages, lesion development could be prevented by the administration of clodronate liposomes. MAYP is expressed in monocytes/macrophages and in a Mac1+ subfraction of granulocytes. LPS stimulation increases its expression in macrophages. Because of the instability of the mutant protein, MAYP expression is reduced 3-fold in Mayp(Lp/Lp) macrophages and, on LPS stimulation, does not rise above the level of unstimulated wild-type (WT) cells. Mayp(Lp/Lp) mice expressed elevated circulating levels of several cytokines, including MCP-1; their macrophages exhibited altered cytokine production in vitro. These studies suggest that MAYP plays an anti-inflammatory role in macrophages.

Binding of the Intracellular Fas Ligand (FasL) Domain to the Adaptor Protein PSTPIP Results in a Cytoplasmic Localization of FasL

The tumor necrosis factor family member Fas ligand (FasL) induces apoptosis in Fas receptor-expressing target cells and is an important cytotoxic effector molecule used by CTL- and NK-cells. In these hematopoietic cells, newly synthesized FasL is stored in specialized secretory lysosomes and only delivered to the cell surface upon activation and target cell recognition. FasL contains an 80-amino acid-long cytoplasmic tail, which includes a proline-rich domain as a bona fide Src homology 3 domain-binding site. This proline-rich domain has been implicated in FasL sorting to secretory lysosomes, and it may also be important for reverse signaling via FasL, which has been described to influence T-cell activation. Here we report the identification of the Src homology 3 domain-containing adaptor protein PSTPIP as a FasL-interacting partner, which binds to the proline-rich domain. PSTPIP co-expression leads to an increased intracellular localization of Fas ligand, thereby regulating extracellular availability and cytotoxic activity of the molecule. In addition, we demonstrate recruitment of the tyrosine phosphatase PTP-PEST by PSTPIP into FasL.PSTPIP.PTP-PEST complexes which may contribute to FasL reverse signaling.

A common pathway in periodic fever syndromes

Familial Mediterranean fever (FMF) is an autosomal recessive disease due to mutations in pyrin, which normally inhibits pro-interleukin-1beta (IL-1beta) cytokine processing to the active form. A novel role for pyrin has been proposed by Shoham et al., who studied patients with an autosomal dominant disease called pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome. They demonstrated an interaction between pyrin and proline serine threonine phosphatase-interacting protein 1 (PSTPIP1), the protein involved in PAPA, and thus revealed a biochemical pathway common to both FMF and PAPA.

Pyrin binds the PSTPIP1/CD2BP1 protein, defining familial Mediterranean fever and PAPA syndrome as disorders in the same pathway

Pyrin, the familial Mediterranean fever protein, is found in association with the cytoskeleton in myeloid/monocytic cells and modulates IL-1beta processing, NF-kappaB activation, and apoptosis. These effects are mediated in part through cognate interactions with the adaptor protein ASC, which shares an N-terminal motif with pyrin. We sought additional upstream regulators of inflammation by using pyrin as the bait in yeast two-hybrid assays. We now show that proline serine threonine phosphatase-interacting protein [PSTPIP1, or CD2-binding protein 1 (CD2BP1)], a tyrosine-phosphorylated protein involved in cytoskeletal organization, also interacts with pyrin. Recently, PSTPIP1/CD2BP1 mutations were shown to cause the syndrome of pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA), a dominantly inherited autoinflammatory disorder mediated predominantly by granulocytes. Endogenous PSTPIP1/CD2BP1 and pyrin are coexpressed in monocytes and granulocytes and can be coimmunoprecipitated from THP-1 cells. The B box segment of pyrin was necessary and the B box/coiled-coil segment sufficient for this interaction, whereas the SH3 and coiled-coil domains of PSTPIP1/CD2BP1 were both necessary, but neither was sufficient, for pyrin binding. The Y344F PSTPIP1/CD2BP1 mutation, which blocks tyrosine phosphorylation, was associated with a marked reduction in pyrin binding in pervanadate-treated cells. PAPA-associated A230T and E250Q PSTPIP1/CD2BP1 mutations markedly increased pyrin binding as assayed by immunoprecipitation and, relative to WT, these mutants were hyperphosphorylated when coexpressed with c-Abl kinase. Consistent with the hypothesis that these mutations exert a dominant-negative effect on the previously reported activity of pyrin, we found increased IL-1beta production by peripheral blood leukocytes from a clinically active PAPA patient with the A230T PSTPIP1/CD2BP1 mutation and in cell lines transfected with both PAPA-associated mutants.

