PTEN is a tumor suppressor that primarily dephosphorylates phosphatidylinositol 3,4,5-trisphosphate to down-regulate the phosphoinositide 3-kinase/Akt signaling pathway. Although the cellular functions of PTEN as a tumor suppressor have been well characterized, the mechanism by which PTEN activity is modulated by other signal molecules in vivo remains poorly understood. In searching for potential PTEN modulators through protein-protein interaction, we identified the major vault protein (MVP) as a dominant PTENbinding protein in a yeast two-hybrid screen. MVP is the major structural component of vault, the largest intracellular ribonucleoprotein particle. PTEN was originally identified as a tumor suppressor gene based on its high frequency of mutation in a variety of tumors (1-3). Germ-line mutations of PTEN are the cause of Cowden disease, an autosomal-dominant hamartoma syndrome that results in an increased risk for development of tumors in a variety of tissues (4 -7). The genetic evidence that PTEN is an important tumor suppressor is supported by the fact that heterozygous disruption of the PTEN gene in knockout mice results in the spontaneous development of tumors (8 -10). Although PTEN as a protein phosphatase is capable of dephosphorylating tyrosine and threonine/serine residues (11, 12), the primary substrates of PTEN are 3Ј-phosphoinositides, PtdIns-3,4-P 2 and PtdIns-3,4,5-P 3 (13). Genetic and biochemical studies have demonstrated that the tumor suppressor functions of PTEN are linked primarily with the lipid phosphatase activity and its association with the well defined phosphoinositide 3-kinase pathway (reviewed in Refs. 14 -20). Substantial progress has been made in the characterization of PTEN as a tumor suppressor as well as in the regulation of many cellular processes including growth, adhesion, migration, invasion, and apoptosis. Nevertheless, the mechanism by which PTEN activity is modulated in various cellular signaling complexes remains elusive. It is assumed that the activity and the cellular function of PTEN may be regulated through in vivo proteinprotein interactions. PTEN contains a number of putative regulatory modules, including the N-terminal phosphoinositide binding motif, a C2 domain, a PDZ-binding site, and two proline-, glutamic acid-, serine-, and threonine-rich segments (21). The C2 domain of PTEN has been implicated in mediating membrane association (22). The C-terminal tail of PTEN interacts with several PDZ domain-containing proteins such as hDLG, hMAST205, MAGI-2, and MAGI-3 (23-25). The interaction of PTEN with these proteins may be important for its biological function, as it has been reported that MAGI-2 and MAGI-3 can enhance the activity of PTEN (23, 24). In contrast, several groups found that the PDZ-binding site of PTEN is not required for tumor suppression or other biological activities (21,(25)(26)(27)(28). Therefore, the complete spectrum of PTEN-interacting proteins and the effects of the interactions on PTEN function are not yet defined.To date, the vault complex w...

The sialic acid-binding immunoglobulin-like lectins (siglecs) represent a recently defined distinct subset of the immunoglobulin superfamily. By using the Src homology 2 (SH2)-domain-containing protein tyrosine phosphatase SHP-1 as bait in a yeast two-hybrid screen, we have identified a new member of the mouse siglec family, mSiglec-E. The mSiglec-E cDNA encodes a protein of 467 amino acids that contains three extracellular immunoglobulin-like domains, a transmembrane region and a cytoplasmic tail bearing two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). mSiglec-E is highly expressed in mouse spleen, a tissue rich in leucocytes. The ITIMs of mSiglec-E can recruit SHP-1 and SHP-2, two inhibitory regulators of immunoreceptor signal transduction. This suggests that the function of mSiglec-E is probably an involvement in haematopoietic cells and the immune system as an inhibitory receptor. When expressed in COS-7 cells, mSiglec-E was able to mediate sialic acid-dependent binding to human red blood cells, suggesting that mSiglec-E may function through cell-cell interactions. In comparison with the known members of the siglec family, mSiglec-E exhibits a high degree of sequence similarity to both human siglec-7 and siglec-9. The gene encoding mSiglec-E is localized in the same chromosome as that encoding mouse CD33. Phylogenetic analysis reveals that neither mouse mSiglec-E nor CD33 shows a clear relationship with any human siglecs so far identified.

