HEAT repeats mediate plasma membrane localization of Tor2p in yeast

The subcellular distribution of Tor1p and Tor2p, two phosphatidylinositol k inase homologs and targets of the immunosuppressive drug rapamycin in Sacch aromyces cerevisiae, was analyzed. We found that Tor protein is peripherall y associated with membranes. Subcellular fractionation and immunofluorescen ce studies showed that Tor1p and Tor2p associate with the plasma membrane a nd a second fraction that is distinct from Golgi, vacuoles, mitochondria, a nd nucleus and may represent vesicular structures. Pulse-chase experiments showed that association of Tor protein with plasma membrane and the second compartment is fast, does not appear to involve components of endocytic, se cretory, or Golgi to vacuole transport pathways, and is not affected by the immunosuppressive drag rapamycin. Deletion analysis reveals that two domai ns within Tor2p independently mediate localization to both compartments. Th ese domains are composed of HEAT repeats that are thought to act as protein -protein interaction surfaces. Our studies therefore place Tor proteins at the site of action of their known downstream effecters and suggest that the y may be part of a multiprotein complex.