A Rab1 GTPase is required for transport between the endoplasmic reticulum and Golgi apparatus and for normal Golgi movement in plants

We describe a green fluorescent protein (GFP)-based assay for investigating membrane traffic on the secretory pathway in plants. Expression of AtRab1b (N121I), predicted to be a dominant inhibitory mutant of the Arabidopsis Ra b GTPase AtRab1b, resulted in accumulation of a secreted GFP marker in an i ntracellular reticulate compartment reminiscent of the endoplasmic reticulu m. This accumulation was alleviated by coexpressing wild-type AtRab1b but n ot AtRab8c. When a Golgi-targeted and N-glycosylated variant of GFP was coe xpressed with AtRab1b(N121I), the variant also accumulated in a reticulate network and an endoglycosidase H-sensitive population appeared. Unexpectedl y, expression of AtRab1b(N121I), but not of the wild-type AtRab1b, resulted in a reduction or cessation of vectorial Golgi movement, an effect that wa s reversed by coexpression of the wild type. We conclude that AtRab1b funct ion is required for transport from the endoplasmic reticulum to the Golgi a pparatus and suggest that this process may be coupled to the control of Gol gi movement.