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 behavior of Golgi resident proteins in HeLa cells overexpressing wild-type rab6 and GTP- and GDP-bound mutants of rab6 (rab6 Q72L and rab6 T27N, respectively). We show that wild-type rab6 and rab6 Q72L overexpression induces the redistribution of the trans-Golgi protein beta-1,4-galactosyltransferase 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. Remarkably, rab6 Q72L effects, which require the integrity of microtubules, were almost indistinguishable from those induced by brefeldin A, a fungic metabolite that causes a mixing of Golgi and ER membranes. In contrast, overexpression of rab6 T27N does not cause the redistribution of Golgi proteins, but inhibits basal O-glycosylation of the major histocompatibility complex class II-associated invariant chain.