Intracellular membrane fusion requires Rab GTPases, tethers, SNAREs of the R, Qa, Qb, and Qc families, and SNARE chaperones of the Sec17 (SNAP), Sec18 (NSF), and SM (Sec1/Munc18) families. The vacuolar HOPS complex combines the functions of membrane tethering and SM catalysis of SNARE assembly. HOPS is activated for this catalysis by binding to the vacuolar lipids and Rab. Of the 8 major vacuolar lipids, we now report that phosphatidylinositol and phosphatidylinositol-3-phosphate are required to activate HOPS for SNARE complex assembly. These lipids plus ergosterol also allow full trans-SNARE complex assembly, yet do not support fusion, which is reliant on either phosphatidylethanolamine (PE) or on phosphatidic acid (PA), phosphatidylserine (PS), and diacylglycerol (DAG). Fusion with a synthetic tether and without HOPS, or even without SNAREs, still relies on either PE or on PS, PA, and DAG. These lipids are thus required for the terminal bilayer rearrangement step of fusion, distinct from the lipid requirements for the earlier step of activating HOPS for trans-SNARE assembly.

细胞内膜融合需要 R、Qa、Qb 和 Qc 家族的 Rab GTP 酶、系链、SNARE，以及 Sec17 (SNAP)、Sec18 (NSF) 和 SM (Sec1/Munc18) 家族的 SNARE 伴侣。液泡 HOPS 复合物结合了膜束缚和 SM 催化 SNARE 组装的功能。HOPS 通过与液泡脂质和 Rab 结合而被激活用于这种催化。在 8 种主要液泡脂质中，我们现在报告需要磷脂酰肌醇和 3-磷酸磷脂酰肌醇来激活 HOPS 以进行 SNARE 复合物组装。这些脂质加上麦角甾醇还允许完全反式SNARE 复合物组装，但不支持融合，它依赖于磷脂酰乙醇胺 (PE) 或磷脂酸 (PA)、磷脂酰丝氨酸 (PS) 和二酰基甘油 (DAG)。用合成系绳融合，没有 HOPS，甚至没有 SNARE，仍然依赖于 PE 或 PS、PA 和 DAG。因此，这些脂质是融合的末端双层重排步骤所必需的，这与激活 HOPS 以进行反式SNARE 组装的早期步骤所需要的脂质不同。