Arp2/3-dependent actin assembly shapes endosomes and promotes intracellular trafficking in fission yeast

Endosomes serve as crucial sorting centres that streamline the distribution of cell surface proteins. The early endosome receives traffic from both the plasma membrane (PM) and the Golgi, and orchestrates the redistribution of cargoes for recycling to the PM or through retrograde movement to the Golgi, and for degradation to late endosomes and lysosomes ¹ . In animal cells and amoebae, Arp2/3-mediated F-actin assembly plays critical roles in many aspects of endosome function, from capturing cargo to generating the forces required for membrane deformation and creating cargo-enriched transport carriers, thus promoting both recycling and degradative trafficking routes ² . Yeast models, which allowed dissection of the major membrane trafficking routes ³ , exhibit highly simplified endosomes, as shown in Saccharomyces cerevisiae , where the trans-Golgi network (TGN) functions as recycling endosome ⁴ . Furthermore, there is no reported role for Arp2/3 or F-actin in endomembrane remodelling; in fact, the Arp2/3 activators on animal endosomes – the WASH complex promoting recycling of transmembrane receptors towards the PM or TGN ^5,6 , and Annexin A2 necessary for endosome maturation and the movement of cargoes towards the degradative pathway ⁷ – do not exist in yeast cells ^8,9 . Here, we report that Arp2/3 and F-actin are present on fission yeast endosomes. Through live-imaging and correlative light-electron tomography, we demonstrate that Arp2/3 activity controls endosomal morphology and that this activity is essential for trafficking from the endosome to the degradative vacuole. Thus, Arp2/3-dependent actin assembly has a deeply conserved role in shaping and promoting the function of the endomembrane trafficking system.