Regulation of G(i) and G(o) by mastoparan, related amphiphilic peptides, and hydrophobic amines. Mechanism and structural determinants of activity

Mastoparan (MP), a cationic, amphiphilic tetradecapeptide, stimulates guanine nucleotide exchange by GTP-binding regulatory proteins (G proteins) in a manner similar to that of G protein-coupled receptors. 1) MP stimulated exchange by isolated G protein α subunits and αβγ trimers. Relative stimulation was greater with αβγ trimers and βγ subunits could increase net MP-stimulated activity. 2) MP action was enhanced by reconstitution of trimeric G protein into phospholipid vesicles. Hill coefficients for activation were 2-4. The membrane-bound α-helical conformation of MP appeared to be the activating species. 3) MP blocked the ability of G(o) to increase the affinity of muscarinic receptors for agonist ligands, suggesting that MP and the receptor may compete for a common binding site on G(o). 4) MP stimulated steady state GTPase activity at < 1 μM Mg²⁺ and stimulated the dissociation of both GDP and guanosine 5'-O-(3-thiotriphosphate) at < 1 nM Mg²⁺. Millimolar Mg²⁺ blocked the stimulatory effect of MP. Both high and low affinity Mg²⁺ binding sites are on the α subunit. 5) Increasing the amphiphilicity or hydrophobicity of MP enhanced its regulatory activity more than 2-fold and lowered the EC₅₀ more than 10-fold. Several natural amphiphilic peptides also displayed modest stimulatory activity. 6) Benzalkonium chloride competitively antagonized the stimulation of G(i) by MP but potently stimulated nucleotide exchange on G(o). Because cationic, amphiphilic sequences on the cytoplasmic faces of receptors are required for G protein regulation, these findings suggest that nucleotide exchange on G proteins is regulated by the presentation of multiple cationic structures on the inner face of the plasma membrane.