Rw. Peng et al., Evidence for overlapping and distinct functions in protein transport of coat protein Sec24p family members, J BIOL CHEM, 275(15), 2000, pp. 11521-11528
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.