Polymerization of actin branched networks controls the organization of WASH domains at the surface of endosomes
Emmanuèle Helfer (Laboratoire d’Enzymologie et de Biochimie Structurales – Gif-sur-Yvette)
Sorting of cargoes in endosomes occurs through their selective enrichment into sorting platforms, where transport intermediates are generated. The WASH complex, which is recruited from the cytosol to endosomes, activates the Arp2/3 complex and hence actin polymerization onto such sorting platforms.
Here, we analyzed the role of actin polymerization in the physiology of endosomal domains containing WASH using quantitative image analysis. Using a novel colocalization method that is insensitive to the heterogeneity of size and shape of endosomes, we show that preventing the generation of branched actin networks induces endosomal accumulation of the WASH complex. Moreover, we found that actin depolymerization induces a dramatic decrease in the recovery of endosomal WASH after photobleaching. These results suggest a built-in turnover, where the actin network, i.e. the product of the WASH complex, contributes to the dynamic cytosol/endosome exchange of the WASH complex. Our data also suggest a role of actin in the lateral compartmentalization of endosomes: discrete WASH domains coalesce upon actin depolymerization or Arp2/3 depletion. Thus, branched actin networks are involved in the regulation of WASH domain size.
I will finally discuss the potential role of lipid repartitioning in these sorting platforms and of the ensuing line tension that could develop at the domain boundary. This may provide a dynamin-independent contribution to membrane scission.