PSTPIP is a substrate of PTP-PEST and serves as a scaffold guiding PTP-PEST toward a specific dephosphorylation of WASP

PSTPIP is a tyrosine-phosphorylated protein involved in the organization of the cytoskeleton. Its ectopic expression induces filipodial-like membrane extensions in NIH 3T3 cells. We previously observed a defect in cytokinesis and an increase in the tyrosine phosphorylation of PSTPIP in PTP-PEST-deficient fibroblasts. In this article, we demonstrate that PTP-PEST and PSTPIP are found in the same complexes in vivo and that they interact directly through the CTH domain of PTP-PEST and the coiled-coil domain of PSTPIP. We tested pathways that could regulate the tyrosine phosphorylation of PSTPIP. We found that the activation of the epidermal growth factor and platelet-derived growth factor receptors can induce PSTPIP phosphorylation. With the use of the PP2 inhibitor, we demonstrate that Src kinases are not involved in the epidermal growth factor-mediated phosphorylation of PSTPIP. Together with previous results, this suggests that c-Abl is the critical tyrosine kinase downstream of growth factor receptors responsible for PSTPIP phosphorylation. We also demonstrate that PTP-PEST dephosphorylates PSTPIP at tyrosine 344. Importantly, we identified tyrosine 344 as the main phosphorylation site of PSTPIP by performing tryptic phosphopeptide maps. This is an important finding since tyrosine 367 of PSTPIP was also proposed as a candidate phosphorylation site involved in the negative regulation of the association between PSTPIP and WASP. In this respect, we observed that the PSTPIP.WASP complex is stable in vivo and is not affected by the phosphorylation of PSTPIP. Furthermore, we demonstrate that PSTPIP serves as a scaffold protein between PTP-PEST and WASP and allows PTP-PEST to dephosphorylate WASP. This finding suggests a possible mechanism for PTP-PEST to directly modulate actin remodeling through the PSTPIP-WASP interaction.

Regulation of the association between PSTPIP and CD2 in murine T cells

Prominent in T cells and natural killer cells, CD2 binding protein 1 (CD2BP1) plays an important role in CD2-mediated adhesion and signal transduction. In the current study, we investigated CD2 and PSTPIP (proline, serine, threonine phosphatase interacting protein, murine homologue of CD2BP1) interactions in purified mouse splenic T cells. PSTPIP associated with CD2 in both resting and activated T cells. Following various stimuli, such as concanavalin A, anti-TCRbeta, anti-CD3epsilon, anti-CD3epsilon/phorbol myristate acetate (PMA), IL-2, or PMA/ionomycin, PSTPIP and CD2 expression, as well as their association, increased in a time-dependent fashion. While PSTPIP expression and CD2 expression were comparable across most groups, the PSTPIP-CD2 association stimulated by anti-CD3epsilon alone was significantly greater than with other stimuli. Stimulation by anti-CD3epsilon plus anti-CD28 induced even greater PSTPIP-CD2 association than anti-CD3epsilon treatment alone, indicating that CD28 initiated signals are involved in regulating this interaction. There was no direct association between CD3epsilon or CD28 and PSTPIP. Tyrosine phosphorylated PSTPIP bound poorly to CD2 compared to dephosphorylated PSTPIP, and protein tyrosine phosphatase was shown to affect both phosphorylation of PSTPIP and the CD2-PSTPIP association. In addition to CD2, PSTPIP associated with CD4, CD8, CD54, and CD62L. CD2 and CD4 ligation reciprocally regulated their association with PSTPIP. These findings indicate that T cell activation, particularly through the CD3 and CD28 signal transduction pathways, regulates PSTPIP-CD2 interactions. PSTPIP likely has additional broader effects through interactions with CD4, CD8, CD54, and CD62L, and this may influence T cell responses to antigen.

