Sfb2p, a yeast protein related to sec24p, can function as a constituent ofCOPII coats required for vesicle budding from the endoplasmic reticulum

The COPII coat is required for vesicle budding from the endoplasmic reticul um (ER), and consists of two heterodimeric subcomplexes, Sec23p/Sec24p, Sec 13p/Sec31p, and a small GTPase, Sar1p. We characterized a yeast mutant, anu 1 (abnormal nuclear morphology) exhibiting proliferated ER as well as abnor mal nuclear morphology at the restrictive temperature. Based on the finding that ANU1 is identical to SEC24, we confirmed a temperature-sensitive prot ein transport from the ER to the Golgi in anu1-1/sec24-20 cells. Overexpres sion of SFB2, a SEC24 homologue with 56% identity, partially suppressed not only the mutant phenotype of sec24-20 cells but also rescued the SEC24-dis rupted cells. Moreover, the yeast two-hybrid assay revealed that Sfb2p, sim ilarly to Sec24p, interacted with Sec23p. In SEC24-disrupted cells rescued by overexpression of SFB2, some cargo proteins were still retained in the E R, while most of the protein transport was restored. Together, these findin gs strongly suggest that Sfb2p functions as the component of COPII coats in place of Sec24p, and raise the possibility that each member of the SEC24 f amily of proteins participates directly and/or indirectly in cargo-recognit ion events with its own cargo specificity at forming ER-derived vesicles.