Phox homology domains specifically bind phosphatidylinositol phosphates

The recruitment of specific cytosolic proteins to intracellular membranes t hrough binding phosphorylated derivatives of phosphatidylinositol (PtdIns) controls such processes as endocytosis, regulated exocytosis, cytoskeletal organization, and cell signaling. Protein modules such as FVYE domains and PH domains that bind specifically to PtdIns 3-phosphate (PtdIns-3-P) and po lyphosphoinositides, respectively, can direct such membrane targeting. Here we show that two representative Phox homology (PX) domains selectively bin d to specific phosphatidylinositol phosphates. The PX domain of Vam7p selec tively binds Ptdlns-3-P, while the PX domain of the CPK PI-3 kinase selecti vely binds PtdIns-4,5-P-2. In contrast, the PX domain of Vps5p displays no binding to any PtdInsPs that were tested. In addition, the double mutant (Y 42A/L48Q) of the PX domain of Vam7p, reported to cause vacuolar trafficking defects in yeast, has a dramatically decreased level of binding to Ptdlns- 3-P. These data reveal that the membrane targeting function of the Vam7p PX domain is based on its ability to associate with PtdIns-3-P, analogous to the function of FYVE domains.