A ROLE FOR PHOSPHATIDYLINOSITOL TRANSFER PROTEIN IN SECRETORY VESICLEFORMATION

VESICULAR traffic in eukaryotic cells is characterized by two steps of membrane rearrangement: the formation of vesicles from donor membrane s and their fusion with acceptor membranes. With respect to vesicle fo rmation, several of the cytosolic proteins implicated in budding and f ission have been identified. A feature common to all these proteins is that their targets, when known, are other proteins rather than lipids . Here we report, using a previously established cell-free system(1,2) derived from a neuroendocrine cell line, the purification of cytosoli c factors that stimulate the formation of constitutive secretory vesic les and immature secretory granules from the trans-Golgi network. One such factor, referred to as CAST1, was identified as the alpha and bet a isoforms of the mammalian phosphatidylinositol transfer protein (Ptd Ins-TP) (refs 3-5). The yeast PtdIns-TP, SEC14p (ref. 6), which has no sequence homology to mammalian PtdIns-TP (refs 7,8), was able to subs titute for the mammalian PtdIns-TP in secretory vesicle formation. Our results suggest a highly conserved role for phosphoinositides in vesi cle formation.