Functional importance of the amino terminus of G sub(q alpha )

G sub(q alpha ) is palmitoylated at residues Cys super(9) and Cys super(10). Removal of palmitate from purified G sub(q alpha ) with palmitoyl-thioesterase in vitro failed to alter interactions of G sub(q alpha ) with phospholipase C- beta 1, the G protein beta gamma subunit complex, or m1 muscarinic cholinergic receptors. Mutants C9A, C10A, C9A/C10A, C9S/C10S, and truncated G sub(q alpha ) (removal of residues 1-6) were synthesized in Sf9 cells and purified. Loss of both Cys residues or truncation prevented palmitoylation of G sub(q alpha ). However, truncated G sub(q alpha ) and the single Cys mutants activated phospholipase C- beta 1 normally, while the double Cys mutants were poor activators. Loss of both Cys residues impaired but did not abolish interaction of G sub(q alpha ) with m1 receptors. These Cys residues are thus important regardless of their state of palmitoylation. When expressed in HEK-293 or Sf9 cells, all of the proteins studied associated entirely or predominantly with membranes, although a minor fraction of nonpalmitoylated G sub(q alpha ) proteins accumulated in the cytosol of HEK-293 cells. When subjected to TX-114 phase partitioning, a significant fraction of all of the proteins, including those with no palmitate, was found in the detergent-rich phase. Removal of residues 1-34 of G sub(q alpha ) caused a loss of surface hydrophobicity as evidenced by complete partitioning into the aqueous phase. The Cys residues at the amino terminus of G sub(q alpha ) are thus important for its interactions with effector and receptor, and the amino terminus conveys a hydrophobic character to the protein distinct from that contributed by palmitate.