SAR1 PROMOTES VESICLE BUDDING FROM THE ENDOPLASMIC-RETICULUM BUT NOT GOLGI COMPARTMENTS

Two new members (Sar1a and Sar1b) of the SARI gene family have been id entified in mammalian cells. Using immunoelectron microscopy, Sar1 was found to be restricted to the transitional region where the protein w as enriched 20-40-fold in vesicular carriers mediating ER to Golgi tra ffic. Biochemical analysis revealed that Sar1 was essential for an ear ly step in vesicle budding. A Sar1-specific antibody potently inhibite d export of vesicular stomatitis virus glycoprotein (VSV-G) from the E R in vitro. Consistent with the role of guanine nucleotide exchange in Sar1 function, a trans-dominant mutant (Sar1a[T39N]) with a preferent ial affinity for GDP also strongly inhibited vesicle budding from the ER. In contrast, Sar1 was not found to be required for the transport o f VSV-G between sequential Golgi compartments, suggesting that compone nts active in formation of vesicular carriers mediating ER to Golgi tr affic may differ, at least in part, from those involved in intra-Golgi transport. The requirement for novel components at different stages o f the secretory pathway may reflect the recently recognized difference s in protein transport between the Golgi stacks as opposed to the sele ctive sorting and concentration of protein during export from the ER.