Clathrin-mediated endocytosis is a highly conserved process that mediates the internalisation of extracellular matter into cells. Endocytosis requires the assembly of actin into patches, which allow efficient membrane invagination and scission. In their paper, Vladimir Sirotkin and co-workers (MacQuarrie et al., 2019) study the function of the previously uncharacterised fission yeast protein Bbc1 in actin assembly at endocytic sites. They find that Bbc1 localises to endocytic actin patches. The dynamics of Bbc1 are similar to those of Myo1, an actin nucleation-promoting factor (NPF) involved in activating the Arp2/3 complex, which nucleates branched actin networks. Bbc1 directly interacts with the SH3 domain of Myo1 and competes with the NPF Vrp1 for binding to this domain, suggesting that Bbc1 counteracts the Myo1–Vrp1 interaction. Indeed, the authors show that Myo1 association with Vrp1 is increased in the absence of Bbc1. They also reveal that Bbc1 cooperates with the adaptor protein Sla1 to inhibit the assembly of NPFs and actin at endocytic patches. Consequently, the absence of Bbc1 causes the retention of the NPFs Wsp1 and Vrp1 at the base of endocytic invaginations, resulting in an increase in the length of these invaginations. Collectively, these results suggest that Bbc1 associates with Myo1 to reduce Myo1 association with Wsp1 and Vrp1 at endocytic structures and thus restrict Arp2/3-mediated actin nucleation at the plasma membrane.
