Asymmetric Requirements for a Rab Gtpase and Snare Proteins in Fusion of Copii Vesicles with Acceptor Membranes

Soluble NSF attachment protein receptor (SNARE) proteins are essential for membrane fusion in transport between the yeast ER and Golgi compartments. Subcellular fractionation experiments demonstrate that the ER/Golgi SNAREs Bos1p, Sec22p, Bet1p, Sed5p, and the Rab protein, Ypt1p, are distributed similarly but localize primarily with Golgi membranes. All of these SNARE proteins are efficiently packaged into COPII vesicles and suggest a dynamic cycling of SNARE machinery between ER and Golgi compartments. Ypt1p is not efficiently packaged into vesicles under these conditions. To determine in which membranes protein function is required, temperature-sensitive alleles of BOS1, BET1, SED5, SLY1, and YPT1 that prevent ER/Golgi transport in vitro at restrictive temperatures were used to selectively inactivate these gene products on vesicles or on Golgi membranes. Vesicles bearing mutations in Bet1p or Bos1p inhibit fusion with wild-type acceptor membranes, but acceptor membranes containing these mutations are fully functional. In contrast, vesicles bearing mutations in Sed5p, Sly1p, or Ypt1p are functional, whereas acceptor membranes containing these mutations block fusion. Thus, this set of SNARE proteins is symmetrically distributed between vesicle and acceptor compartments, but they function asymmetrically such that Bet1p and Bos1p are required on vesicles and Sed5p activity is required on acceptor membranes. We propose the asymmetry in SNARE protein function is maintained by an asymmetric distribution and requirement for the Ypt1p GTPase in this fusion event. When a transmembrane-anchored form of Ypt1p is used to restrict this GTPase to the acceptor compartment, vesicles depleted of Ypt1p remain competent for fusion.