Sec7p directs the transitions required for yeast Golgi biogenesis

Endoplasmic reticulum (ER)-to-Golgi traffic in yeast proceeds by the matura tion of membrane compartments from post-ER vesicles to intermediate small v esicle tubular clusters (VTCs) to Golgi nodular membrane networks (Morin-Ga net et al.. Traffic 2000; 1: 56-68). The balance between ER and Golgi compa rtments is maintained by COPII- and COPI-mediated anterograde and retrograd e traffic, which are dependent on Sec7p and ARF function. The sec7-4 temper ature-sensitive allele is a mutation in the highly conserved Sec7 domain (S ec7d) found in all ARF-guanine nucleotide exchange factor proteins. Post-ER trafficking is rapidly inactivated in sec7-4 mutant yeast at the restricti ve temperature. This conditional defect prevented the normal production of VTCs and instead generated Golgi-like tubes emanating from the ER exit site s. These tubes progressively developed into stacked cisternae defining the landmark sec7 mutant phenotype. Consistent with the in vivo results, a Sec7 d peptide inhibited ER-to-Golgi transport and displaced Sec7p from its memb rane anchor in vitro. The similarities in the consequences of inactivating Sec7p or ARFs in vivo was revealed by genetic disruption of yeast ARFs or b y addition of brefeldin A (BFA) to whole cells. These treatments, as in sec 7-4 yeast, affected the morphology of membrane compartments in the ER-Golgi transition. Further evidence for Sec7p involvement in the transition for G olgi biogenesis was revealed by in vitro binding between distinct domains o f Sec7p with ARFs. COPI and COPII coat proteins. These results suggest that Sec7p coordinates membrane transitions in Golgi biogenesis by directing an d scaffolding the binding and disassembly of coat protein complexes to memb ranes, both at the VTC transition from ER exit sites to form Golgi elements and for later events in Golgi maturation.