GTP-BOUND FORMS OF RAB6 INDUCE THE REDISTRIBUTION OF GOLGI PROTEINS INTO THE ENDOPLASMIC-RETICULUM

rab6 is a ubiquitous ras-like GTPase involved in intra-Golgi transport , We have studied at both morphological and biochemical levels the beh avior of Golgi resident proteins in HeLa cells overexpressing wild-typ e rab6 and GTP- and GDP-bound mutants of rab6 (rab6 Q72L and rab6 T27N , respectively), We show that wild-type rab6 and rab6 Q72L overexpress ion induces the redistribution of the trans-Golgi protein beta-1,4-gal actosyltransferase into the endoplasmic reticulum (ER) and allows the addition of sialylated O-glycans on an ER-retained protein, the major histocompatibility complex class II-associated invariant chain. Remark ably, rab6 Q72L effects, which require the integrity of microtubules, were almost indistinguishable from those induced by brefeldin A, a fun gic metabolite that causes a mixing of Golgi and ER membranes. In cont rast, overexpression of rab6 T27N does not cause the redistribution of Golgi proteins, but inhibits basal O-glycosylation of the major histo compatibility complex class II-associated invariant chain.