Coordination of the Rab5 Cycle on Macropinosomes

The GTPase Rab5a regulates the homotypic and heterotypic fusion of membranous organelles during the early stages of endocytosis. Many of the molecules which regulate the Rab5a cycle of association with membranes, activation, deactivation and dissociation are known. However, the extent to which these molecular scale activities are coordinated on membranes to affect the behavior of individual organelles has not been determined. This study used novel Forster resonance energy transfer (FRET) microscopic methods to analyze the Rab5a cycle on macropinosomes, which are large endocytic vesicles that form in ruffled regions of cell membranes. In Cos-7 cells and mouse macrophages stimulated with growth factors, Rab5a activation followed immediately after its recruitment to newly formed macropinosomes. Rab5a activity increased continuously and uniformly over macropinosome membranes then decreased continuously, with Rab5a deactivation preceding dissociation by 1-12 min. Although the maximal levels of Rab5a activity were independent of organelle size, Rab5a cycles were longer on larger macropinosomes, consistent with an integrative activity governing Rab5a dynamics on individual organelles. The Rab5a cycle was destabilized by microtubule depolymerization and by bafilomycin A1. Overexpression of activating and inhibitory proteins indicated that active Rab5a stabilized macropinosomes. Thus, overall Rab5a activity on macropinosomes is coordinated by macropinosome structure and physiology.