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-hug 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⁺/calmodium-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.