Mutations of the SM protein Sly1 resulting in bypass of GTPase requirement in vesicular transport are confined to a short helical region

Ypt/Rab GTPases and Sec1/Munc18 (SM) proteins are key components of the membrane fusion machinery. Here, we describe new mutants of the yeast SM protein Sly1 that specifically bypass the need for GTPases Ypt1 and Ypt6 in vesicular transport. All sequence alterations are confined to a short α-helix (α-20), which is conserved in fungal Sly1 proteins and, when deleted, results in GTPase suppression. Whereas Sly1p of the evolutionarily distant fission yeast Schizosaccharomyces pombe can functionally replace Sly1p in Sacchromyces cerevisiae, mammalian homologues cannot. This indicates that α-20 in fungal Sly1p plays an important role in mediating Ypt/Rab-regulated Sly1p function in membrane fusion.