The mechanism of inhibition of the Ca2+-ATPase by mastoparan. Mastoparan abolishes cooperative ca2+ binding.

The amphiphilic peptide mastoparan, isolated from wasp venom, is a potent inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. At pH 7. 2, ATPase activity is inhibited with an inhibitory constant (Ki) of 1 +/- 0.13 microM. Mastoparan shifts the E2-E1 equilibrium toward E1 and may affect the regulatory ATP binding site. The peptide also decreases the affinity of the ATPase for Ca2+ and abolishes the cooperativity of Ca2+ binding. In the presence of mastoparan, the two Ca2+ ions bind independently of one another. Our results appear to support the model that describes the relationship between the two Ca2+ binding sites as "side-by-side," because this model allows the possibility of independent Ca2+ entry to the two sites. Mastoparan shifts the steady-state equilibrium between E1'Ca2 and E1'Ca2.P toward E1'Ca2.P, by possibly affecting the conformational change that follows ATP binding. The peptide also causes a reduction in the levels of phosphoenzyme formed from [32P]Pi. Some analogues of mastoparan were also tested and were found to cause inhibition of the Ca2+-ATPase in the range of 2-4 microM. The inhibitory action of mastoparan and its analogues appears dependent on their ability to form alpha-helices in membranes.