Interactions between proteins and phosphoinositides can affect subcellular localization, protein-protein interactions, enzymatic activity, and downstream signaling events in a spatially and temporally controlled manner. Intracellular phosphoinositide levels are tightly controlled by specific phosphoinositide kinases and phosphatases and the importance of this regulation is reflected by their association with many human diseases. Several forms of cancer, including breast, colon, ovarian, pancreatic, lymphoid, prostate, multiple myeloma, and head and neck cancers are associated with dysregulation of phosphoinositide-mediated signaling pathways via mutations in phosphoinositide kinases and phosphatases such as phosphatidylinositol 3-kinase (PI3K) and the phosphatase and tensin homolog (PTEN). Pleckstrin homology (PH) domains play a key role in mediating phosphoinositide binding of proteins with diverse cellular functions, including cell proliferation, apoptosis, cytoskeletal remodeling, cell motility, vesicular transport, and metabolism. Human DIP13α/APPL and DIP13β are members of a novel family of proteins that contain a central PH domain and a C-terminal phosphotyrosine binding (PTB) domain. DIP13α was identified by our lab as a protein that interacts with the intracellular domain of the deleted in colorectal cancer (DCC) protein via its PTB domain and is required for DCC-mediated apoptosis. DIP13α has also been identified by other labs as a protein that interacts with AKT protein kinases and with PI3K regulatory and catalytic subunits. Our in vitro phosphoinositide binding studies demonstrate that isolated PH domains from DIP13α and DIP13β bind immobilized PtdIns(4)P, PtdIns(5)P, PtdIns(3,4)P2, and PtdIns(3,5)P2. To our knowledge, this is a novel PH domain-mediated phosphoinositide binding profile. We have expressed enhanced cyan fluorescence protein (ECFP) fusions of isolated PH domains and full-length DIP13α and DIP13β proteins in human cell lines and observe patterns of subcellular localization in living cells consistent with membrane and phosphoinositide association in vivo. Therefore, DIP13α and DIP13β may act as adapter proteins by linking proteins with phosphoinositides and play a role in phosphoinositide-mediated signaling pathways.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]