Phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P 3 ) is a key molecule involved in cell growth signaling. We demonstrated that overexpression of PTEN, a putative tumor suppressor, reduced insulininduced PtdIns(3,4,5)P 3 production in human 293 cells without effecting insulin-induced phosphoinositide 3-kinase activation. Further, transfection of the catalytically inactive mutant of PTEN (C124S) caused PtdIns(3,4,5)P 3 accumulation in the absence of insulin stimulation. Purified recombinant PTEN catalyzed dephosphorylation of PtdIns(3,4,5)P 3 , specifically at position 3 on the inositol ring. PTEN also exhibited 3-phosphatase activity toward inositol 1,3,4,5-tetrakisphosphate. Our results raise the possibility that PTEN acts in vivo as a phosphoinositide 3-phosphatase by regulating PtdIns(3,4,5)P 3 levels. As expected, the C124S mutant of PTEN was incapable of catalyzing dephosphorylation of PtdIns(3,4,5)P 3 consistent with the mechanism observed in protein-tyrosine phosphatase-catalyzed reactions.A recently identified candidate tumor suppressor gene, PTEN/MMAC1, shares sequence identity with the family of protein-tyrosine phosphatases (PTPases) 1 (1). Deletions and mutations within the PTEN gene have been observed in several cancer cell types and tumor cell lines (2, 3). Additional evidence that PTEN functions as a tumor suppressor was obtained by Furnari et al. (4), who showed that PTEN had a growth suppressor activity in glioma cells. PTEN encodes the active site consensus motif HCXXGXXR(S/T) found in all PTPases. In contrast, the recombinant protein is a poor catalyst toward both phosphoproteins and peptide substrates with the highest activity of PTEN observed toward the highly negatively charged, multiply phosphorylated polymer of (Glu-Tyr) n (5, 6). Based on these observations we thought it possible that PTEN could catalyze the dephosphorylation of acidic nonproteinaceous substrate. Identification of possible in vivo substrates would not only suggest a possible physiological function of PTEN, but they might also provide insight into how PTEN functions as a tumor suppressor.PtdIns(3,4,5)P 3 is an important second messenger involved in cell growth signaling (7). PtdIns(3,4,5)P 3 is specifically produced from PtdIns(4,5)P 2 by PI 3-kinase upon stimulation by a variety of ligands (7). Recent studies have identified that PtdIns(3,4,5)P 3 can directly activate Akt, which in turn activates p70 S6 kinase and inhibits glycogen synthase kinase-3 (8, 9). Although there are several phosphoinositide 5-phosphatases, the mechanism of regulation and particularly the degradation pathway of PtdIns(3,4,5)P 3 in vivo is still unclear (10,11). In the present study we demonstrate that recombinant PTEN has PtdIns(3,4,5)P 3 3-phosphatase activity. In addition, we provide evidence that PTEN may act in vivo as a regulator of PtdIns(3,4,5)P 3 , which produces a substrate that can be recycled by PI 3-kinase. EXPERIMENTAL PROCEDURESDetermination of PtdIns(3,4,5)P 3 in Vivo-The coding sequence of human PTEN and the C124S mutant o...

Tyrosine phosphorylation is catalyzed by protein tyrosine kinases, which are represented by 90 genes in the human genome. Here, we present the set of 107 genes in the human genome that encode members of the four protein tyrosine phosphatase (PTP) families. The four families of PTPases, their substrates, structure, function, regulation, and the role of these enzymes in human disease will be discussed.

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A Drosophila homolog of human Down syndrome cell adhesion molecule (DSCAM), an immunoglobulin superfamily member, was isolated by its affinity to Dock, an SH3/SH2 adaptor protein required for axon guidance. Dscam binds directly to both Dock's SH2 and SH3 domains. Genetic studies revealed that Dscam, Dock and Pak, a serine/threonine kinase, act together to direct pathfinding of Bolwig's nerve, containing a subclass of sensory axons, to an intermediate target in the embryo. Dscam also is required for the formation of axon pathways in the embryonic central nervous system. cDNA and genomic analyses reveal the existence of multiple forms of Dscam with a conserved architecture containing variable Ig and transmembrane domains. Alternative splicing can potentially generate more than 38,000 Dscam isoforms. This molecular diversity may contribute to the specificity of neuronal connectivity.