THE FIRST 5 AMINO-ACIDS OF THE CARBOXYL-TERMINUS OF PHOSPHATIDYLINOSITOL TRANSFER PROTEIN (PITP)-ALPHA PLAY A CRITICAL ROLE IN INOSITOL LIPID SIGNALING - TRANSFER ACTIVITY OF PITP IS ESSENTIAL BUT NOT SUFFICIENT FOR RESTORATION OF PHOSPHOLIPASE-C SIGNALING

Phosphatidylinositol transfer protein (PITP) is essential for phosphol ipase C signaling and for constitutive and regulated vesicular traffic . PITP has a single lipid-binding site that can reversibly bind phosph atidylinositol (PI) and phosphatidylcholine (PC) and transfer these li pids between membrane compartments in vitro. The role of the carboxyl terminus was examined by comparing wild-type PITP alpha with PITP alph a in which 5, 10, and 20 amino acids were deleted from the C terminus. Delta 5- and Delta 10-PITP had reduced PI and PC transfer activities compared with wild-type PITP, with the effect on PI transfer being mor e marked than that on PC transfer. Delta 20-PITP was inactive at all c oncentrations tested. All three truncated mutants were unable to resto re G-protein-mediated phospholipase C beta stimulation in HL-60 cells. Delta 5- and Delta 10-PITP, but not Delta 20-PITP, inhibited the sign aling function of wild-type protein without any effect on lipid transf er in vitro. We conclude that (a) the carboxyl terminus of PITP plays a critical role in phospholipase C signaling; (b) the transfer activit y is not the only determining factor that dictates the restorative fun ction of PITP in inositol lipid signaling; and (c) the dominant inhibi tory effects of Delta 5- and Delta 10-PITP on wild-type PITP in phosph olipase C signaling suggest the existence of a receptor for PITP.