pH-sensitive, Ca2+/calmodulin-dependent phosphorylation of unique protein in molluscan nervous system

Intracellular pH and Ca2+ are prominent co-regulators of neuron excitability that act on ion channels. In looking for a possible mechanism of their action, we tested their combinatorial effect on the phosphorylation state of nervous system proteins. 32PO4 labelling in endogenous phosphorylation reactions of homogenates of nervous tissue of the sea-slug Pleurobranchaea showed steep pH sensitivity in protein migrating at a molecular mass of 108 kDa with pI 6.9-7.0 (pp108). Phosphorylation of pp108 was highest below reaction pH 7.0 and declined steeply as pH rose to 7.4 pp108 phosphorylation was Ca2+/calmodulin-dependent. pp108 constituted a significant part of the total protein (0.15%) and phosphoprotein (8.9%) of the nervous system. The specifically and uniquely combinatorial pH and Ca2+ sensitivity of the phosphorylation of pp108, and its relative abundance, suggest that it could mediate integrated actions of H+ and Ca2+ in the molluscan neuron.