All three PACSIN isoforms bind to endocytic proteins and inhibit endocytosis

The PACSINs are a family of cytoplasmic phosphoproteins that play a role in vesicle formation and transport. We report the cloning and cDNA sequencing of PACSIN 3 and the analysis of all three PACSIN isoforms with regard to tissue distribution, ligand binding properties and influence on endocytosis. PACSIN 3 differs from the other family members in having a short proline-rich region and lacking asparagine-proline-phenylalanine motifs. In contrast to the neurospecific PACSIN 1 and the ubiquitously expressed PACSIN 2, PACSIN 3 is mainly detected in lung and muscle tissues. All isoforms potentially oligomerize and bind to dynamin, synaptojanin 1 and N-WASP via their Src homology 3 domains. The PACSIN proteins colocalize with dynamin, but not with clathrin, implying a specific role with a distinct subpopulation of dynamin at defined cellular sites. Transferrin endocytosis is blocked in a dose-dependent manner in cells overexpressing the PACSIN variants, but the inhibitory effect can be abolished by mutating specific amino acid residues in the Src homology 3 domains. These characteristics of the PACSIN protein family suggest a general function in recruitment of the interacting proteins to sites of endocytosis.

Cytoskeletal protein PSTPIP1 directs the PEST-type protein tyrosine phosphatase to the c-Abl kinase to mediate Abl dephosphorylation

A search for c-Abl interacting proteins resulted in the recovery of PSTPIP1, originally identified as a binding protein of the PEST-type protein tyrosine phosphatases (PTP). PSTPIP1 was phosphorylated by c-Abl, and growth factor-induced PSTPIP1 phosphorylation was diminished in Abl null fibroblasts. PSTPIP1 was able to bridge c-Abl to the PEST-type PTPs. Several experiments suggest that the PEST-type PTPs negatively regulate c-Abl activity: c-Abl was hyperphosphorylated in PTP-PEST-deficient cells; disruption of the c-Abl-PSTPIP1-PEST-type PTP ternary complex by overexpression of PSTPIP1 mutants increased c-Abl phosphotyrosine content; and PDGF-induced c-Abl kinase activation was prolonged in PTP-PEST-deficient cells. Dephosphorylation of c-Abl by PEST-type PTP represents a novel mechanism by which c-Abl activity is regulated.

Involvement of PCH family proteins in cytokinesis and actin distribution

Pombe Cdc15 homology (PCH) proteins constitute an extensive protein family whose members have been found in diverse eukaryotic organisms. These proteins are characterized by the presence of several conserved sequence and structural motifs. Recent studies in yeast and mammalian cultured cells have implicated these proteins in actin-based processes, in particular, cytokinesis. Here we review the recent findings on the in vivo localization, function, and binding partners of PCH family members. We also provide new microscopy data regarding the in vivo dynamics of a budding yeast PCH protein involved in cytokinesis.

Cdc42-interacting protein 4 mediates binding of the Wiskott-Aldrich syndrome protein to microtubules

