The PTEN gene is a tumor suppressor localized in the frequently altered chromosomal region 10q23. The tumor suppressor function of the PTEN protein (PTEN) has been linked to its ability to dephosphorylate the lipid second-messenger phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate and, by doing so, to antagonize the phosphoinositide 3-kinase pathway. The PTEN protein consists of an amino-terminal phosphatase domain, a lipid binding C2 domain, and a 50-amino-acid C-terminal domain (the "tail") of unknown function. A number of studies have shown that the tail is dispensable for both phosphatase activity and blocking cell growth. Here, we show that the PTEN tail is necessary for maintaining protein stability and that it also acts to inhibit PTEN function. Thus, removing the tail results in a loss of stability but does not result in a loss of function because the resultant protein is more active. Furthermore, tail-dependent regulation of stability and activity is linked to the phosphorylation of three residues (S380, T382, and T383) within the tail. Therefore, the tail is likely to mediate the regulation of PTEN function through phosphorylation.The PTEN gene was cloned as a candidate tumor suppressor gene from the chromosome 10q23 region, a locus frequently targeted for genetic loss in tumors (24,26,42). Somatic inactivation of both PTEN alleles and loss of heterozygosity have been demonstrated in a number of tumors including glioblastoma, melanoma, and prostate, breast, and endometrial carcinomas (reviewed in reference 46). Germ line PTEN mutations are associated with the development of the related dominantly inherited disorders known as Cowden disease and BannayanZonana syndrome (28)(29)(30)34). These disorders are characterized by the presence of benign hamartomas of the skin, intestinal tract, and central nervous system and by an increased incidence of cancers of the thyroid and breast (28,29,34). Similarly, heterozygous PTEN mice develop a variety of tumors and proliferative lesions of multiple tissues (10,11,37,43).Reconstitution of PTEN expression to certain PTEN null cells results in an increase in the population of cells in the G 1 phase of the cell cycle (13,23,39); in other PTEN null cells it results in the induction of apoptosis or anoikis (9,25,32). Accumulating evidence suggests that these functions are linked to the lipid phosphatase activity of PTEN, which allows PTEN to antagonize the phosphatidylinositol 3-kinase (PI3K) pathway (reviewed in references 4 and 46). A number of downstream targets of phosphatidylinositol 3,4,5-triphosphate and phosphatidylinositol 3,4-bisphosphate including the serinethreonine kinase Akt, BTK, SGK, and p70 S6K

PTEN͞MMAC1 is a tumor suppressor gene located on chromosome 10q23. Inherited PTEN͞MMAC1 mutations are associated with a cancer predisposition syndrome known as Cowden's disease. Somatic mutation of PTEN has been found in a number of malignancies, including glioblastoma, melanoma, and carcinoma of the prostate and endometrium. The protein product (PTEN) encodes a dualspecificity protein phosphatase and in addition can dephosphorylate certain lipid substrates. Herein, we show that PTEN protein induces a G 1 block when reconstituted in PTEN-null cells. A PTEN mutant associated with Cowden's disease (PTEN;G129E) has protein phosphatase activity yet is defective in dephosphorylating inositol 1,3,4,5-tetrakisphosphate in vitro and fails to arrest cells in G 1 . These data suggest a link between induction of a cell-cycle block by PTEN and its ability to dephosphorylate, in vivo, phosphatidylinositol 3,4,5-trisphosphate. In keeping with this notion, PTEN can inhibit the phosphatidylinositol 3,4,5-trisphosphate-dependent Akt kinase, a downstream target of phosphatidylinositol 3-kinase, and constitutively active, but not wild-type, Akt overrides a PTEN G 1 arrest. Finally, tumor cells lacking PTEN contain high levels of activated Akt, suggesting that PTEN is necessary for the appropriate regulation of the phosphatidylinositol 3-kinase͞Akt pathway.