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 requi res the formation, at the budding site, of a coat protein complex (COPII) t hat consists of two heterodimeric subcomplexes (Sec23p/Sec24p and Sec13p/Se c31p) and the Sari GTPase. Sec24p is an essential protein and involved in c argo selection. In addition to Sec24p, the yeast Saccharomyces cerevisiae e xpresses two non-essential Sec24p-related proteins, termed Sfb2p (product o f YNL049c) and Sfb3p/Lst1p (product of YHR098c). We here show that Sfb2p an d, less efficiently, Sfb3p/Lst1p are able to bind, like Sec24p, the integra l 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 maturat ion of the glycolipid-anchored plasma membrane protein Gas1p was differenti ally impaired in sfb3 knock-out cells. We generated several conditional-let hal sec24 mutants that, combined with null alleles of SFB2 and SFB3/LST1, l ed to a complete block of transport between the endoplasmic reticulum and t he Golgi (sec24-11/Delta sfb2) or to cell death (sec24-11/Delta sfb3). Of t he Sec24p family members, Sfb2p is the least abundant at steady state, but high intracellular concentrations of Sfb2p can rescue sec24 mutants under r estrictive conditions. The data presented strongly suggest that the Sec24p- related proteins function as COPII components.