The tumor suppressor PTEN (phosphatase and tensin homologue) negatively regulates the PI3K pathway through its lipid phosphatase activity and is one of the most commonly lost tumor suppressors in human cancers. Though the tumor suppressive function involves the lipid phosphatase-dependent and -independent activities of PTEN, the mechanism leading to the phosphatase-independent function of PTEN is understood poorly. Some PTEN mutants have lipid phosphatase activity but fail to suppress cell growth. Here, we use a cancer-associated mutant, G20E, to gain insight into the phosphatase-independent function of PTEN by investigating protein-protein interactions using MS-based stable isotope labeling by amino acids in cell culture (SILAC). A strategy named parallel affinity purification (PAP) and SILAC has been developed to prioritize interactors and to compare the interactions between wild-type and G20E PTEN. Clustering of the prioritized interactors acquired by the PAP-SILAC approach shows three distinct clusters: 1) wild-type-specific interactors, 2) interactors unique to the G20E mutant, and 3) proteins common to wild-type and mutant. These interactors are involved mainly in cell migration and apoptosis pathways. We further demonstrate that the wild-typespecific interactor, NUDTL16L1, is required for the regulatory function of wild-type PTEN in cell migration. These findings contribute to a better understanding of the mechanisms of the phosphatase-dependent and -independent functions of PTEN.PTEN is a tumor suppressor gene that frequently is somatically deleted or mutated in a variety of human cancers, including those of the brain, endometrium, prostate, and lung (1, 2). Germ line mutations of PTEN are the cause of Cowden disease and Bannayan-Riley-Ruvalcaba syndrome, an autosomal dominant hamartoma syndrome with increased risk for the development of tumors in a variety of tissues (1, 2). The PTEN protein consists of N-terminal phosphatase, and C-terminal C2, phosphorylation, and PDZ (PSD-95, DLG1, and ZO-1) binding domains (2). The catalytic domain of PTEN functions to dephosphorylate the 3Ј position of the phospholipids PI(3,4,5)P3 (PIP 3 ) and PI(3,4)P2 (3). As such, PTEN lipid phosphatase activity regulates the Akt serine/threonine kinase pathway through modulation of PIP 3 levels and hence, regulates cell cycle progression, apoptosis, and migration, cell proliferation and motility, which also are critical for tumor development (4). Thus, deregulation of the PI3K/PTEN/Akt pathway has been found in many malignant cancers.More than 20% of all known PTEN mutations in tumors are located outside of the catalytic site, and these mutants exhibit normal phosphatase activity (1), suggesting two possibilities. First, mutations in the interacting domain for the regulatory proteins of PTEN may affect its ability for tumor suppression through PTEN phosphatase activity, which is regulated by PTEN interactors (5-10). Indeed, a PTEN mutation within a binding site of PICT1, which controls stability of PTEN, affects the protein le...