Receptor and membrane recycling can occur with unaltered efficiency despite dramatic Rab5(q79l)-induced changes in endosome geometry.

Current models for sorting in the endosomal compartment suggest that endosomal geometry plays a significant role as membrane-bound proteins accumulate in tubular regions for recycling, and lumenal markers accumulate in large vacuolar portions for delivery to lysosomes. Rab5, a small molecular weight GTPase, functions in the formation and maintenance of the early/sorting endosome. Overexpression of the constitutively active form, Rab5(Q79L), leads to enhanced endosome fusion resulting in the enlargement of early endosomes. Using an adenoviral expression system to regulate the time and level of Rab5(Q79L) overexpression in HeLa cells, we find that although endosomes are dramatically enlarged, the rates of transferrin receptor-mediated endocytosis and recycling are unaffected. Moreover, despite the enlarged endosome phenotype, neither the rate of internalization of a fluid phase marker nor the rate of recycling of a bulk lipid marker were affected. These results suggest that GTP hydrolysis by Rab5 is rate-limiting for endosome fusion but not for endocytic trafficking and that early endosome geometry may be a less critical determinant of sorting efficiencies than previously thought.