Yeast Snc proteins complex with Sec9. Functional interactions between putative SNARE proteins.

Yeast possess two homologs of the synaptobrevin family of vesicle-associated proteins that are proposed to be involved in membrane recognition and to act as receptors for components of the fusion machinery in neurons. We have previously described the yeast homologs, Snc1 and Snc2, and demonstrated that they localize to secretory vesicles and are required for normal secretion. Yeast lacking Snc protein expression accumulate post-Golgi transport vesicles that contain secretory proteins. Therefore, Snc proteins are essential for the fusion of carrier vesicles with the plasma membrane, and this property appears to have been conserved in evolution. We have now examined whether Snc proteins interact with other components of the late secretory pathway in yeast. Here we show that Snc proteins form a tight genetic and physical interaction with a plasma membrane protein, Sec9. Sec9 is the yeast equivalent of SNAP-25, a second receptor protein from neurons that has been shown to interact with synaptobrevin. We suggest, then, that recognition of the plasma membrane by secretory vesicles may involve the formation of a Snc-Sec9 complex and that this interaction has evolved as a fundamental step in secretory processes.