Evolutionarily diverged on-switch for actin assembly in fungal endocytosis

Clathrin-mediated endocytosis is a conserved eukaryotic trafficking process where an Arp2/3 complex nucleated branched actin network provides force for vesicle formation. The mechanisms that initiate endocytic actin assembly are incompletely understood. In the fission yeast, Schizosaccharomyces pombe, actin assembly is initiated by Dip1, an Arp2/3 activator. In the budding yeast, Saccharomyces cerevisiae, the initiation of actin assembly has remained a mystery. Here we show that S. cerevisiae Ldb17, the homolog of Dip1, functions as an on-switch for endocytic actin assembly. Unexpectedly, the regulation of Ldb17 is more complicated than that of constitutively active Dip1. Ldb17 is controlled by a coat protein, Sla1, via separate recruitment and activation steps. This regulation was likely lost in the S. pombe lineage and this simplification may be related to other changes in actin assembly between these species. Our findings add a key missing piece in the understanding of endocytosis in S. cerevisiae and reveal an intriguing evolutionary tinkering of the actin on-switch.