The soluble hydrolases of the mammalian lysosome are marked for delivery to this 
organelle by the addition of mannose 6-phosphate to their N-glycans. Two related 
mannose 6-phosphate receptors (MPRs) recognize this feature in the trans Golgi 
network (TGN) and deliver the hydrolases to the late endosome. In contrast, the 
vacuolar hydrolases of the yeast Saccharomyces cerevisiae do not contain 
6-phosphate monoesters on their N-glycans, and the only sorting receptor so far 
identified in this organism is the product of the VPS10 gene. This protein also 
cycles between the Golgi and the late endosome, but is unrelated to the 
vertebrate MPRs, and recognizes a specific amino acid sequence of 
carboxypeptidase Y (CPY). This has led to the notion that although yeast and 
mammals share many components in Golgi to endosome traffic, they use unrelated 
receptor systems to sort their abundant soluble hydrolases. In this paper, we 
report that the yeast genome does in fact contain an uncharacterized ORF 
(YPR079w) that encodes a membrane protein that is distantly related to mammalian 
MPRs. The protein encoded by this gene (which we term MRL1) cycles through the 
late endosome. Moreover, there is a strong synergistic effect on the maturation 
of proteinases A and B when both MRL1 and VPS10 are deleted, which suggests that 
Mrl1p may serve as a sorting receptor in the delivery of vacuolar hydrolases.