Comparable affinity of RabGDIα for GTP- and GDP-bound forms of Rab7 supports a four-state transition model for Rab7 subcellular localization

Endolysosomal system is linked to almost all aspects of cell life and diseases, and Rab7 occupies a critical node in this crucial pathway. However, there have been conflicting views about the exact role of Rab7 in membrane trafficking, since some studies have reported that Rab7 regulates the trafficking from early to late endosomes, while others highlighted its role in late endosomes to lysosomes progression. In the present study, we have revisited this issue from a new viewpoint. In COS-7 cells, a GDP-bound Rab7 mutant, T22N, was located to vesicular membranes as well as in cytoplasm. Similarly, the GTPase-deficient Q67L mutant of Rab7 resided in cytoplasm as well as on membranes. Additionally, we found that RabGDI interacted with both GTP- and GDP-bound forms of Rab7 in vitro. These results have prompted us to propose a four-state transition model for Rab7. This four-state model matches with our recent findings that Rab7 was initially recruited to macropinosomes in a GDP-bound inactive form and subsequently became activated during endocytic maturation in EGF-stimulated COS-7 cells.