The SMS domain of Trs23p is responsible for the in vitro appearance of the TRAPP I complex in Saccharomyces cerevisiae

Saccharomyces cerevisiae transport protein particle (TRAPP) is a family of related multisubunit complexes required for endoplasmic reticulum-to-Golgi transport (TRAPP I), endosome-to-Golgi transport (TRAPP II) or cytosol to vacuole targeting (TRAPP III). To gain insight into the relationship between these complexes, we generated random and targeted mutations in the Trs23p core subunit. Remarkably, at physiological salt concentrations only two peaks (TRAPP I and a high molecular weight peak) are detected in wild-type cells. As the salt was raised, the high molecular weight peak resolved into TRAPP II and III peaks. Deletion of a Saccharomycotina-specific domain of Trs23p resulted in destabilization of TRAPP I but had no effect on TRAPP II or III. This mutation had no observable growth phenotype, normal levels of Ypt1p-directed guanine nucleotide exchange factor activity in vivo and did not display any in vivo nor in vitro blocks in membrane traffic. Biochemical analysis indicated that TRAPP I could be produced from the TRAPP II/III peak in vitro by increasing the salt concentration. Our data suggest that the SMS domain of Trs23p is responsible for the in vitro appearance of TRAPP I in S. cerevisiae. The implications of these findings are discussed.