Dynamics of rab5 activation in endocytosis and phagocytosis

Abstract: Fluid-phase endocytosis is stimulated by H-ras-linked growth factor receptors and this stimu-lation requires activation of rab5. We utilized a GFP-rab5a:wt fusion protein to monitor GFP-rab5a:wt activation in living fibroblasts and in J774 macro-phages. Control CHO cells that expressed GFP-rab5a:wt were cultured in serum-free conditions and showed GFP-rab5a:wt localized to endosomal vesi-cles with a mean diameter of 0.3 6 0.1 mm. Endo-some fusion, membrane ruffling, and pinosome for-mation were rarely detected in these cells. Coexpres-sion of H-ras:G12V, a constitutively active H-ras mutant that activates rab5a, in cells resulted in marked enlargement of labeled endosomes (mean diameter 0.7 6 0.2 mm) and large numbers of gian