Structures of yeast vesicle trafficking proteins

Tn protein transport between organelles, interactions of v- and t-SNARE pro teins are required for fusion of protein-containing vesicles with appropria te target compartments. Mammalian SNARE proteins have been observed to inte ract with NSF and SNAP, and yeast SNAREs with yeast homologues of NSF and S NAP proteins. This observation led to the hypothesis that, despite low sequ ence homology, SNARE proteins are structurally similar among eukaryotes. SN ARE proteins can be classified into two groups depending on whether they in teract with SNARE binding partners via conserved glutamine (Q-SNAREs) or ar ginine (R-SNAREs). Much of the published structural data available is for S NAREs involved in exocytosis (either in yeast or synaptic vesicles). This p aper describes circular dichroism, Fourier transform infrared spectroscopy, and dynamic light scattering data for a set of yeast v- and t-SNARE protei ns, Vti1p and Pep12p, that an Q-SNAREs involved in intracellular traffickin g. Our results suggest that the secondary structure of Vti1p is highly alph a-helical and that Vti1p forms multimers under a variety of solution condit ions. In these respects, Vti1p appears to be distinct from R-SNARE proteins characterized previously. The alpha-helicity of Vti1p is similar to that o f Q-SNARE proteins characterized previously. Pep12p, a Q-SNARE, is highly a lpha-helical. It is distinct from other Q-SNAREs in that it forms dimers un der many of the solution conditions tested in our experiments. The results presented in this paper are among the first to suggest heterogeneity in the functioning of SNARE complexes.