The interaction of coatomer with inositol polyphosphates is conserved in Saccharomyces cerevisiae

Coatomer is an oligomeric complex of coat proteins that regulates vesicular traffic through the Golgi complex and from the Golgi to the endoplasmic reticulum [Pelham (1994) Cell 79, 1125-1127]. We have investigated whether the binding of InsP6 to mammalian coatomer [Fleischer, Xie, Mayrleitner, Shears and Fleischer (1994) J. Biol. Chem. 269, 17826-17832] is conserved in the genetically amenable model Saccharomyces cerevisiae. We have isolated coatomer from S. cerevisiae and found it to bind InsP6 at two apparent classes of binding sites (KD1 = 0.8 +/- 0.2 nM; KD2 = 361 +/- 102 nM). Ligand specificity was studied by displacing 4.5 nM [3H]InsP6 from coatomer with various Ins derivatives. The following IC50 values (nM) were obtained: myo-InsP6 = 6; bis(diphospho)inositol tetrakisphosphate = 6; diphosphoinositol pentakisphosphate = 6; scyllo-InsP6 = 12; Ins(1,3,4,5,6)P5 = 13; Ins(1,2,4,5,6)P5 = 22; Ins(1,3,4,5)P4 = 22; 1-O-(1,2-di-O-octanoyl-sn-glycero-3-phospho)-D-Ins(3,4,5)P3 = 290. Less than 10% of the 3H label was displaced by 1 microM of either Ins(1,4,5)P3 or inositol hexakis-sulphate. A cell-free lysate of S. cerevisiae synthesized diphosphoinositol polyphosphates (PP-InsPn) from InsP6, but our binding data, plus measurements of the relative levels of inositol polyphosphates in intact yeast [Hawkins, Stephens and Piggott (1993) J. Biol. Chem. 268, 3374-3383], indicate that InsP6 is the major physiologically relevant ligand. Thus a reconstituted vesicle trafficking system using coatomer and other functionally related components isolated from yeast should be a useful model for elucidating the functional significance of the binding of InsP6 by coatomer.