Regulated fusion of secretory granules with the plasma membrane in secretory cells requires ATP, Ca2+ and cytosolic as well as membrane proteins. ATP-dependent steps in Ca(2+)-activated secretion from PC12 cells require three cytosolic PEP proteins (priming in exocytosis proteins, PEP1-3), the identity of which will provide insights into the required ATP-using reactions. PEP3 was recently identified as phosphatidylinositol transfer protein (PtdInsTP), and here we report that PEP1 consists of the type I phosphatidylinositol-4-phosphate 5-kinase (PtdInsP5K). The roles of PEP3/PtdInsTP and PEP1/PtdInsP5K in sequential phosphoinositide recruitment and phosphorylation explains their synergistic activity in ATP-dependent priming. Moreover, inhibition of Ca(2+)-activated secretion by PtdIns(4,5)P2-specific antibodies and phospholipase C implies that 5-phosphorylated inositides play a novel, necessary role in the regulated secretory pathway. The results indicate that lipid kinase-mediated phosphorylation is an important basis for ATP use in the exocytotic pathway.

The budding of endoplasmic reticulum (ER)-derived vesicles is dependent on the COPII coat complex. Coat assembly is initiated when Sar1-GTP recruits the cargo adaptor complex, Sec23/Sec24, by binding to its GTPase-activating protein (GAP) Sec23 (ref. 2). This leads to the capture of transmembrane cargo by Sec24 (refs 3, 4) before the coat is polymerized by the Sec13/Sec31 complex. The initial interaction of a vesicle with its target membrane is mediated by tethers. We report here that in yeast and mammalian cells the tethering complex TRAPPI (ref. 7) binds to the coat subunit Sec23. This event requires the Bet3 subunit. In vitro studies demonstrate that the interaction between Sec23 and Bet3 targets TRAPPI to COPII vesicles to mediate vesicle tethering. We propose that the binding of TRAPPI to Sec23 marks a coated vesicle for fusion with another COPII vesicle or the Golgi apparatus. An implication of these findings is that the intracellular destination of a transport vesicle may be determined in part by its coat and its associated cargo.

Elucidation of the reactions responsible for the calcium-regulated fusion of secretory granules with the plasma membrane in secretory cells would be facilitated by the identification of participant proteins having known biochemical activities. The successful characterization of cytosolic and vesicle proteins that may function in calcium-regulated secretion has not yet revealed the molecular events underlying this process. Regulated secretion consists of sequential priming and triggering steps which depend on ATP and Ca2+, respectively, and require distinct cytosolic proteins. Characterization of priming-specific factors (PEP proteins) should enable the ATP-requiring reactions to be identified. Here we show that one of the mammalian priming factors (PEP3) is identical to phosphatidylinositol transfer protein (PITP). The physiological role of PITP was previously unknown. We also find that SEC14p, the yeast phosphatidylinositol transfer protein which is essential for constitutive secretion, can substitute for PEP3/PITP in priming. Our results indicate that a role for phospholipid transfer proteins is conserved in the constitutive and regulated secretory pathways.

How the directionality of vesicle traffic is achieved remains an important unanswered question in cell biology. The Sec23p/Sec24p coat complex sorts the fusion machinery (SNAREs) into vesicles as they bud from the endoplasmic reticulum. Vesicle tethering to the Golgi begins when the tethering factor TRAPPI binds to Sec23p. Where the coat is released and how this event relates to membrane fusion is unknown. Here we use a yeast transport assay to demonstrate that an ER-derived vesicle retains its coat until it reaches the Golgi. A Golgi-associated kinase, Hrr25p (CK1δ ortholog), then phosphorylates the Sec23p/Sec24p complex. Coat phosphorylation and dephosphorylation are needed for vesicle fusion and budding, respectively. Additionally, we show that Sec23p interacts in a sequential manner with different binding partners, including TRAPPI and Hrr25p, to ensure the directionality of ER-Golgi traffic and prevent the back-fusion of a COPII vesicle with the ER. These events are conserved in mammalian cells.