Geranylgeranylated SNAREs are dominant inhibitors of membrane fusion

Exocytosis in yeast requires the assembly of the secretory vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (V-SNARE) Snc p and the plasma membrane t-SNAREs Ssop and Sec9p into a SNA RE complex, Hi gh-level expression of mutant Snc1 or Sso2 proteins that have a COOH-termin al geranylgeranylation signal instead of a transmembrane domain inhibits ex ocytosis at a stage after vesicle docking. The mutant SNARE proteins are me mbrane associated, correctly targeted, assemble into SNARE complexes, and d o not interfere with the incorporation of wild-type SNARE proteins into com plexes, Mutant SNARE complexes recruit GFP-Sec1p to sites of exocytosis and can be disassembled by the Sec18p ATPase. Heterotrimeric SNARE complexes a ssembled from both wild-type and mutant SNAREs are present in heterogeneous higher-order complexes containing Sec1p that sediment at greater than 20S, Based on a structural analogy between geranylgeranylated SNAREs and the GP I-HA mutant influenza virus fusion protein, we propose that the mutant SNAR Es are fusion proteins unable to catalyze fusion of the distal leaflets of the secretory vesicle and plasma membrane. In support of this model, the in verted cone-shaped lipid lysophosphatidylcholine rescues secretion from SNA RE mutant cells.