The Wiskott-Aldrich syndrome is an inherited X-linked immunodeficiency characterized by thrombocytopenia, eczema, and a tendency toward lymphoid malignancy. Lymphocytes from affected individuals have cytoskeletal abnormalities, and monocytes show impaired motility. The Wiskott-Aldrich syndrome protein (WASP) is a multi-domain protein involved in cytoskeletal organization. In a two-hybrid screen, we identified the protein Cdc42-interacting protein 4 (CIP4) as a WASP interactor. CIP4, like WASP, is a Cdc42 effector protein involved in cytoskeletal organization. We found that the WASP-CIP4 interaction is mediated by the binding of the Src homology 3 domain of CIP4 to the proline-rich segment of WASP. Cdc42 was not required for this interaction. Co-expression of CIP4 and green fluorescent protein-WASP in COS-7 cells led to the association of WASP with microtubules. In vitro experiments showed that CIP4 binds to microtubules via its NH(2) terminus. The region of CIP4 responsible for binding to active Cdc42 was localized to amino acids 383-417, and the mutation I398S abrogated binding. Deletion of the Cdc42-binding domain of CIP4 did not affect the colocalization of WASP with microtubules in vivo. We conclude that CIP4 can mediate the association of WASP with microtubules. This may facilitate transport of WASP to sites of substrate adhesion in hematopoietic cells.

JAK1-dependent phosphorylation of insulin receptor substrate-1 (IRS-1) is inhibited by IRS-1 serine phosphorylation

Serine phosphorylation of insulin receptor substrate-1 (IRS-1) reduces its ability to act as an insulin receptor substrate and inhibits insulin receptor signal transduction. Here, we report that serine phosphorylation of IRS-1 induced by either okadaic acid (OA) or chronic insulin stimulation prevents interferon-alpha (IFN-alpha)-dependent IRS-1 tyrosine phosphorylation and IFN-alpha-dependent IRS-1/phosphatidylinositol 3'-kinase (PI3K) association. In addition, we demonstrate that serine phosphorylation of IRS-1 renders it a poorer substrate for JAK1 (Janus kinase-1). We found that treatment of U266 cells with OA induced serine phosphorylation of IRS-1 and completely blocked IFN-alpha-dependent tyrosine phosphorylation of IRS-1 and IFN-alpha-dependent IRS-1/PI3K association. Additionally, IRS-1 from OA-treated cells could not be phosphorylated in vitro by IFN-alpha-activated JAK1. Chronic treatment of U266 cells with insulin led to a 50% reduction in IFN-alpha-dependent tyrosine phosphorylation of IRS-1 and IRS-1/PI3K association. More importantly, serine-phosphorylated IRS-1-(511-722) could not be phosphorylated in vitro by IFN-alpha-activated JAK1. Taken together, these data indicate that serine phosphorylation of IRS-1 prevents its subsequent tyrosine phosphorylation by JAK1 and suggest that IRS-1 serine phosphorylation may play a counter-regulatory role in pathways outside the insulin signaling system.

Protein tyrosine phosphatase-PEST regulates focal adhesion disassembly, migration, and cytokinesis in fibroblasts

In this article, we show that, in transfected COS-1 cells, protein tyrosine phosphatase (PTP)-PEST translocates to the membrane periphery following stimulation by the extracellular matrix protein fibronectin. When plated on fibronectin, PTP-PEST (-/-) fibroblasts display a strong defect in motility. 3 h after plating on fibronectin, the number and size of vinculin containing focal adhesions were greatly increased in the homozygous PTP-PEST mutant cells as compared with heterozygous cells. This phenomenon appears to be due in part to a constitutive increase in tyrosine phosphorylation of p130(CAS), a known PTP-PEST substrate, paxillin, which associates with PTP-PEST in vitro, and focal adhesion kinase (FAK). Another effect of this constitutive hyperphosphorylation, consistent with the focal adhesion regulation defect, is that (-/-) cells spread faster than the control cell line when plated on fibronectin. In the PTP-PEST (-/-) cells, an increase in affinity for the SH2 domains of Src and Crk towards p130(CAS) was also observed. In (-/-) cells, we found a significant increase in the level of tyrosine phosphorylation of PSTPIP, a cleavage furrow-associated protein that interacts physically with all PEST family members. An effect of PSTPIP hyperphosphorylation appears to be that some cells remain attached at the site of the cleavage furrow for an extended period of time. In conclusion, our data suggest PTP-PEST plays a dual role in cell cytoskeleton organization, by promoting the turnover of focal adhesions required for cell migration, and by directly or indirectly regulating the proline, serine, threonine phosphatase interacting protein (PSTPIP) tyrosine phosphorylation level which may be involved in regulating cleavage furrow formation or disassembly during normal cell division.

