Evidence for overlapping and distinct functions in protein transport of coat protein Sec24p family members

Budding of transport vesicles from the endoplasmic reticulum in yeast requires the formation, at the budding site, of a coat protein complex (COPII) that consists of two heterodimeric subcomplexes (Sec23p/Sec24p and Sec13p/Sec31p) and the Sar1 GTPase. Sec24p is an essential protein and involved in cargo selection. In addition to Sec24p, the yeast Saccharomyces cerevisiae expresses two non-essential Sec24p-related proteins, termed Sfb2p (product of YNL049c) and Sfb3p/Lst1p (product of YHR098c). We here show that Sfb2p and, less efficiently, Sfb3p/Lst1p are able to bind, like Sec24p, the integral membrane cargo protein Sed5p. We also demonstrate that Sfb2p, like Sec24p and Sfb3p/Lst1p, forms a complex with Sec23p in vivo. Whereas the deletion of SFB2 did not affect transport kinetics of various proteins, the maturation of the glycolipid-anchored plasma membrane protein Gas1p was differentially impaired in sfb3 knock-out cells. We generated several conditional-lethal sec24 mutants that, combined with null alleles of SFB2 and SFB3/LST1, led to a complete block of transport between the endoplasmic reticulum and the Golgi (sec24-11/Deltasfb2) or to cell death (sec24-11/Deltasfb3). Of the Sec24p family members, Sfb2p is the least abundant at steady state, but high intracellular concentrations of Sfb2p can rescue sec24 mutants under restrictive conditions. The data presented strongly suggest that the Sec24p-related proteins function as COPII components.