A key role in the regulation of membrane traffic is played by the rab proteins, members of a family of ras-related small GTP-binding proteins. This family comprises at least 25 identified members, the intracellular localization of only a few of which has been investigated. rab6p has been shown to be distributed along the exocytic pathway in association with the medial and trans regions of the Golgi apparatus. A confocal laser scanning microscopic (CLSM) approach coupled with image analysis was used to compare the localization of rab6p with selected reference Golgi markers by double immunofluorescence on culture cell lines. CLSM analysis shows that, under a set of well-defined conditions, one can investigate the possible colocalization of known markers of Golgi compartments and orientate a couple of labeled Golgi antigens with regard to the polarity of the Golgi apparatus. Thus, having validated the CLSM analysis, the localization of rab6p was studied and compared with some of these markers and the VSV-G protein in VSV (vesicular stomatitis virus)-infected cells blocked at 20 degrees C. rab6p is shown to be associated in all the cell lines used with the last cisternae of the Golgi apparatus and particularly with the trans-Golgi network (TGN), the site of protein sorting at the exit of the Golgi apparatus. These results were supported by an electron microscopic study using double-immunolabeled cryosections: rab6p was found in some flat cisternae of the Golgi stack and colocalized with the VSV-G protein in the TGN. Our results show that the small GTP-binding protein rab6p is distributed from medial Golgi to TGN along the exocytic pathway.

• © 1992 by Company of Biologists