LMA1 binds to vacuoles at Sec18p (NSF), transfers upon ATP hydrolysis to a t-SNARE (Vam3p) complex, and is released during fusion

Abstract

Vacuole fusion requires Sec18p (NSF), Sec17p (alpha-SNAP), Ypt7p (GTP binding protein), Vam3p (t-SNARE), Nyv1p (v-SNARE), and LMA1 (low Mr activity 1, a heterodimer of thioredoxin and I(B)2). LMA1 requires Sec18p for saturable, high-affinity binding to vacuoles, and Sec18p "priming" ATPase requires both Sec17p and LMA1. Either the sec18-1 mutation and deletion of I(B)2, or deletion of both I(B)2 and p13 (an I(B)2 homolog) causes a striking synthetic vacuole fragmentation phenotype. Upon Sec18p ATP hydrolysis, LMA1 transfers to (and stabilizes) a Vam3p complex. LMA1 is released from vacuoles in a phosphatase-regulated reaction. This LMA1 cycle explains how priming by Sec18p is coupled to t-SNARE stabilization and to fusion.