Effects of the wasp venom peptide, mastoparan, on a phosphoinositide-specific phospholipase C purified from rabbit brain membranes

The wasp venom peptide, mastoparan (Ile-Asn-Leu-Lys-Ala-Leu-Ala-Ala-Leu-Ala-Lys-Lys-Ile-LeuNH2), activated phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis as catalyzed by a phosphoinositide-specific phospholipase C (PLC-Im) purified from rabbit brain membranes. This activation was found when the molar ratio of mastoparan to PIP2 was less than 1 and when the concentration of PIP2 exceeded 10 microM. PIP2 breakdown was inhibited at both high and low substrate concentrations if the molar ratio of mastoparan to PIP2 was greater than 1. The stimulatory effect of mastoparan correlated with its ability to restrict aggregation of PIP2 into higher order structures (liposomes or mixed deoxycholate/phospholipid micelles) as the concentration of PIP2 was increased to 10 microM or greater. Mastoparan stimulation of PIP2 breakdown required the presence of a higher calcium concentration than was necessary for detection of enzyme activity. Both the stimulatory and inhibitory effects of mastoparan on PIP2 hydrolysis were lost if 2.5 mM deoxycholate was present in the assays. Hydrolysis of phosphatidylinositol (PI) by PLC-Im was inhibited at all concentrations of mastoparan tested. These results show that both PIP2 and PI are suitable substrates for PLC-Im, depending on the physical characteristics of their aggregates in aqueous suspension. An amphiphilic alpha-helix-forming peptide such as mastoparan may modulate phospholipase C activity due to the peptide's interaction with phospholipid substrates.