Fab1p PtdIns(3)P 5-kinase function essential for protein sorting in the multivesicular body

Sorting of signal-transducing cell surface receptors within multivesicular bodies (MVBs) is required for their rapid down-regulation and degradation w ithin lysosomes. Yeast mutants defective in late stages of transport to the vacuole/lysosome accumulate MVBs. We demonstrate that the membrane glycopr otein carboxypeptidase S and the G protein-coupled receptor Ste2p are targe ted into the vacuole lumen, and this process requires a subset of VPS gene products essential for normal endosome function. The PtdIns(3)P B-kinase ac tivity of Fab1p, which converts the product of the Vps34p PtdIns 3-kinase P tdIns(3)P into PtdIns(3,5)P-2, also is required for cargo-selective sorting into the vacuole lumen. These findings demonstrate a role for phosphoinosi tide signaling at distinct stages of vacuolar/lysosomal protein transport a nd couple PtdIns(3,5)P-2 synthesis to regulation of MVB sorting.