We used in vivo and in vitro strategies to study the mechanisms of multivesicular endosome biogenesis. We found that, whereas annexinA2 and ARP2/3 mediate F-actin nucleation and branching, respectively, the ERM protein moesin supports the formation of F-actin networks on early endosomes. We also found that moesin plays no role during endocytosis and recycling to the plasma membrane but is absolutely required, much like actin, for early-to-late-endosome transport and multivesicular endosome formation. Both actin network formation in vitro and early-to-late endosome transport in vivo also depend on the F-actin-binding protein cortactin. Our data thus show that moesin and cortactin are necessary for formation of F-actin networks that mediate endosome biogenesis or maturation and transport through the degradative pathway. We propose that the primary function of endosomal F-actin is to control the membrane remodeling that accompanies endosome biogenesis. We also speculate that this mechanism helps segregate tubular and multivesicular membranes along the recycling and degradation pathways, respectively.
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