THE SMALL GTP-BINDING PROTEIN RAB6 FUNCTIONS IN INTRA-GOLGI TRANSPORT

Rab6 is a ubiquitous ras-like GTP-binding protein associated with the membranes of the Golgi complex (Goud, B., A. Zahraoui, A. Tavitian, an d J. Saraste. 1990. Nature (Lond.). 345: 553-556; Antony, C., C. Ciber t, G. Geraud, A. Santa Maria, B. Maro, V. Mayau, and B. Goud. 1992. J. Cell Sci. 103: 785-796). We have transiently overexpressed in mouse L cells and human HeLa cells wild-type rab6, GTP (rab6 Q72L), and GDP ( rab6 T27N) -bound mutants of rab6 and analyzed the intracellular trans port of a soluble secreted form of alkaline phosphatase (SEAP) and of a plasma membrane protein, the hemagglutinin protein (HA) of influenza virus. Overexpression of wild-type rab6 and rab6 Q72L greatly reduced transport of both markers between cis/medial (cr-mannosidase II posit ive) and late (sialyl-transferase positive) Golgi compartments, withou t affecting transport from the endoplasmic reticulum (ER) to cis/media l-Golgi or from the trans-Golgi network (TGN) to the plasma membrane. Whereas overexpression of rab6 T27N did not affect the individual step s of transport between ER and the plasma membrane, it caused an appare nt delay in secretion, most likely due to the accumulation of the tran sport markers in late Golgi compartments. Overexpression of both rab6 Q72L, and rab6 T27N altered the morphology of the Golgi apparatus as w ell as that of the TGN, as assessed at the immunofluorescence level wi th several markers. We interpret these results as indicating that rab6 controls intra-Golgi transport, either acting as an inhibitor in ante rograde transport or as a positive regulator of retrograde transport.