A cdc15-like adaptor protein (CD2BP1) interacts with the CD2 cytoplasmic domain and regulates CD2-triggered adhesion

A human CD2 cytoplasmic tail-binding protein, termed CD2BP1, was identified by an interaction trap cloning method. Expression of CD2BP1 is restricted to hematopoietic tissue, being prominent in T and natural killer (NK) cells, with long (CD2BP1L) and short (CD2BP1S) variants arising by alternative RNA splicing. Both CD2BP1 molecules are homologous to Schizosaccharomyces pombe cdc15, and include a helical domain, variable length intervening PEST sequence and C-terminal SH3 domain. Although the CD2BP1 SH3 domain binds directly to the CD2 sequence, KGPPLPRPRV (amino acids 300-309), its association is augmented markedly by the CD2BP1 N-terminal segment. Upon ligand-induced clustering of surface CD2 molecules, CD2BP1 redistributes from a cytosolic to a surface membrane compartment, co-localizing with CD2. In turn, CD2-stimulated adhesion is downregulated by CD2BP1, apparently through coupling of the protein tyrosine phosphatase (PTP)-PEST to CD2. These findings offer the first molecular view into the control processes for T cell adhesion.

PSTPIP 2, a second tyrosine phosphorylated, cytoskeletal-associated protein that binds a PEST-type protein-tyrosine phosphatase

Although cytoskeletal regulation is critical to cell function during interphase and mitosis, the components of the cytoskeleton involved with its control are only beginning to be elucidated. Recently, we reported the identification of a cytoskeletal-associated protein, proline-serine-threonine phosphatase-interacting protein (PSTPIP), whose level of tyrosine phosphorylation was controlled by PEST-type protein-tyrosine phosphatases (PTPs) bound to a novel protein interaction site in the PSTPIP predicted coiled-coil domain. We also showed that the PSTPIP SH3 domain interacts with the Wiskott-Aldrich syndrome protein (WASP), a cytoskeletal regulatory protein, in a manner modulated by tyrosine phosphorylation. Here we describe the identification of PSTPIP 2, a widely expressed protein that is related to PSTPIP. PSTPIP 2 lacks an SH3 domain but contains a region predicted to bind to PEST-type PTPs, and structure-function analyses demonstrate that PSTPIP 2 interacts with the proline-rich C terminus of the PEST-type PTP hematopoietic stem cell factor in a manner similar to that previously demonstrated for PSTPIP. Confocal microscopy revealed that PSTPIP 2 colocalizes with PSTPIP in F actin-rich regions. PSTPIP 2 was found to be efficiently phosphorylated in v-Src-transfected or pervanadate-treated cells at two tyrosines conserved in PSTPIP, but in contrast to PSTPIP, tyrosine phosphorylated PSTPIP 2 was only weakly dephosphorylated in the presence of PTP HSCF. Finally, analysis of oligomer formation demonstrated that PSTPIP and PSTPIP 2 formed homo- but not heterodimers. These data suggest that a family of tyrosine phosphorylated, PEST PTP binding proteins may be implicated in cytoskeletal regulation.

