Cyclic AMP and calcium in the differential control of Mytilus gill cilia

Abstract

Lateral (L) cilia of Mytilus gill are activated by serotonin which, in molluscan systems, is known to activate adenylate cyclase. Triton-extracted models of L-cells, arrested at greater than 10(-6) M Ca++, are stimulated to beat by the addition of 10(-5) M cAMP while still under Ca++ arrest conditions, suggesting that cAMP-activation is not mediated by alterations of Ca++ levels. Using isolated, permeabilized cilia, we find, independent of [Ca++], that cAMP-dependent protein phosphorylation in L-cilia occurs uniquely and reversibly on three low molecular weight polypeptides of 23,000, 18,000, and 14,000 daltons. Phosphorylation is maximal at cAMP concentrations above 0.5 microM. The phosphorylated chains partially co-extract at high salt with a 14S dynein fraction and have approximately the same molecular weights as reported for dynein light chains. Such conditions mainly extract the outer dynein arm, about 40% of the Mg++-ATPase activity, and a corresponding amount of the cAMP phosphorylated chains. However, the three polypeptides sediment together at 10-11S, clearly separable from the 14S dynein ATPase. Using a gel-overlay technique, we find that calmodulin binds to axonemal polypeptides of L-cilia with molecular weights of 18,000 and 13,000, independent of Ca++, while in mixed-population cilia, only a 12,000 dalton chain binds calmodulin, in a Ca++ dependent manner. In neither case are calmodulin binding proteins found in the high salt fraction containing the cAMP-dependent phosphorylated chains, indicating that, in spite of some similarity in molecular weight, the cAMP-phosphorylated and calmodulin binding polypeptides are different. Also, double-labelling indicates that only the 18,000 dalton chains co-migrate.(ABSTRACT TRUNCATED AT 250 WORDS)