We describe the molecular cloning and characterization of S2V, a novel sialic acid binding immunoglobulin-like lectin. The cDNA of S2V encodes a type 1 transmembrane protein with four extracellular immunoglobulin-like (Ig-like) domains and a cytoplasmic tail bearing a typical immunoreceptor tyrosine-based inhibitory motif (ITIM) and an ITIM-like motif. A unique feature of S2V is the presence of two V-set Ig-like domains responsible for the binding to sialic acid, whereas all other known siglecs possess only one. S2V is predominantly expressed in macrophage. In vivo S2V was tyrosine-phosphorylated when co-expressed with exogenous c-Src kinase. Upon tyrosine phosphorylation, S2V recruits both Src homology 2 (SH2) domain-containing protein-tyrosine phosphatases SHP-1 and SHP-2, two important inhibitory regulators of immunoreceptor signal transduction. These findings suggest that S2V is involved in the negative regulation of the signaling in macrophage by functioning as an inhibitory receptor. When expressed in COS-7 cells, S2V was able to mediate sialic acid-dependent binding to human red blood cells, suggesting that S2V may function through cell-cell interaction. SHP-11 is a nonreceptor protein-tyrosine phosphatase that contains tandem Src homology 2 (SH2) domains at its N terminus, enabling its association with tyrosine-phosphorylated proteins. It is expressed primarily in hematopoietic cells and epithelial cells and acts predominantly as a negative regulator of a broad spectrum of receptors, including receptor proteintyrosine kinases such as c-Kit (1), colony-stimulating factor (2), epidermal growth factor (3, 4), and platelet-derived growth factor (5, 6) receptors, cytokine receptors such as erythropoietin (7), interleukin-3 (8) and interferon (9) receptors, and immunoreceptors such as the B cell (10), T cell (11), and natural killer (NK) cell (12, 13) receptors. The importance of the inhibitory signals delivered by SHP-1 is highlighted by the motheaten (me) mouse, which has natural defect in SHP-1 expression (14). Mice with this deficiency display widespread autoimmune phenomena caused by an inability to negatively regulate immune responses (14). The inhibitory effect of SHP-1 appears to reflect its capacity to associate with and dephosphorylate the activated receptors and/or the signaling molecules in the receptor complexes such as receptor protein-tyrosine kinase in c-Kit signaling (1), Jak kinase in cytokine receptor signaling (7), and Lck, ZAP70, and Syk in antigen receptor signaling (10, 11).In addition to direct association with the activated receptor complexes and down-regulation of the signaling of the "growthpromoting" receptors, SHP-1 performs its inhibitory function in many hematopoietic cells by associating with inhibitory receptors that bear the immunoreceptor tyrosine-based inhibitory motifs (ITIMs) (reviewed in Refs. 10, 12, and 13). Among the ITIM-bearing proteins are sialic acid binding immunoglobulinlike lectins (siglecs), a recently designated superfamily of cell surface molecules. The human...

The sialic acid-binding immunoglobulin-like lectins (siglecs) represent a recently defined distinct subset of the immunoglobulin superfamily. By using the Src homology 2 (SH2)-domain-containing protein tyrosine phosphatase SHP-1 as bait in a yeast two-hybrid screen, we have identified a new member of the mouse siglec family, mSiglec-E. The mSiglec-E cDNA encodes a protein of 467 amino acids that contains three extracellular immunoglobulin-like domains, a transmembrane region and a cytoplasmic tail bearing two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). mSiglec-E is highly expressed in mouse spleen, a tissue rich in leucocytes. The ITIMs of mSiglec-E can recruit SHP-1 and SHP-2, two inhibitory regulators of immunoreceptor signal transduction. This suggests that the function of mSiglec-E is probably an involvement in haematopoietic cells and the immune system as an inhibitory receptor. When expressed in COS-7 cells, mSiglec-E was able to mediate sialic acid-dependent binding to human red blood cells, suggesting that mSiglec-E may function through cell–cell interactions. In comparison with the known members of the siglec family, mSiglec-E exhibits a high degree of sequence similarity to both human siglec-7 and siglec-9. The gene encoding mSiglec-E is localized in the same chromosome as that encoding mouse CD33. Phylogenetic analysis reveals that neither mouse mSiglec-E nor CD33 shows a clear relationship with any human siglecs so far identified.

Src homology 2 (SH2) domain‐containing protein tyrosine phosphatase‐1 (SHP‐1) is a critical inhibitory regulator in T cell‐receptor (TCR) signaling. However, the exact molecular mechanism underlying this is poorly defined, largely because the physiological substrates for SHP‐1 in T cells remain elusive. In this study, we showed that adaptor protein 3BP2 serves as a binding protein and a physiological substrate of SHP‐1. 3BP2 is phosphorylated on tyrosyl residue 448 in response to TCR activation, and the phosphorylation is required for T cell signalling, as indicated by transcriptional activation of nuclear factor activated in T cells (NFAT). Concurrently, phosphorylation of Tyr566 at the C‐terminus of SHP‐1 causes specific recruitment of 3BP2 to the phosphatase through the SH2 domain of the adaptor protein. This leads to efficient dephosphorylation of 3BP2 and thereby termination of T cell signaling. The study thus defines a novel function of the C‐terminal segment of SHP‐1 and reveals a new mechanism by which T cell signaling is regulated.