AKT1 and PTEN show the highest affinities among phosphoinositide binding proteins for the second messengers PtdIns(3,4,5)P3 and PtdIns(3,4)P2

Abstract

The phosphoinositides phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3] and phosphatidylinositol-3,4-bisphosphate [PtdIns(3,4)P2] function as second messengers and have been implicated in cancerogenesis. The signalling events downstream of PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are mediated through a complex network of phosphoinositide binding effector proteins and phosphatases. In this study, we compared the phosphoinositide effector proteins AKT1, TAPP1, TAPP2, VAV1 and P-REX1 and the phosphoinositide phosphatases PTEN, SHIP1 and INPP4B for their binding affinities to PtdIns(3,4,5)P3 and/or PtdIns(3,4)P2 using Surface Plasmon Resonance. Our results demonstrate that all measured proteins except P-REX1 and VAV1 showed high affinity phosphoinositide binding with KD values in the nM to sub-nM range. Within the effector proteins, AKT1 showed the highest affinity for both PtdIns(3,4,5)P3 and PtdIns(3,4)P2. Of the phosphoinositide phosphatases PTEN displayed the highest affinity towards PtdIns(3,4,5)P3 and PtdIns(3,4)P2. The SHIP1 mutant E452K detected in carcinoma patients had a 100-fold increased affinity to PtdIns(3,4)P2 but not to PtdIns(3,4,5)P3 compared to SHIP1 WT. Distinct mutations in phosphoinositide binding proteins like the patient-derived SHIP1E452K mutant may be involved in the upregulation of PI(3,4)P2 –mediated signalling in tumor cells due to phosphoinositide trapping. Our results add further information to the complex hierarchy of phosphoinositide binding proteins helping to elucidate their functional role in cellular signal transduction.