Bro1 is an endosome-associated protein that functions in the MVB pathway in Saccharomyces cerevisiae

Multivesicular bodies are late endosomal compartments containing lumenal vesicles that are formed by inward budding of the limiting endosomal membrane. In the yeast Saccharomyces cerevisiae, integral membrane proteins are sorted into the lumenal vesicles of multivesicular bodies, and this process requires the class E subset of VPS genes. We show that one of the class E VPS genes, BRO1/VPS31, encodes a cytoplasmic protein that associates with endosomal compartments. The dissociation of Bro1 from endosomes requires another class E Vps protein, Vps4, which is an ATPase that also regulates the endosomal dissociation of ESCRT-III, a complex of four class E Vps proteins (Vps2, Vps20, Vps24 and Snf7/Vps32) that oligomerize at the endosomal membrane. We also show that the endosomal association of Bro1 is specifically dependent on one of the ESCRT-III components, Snf7. Our data suggest that the function of Bro1 in the MVB pathway takes place on endosomal membranes and occurs in concert with or downstream of the function of the ESCRT-III complex.