Fc receptor-mediated phagocytosis requires CDC42 and Rac1

At the surface of phagocytes, antibody-opsonized particles are recognized by surface receptors for the Fc portion of immunoglobulins (FcRs) that mediate their capture by an actin-driven process called phagocytosis which is poorly defined. We have analyzed the function of the Rho proteins Rac1 and CDC42 in the high affinity receptor for IgE (FcepsilonRI)-mediated phagocytosis using transfected rat basophil leukemia (RBL-2H3) mast cells expressing dominant inhibitory forms of CDC42 and Rac1. Binding of opsonized particles to untransfected RBL-2H3 cells led to the accumulation of F-actin at the site of contact with the particles and further, to particle internalization. This process was inhibited by Clostridium difficile toxin B, a general inhibitor of Rho GTP-binding proteins. Dominant inhibition of Rac1 or CDC42 function severely inhibited particle internalization but not F-actin accumulation. Inhibition of CDC42 function resulted in the appearance of pedestal-like structures with particles at their tips, while particles bound at the surface of the Rac1 mutant cell line were enclosed within thin membrane protrusions that did not fuse. These phenotypic differences indicate that Rac1 and CDC42 have distinct functions and may act cooperatively in the assembly of the phagocytic cup. Inhibition of phagocytosis in the mutant cell lines was accompanied by the persistence of tyrosine-phosphorylated proteins around bound particles. Phagocytic cup closure and particle internalization were also blocked when phosphotyrosine dephosphorylation was inhibited by treatment of RBL-2H3 cells with phenylarsine oxide, an inhibitor of protein phosphotyrosine phosphatases. Altogether, our data show that Rac1 and CDC42 are required to coordinate actin filament organization and membrane extension to form phagocytic cups and to allow particle internalization during FcR-mediated phagocytosis. Our data also suggest that Rac1 and CDC42 are involved in phosphotyrosine dephosphorylation required for particle internalization.

imp2, a new component of the actin ring in the fission yeast Schizosaccharomyces pombe

Cytokinesis is the part of the cell cycle in which the cell is cleaved to form two daughter cells. The unicellular yeast, Schizosaccharomyces pombe is an excellent model organism in which to study cell division, since it shows the general features of eukaryotic cell division and is amenable to genetic analysis. In this manuscript we describe the isolation and characterization of a new protein, imp2, which is required for normal septation in fission yeast. imp2, which colocalizes with the medial ring during septation, is structurally similar to a group of proteins including the S. pombe cdc15 and the mouse PSTPIP that are localized to, and thought to be involved in actin ring organization. Cells in which the imp2 gene is deleted or overexpressed have septation and cell separation defects. An analysis of the actin cytoskeleton shows the lack of a medial ring in septating cells that overexpress imp2, and the appearance of abnormal medial ring structures in septated cells that lack imp2. These observations suggest that imp2 destabilizes the medial ring during septation. imp2 also shows genetic interactions with several, previously characterized septation genes, strengthening the conclusion that it plays a role in normal fission yeast septation.

Tyrosine phosphorylation regulates the SH3-mediated binding of the Wiskott-Aldrich syndrome protein to PSTPIP, a cytoskeletal-associated protein

Wiskott-Aldrich syndrome is an X-linked hematopoietic disease that manifests itself in platelet deficiency and a compromised immune system. Analysis of hematopoietic cells from affected individuals reveals that mutations in the Wiskott-Aldrich syndrome protein (WASP) result in structural and functional abnormalities in the cell cortex, consistent with the suggestion that WASP is involved with regulation of the actin-rich cortical cytoskeleton. Here we report that WASP interacts with a recently described cytoskeletal-associated protein, PSTPIP, a molecule that is related to the Schizosaccharomyces pombe cleavage furrow regulatory protein, CDC15p. This association is mediated by an interaction between the PSTPIP SH3 domain and two polyproline-rich regions in WASP. Co-expression of PSTPIP with WASP in vivo results in a loss of WASP-induced actin bundling activity and co-localization of the two proteins, which requires the PSTPIP SH3 domain. Analysis of tyrosine phosphorylation of PSTPIP reveals that two sites are modified in response to v-Src co-transfection or pervanadate incubation. One of these tyrosines is found in the SH3 domain poly-proline recognition site, and mutation of this tyrosine to aspartate or glutamate to mimic this phosphorylation state results in a loss of WASP binding in vitro and a dissolution of co-localization in vivo. In addition, PSTPIP that is tyrosine phosphorylated in the SH3 domain interacts poorly with WASP in vitro. These data suggest that the PSTPIP and WASP interaction is regulated by tyrosine phosphorylation of the PSTPIP SH3 domain, and this binding event may control aspects of the actin cytoskeleton.