A role for phosphatidylinositol transfer protein in secretory vesicle formation

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Abstract
VESICULAR traffic in eukaryotic cells is characterized by two steps of membrane rearrangement: the formation of vesicles from donor membranes and their fusion with acceptor membranes. With respect to vesicle formation, several of the cytosolic proteins implicated in budding and fission 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 system1,2 derived from a neuroendocrine cell line, the purification of cytosolic factors that stimulate the formation of constitutive secretory vesicles and immature secretory granules from the trans-Golgi network. One such factor, referred to as CAST1, was identified as the α and β isoforms of the mammalian phosphatidylinositol transfer protein (Ptdlns-TP) (refs 3á¤-5). The yeast Ptdlns-TP, SEC14p (ref. 6), which has no sequence homo-logy to mammalian Ptdlns-TP (refs 7,8), was able to substitute for the mammalian Ptdlns-TP in secretory vesicle formation. Our results suggest a highly conserved role for phosphoinositides in vesicle formation.