cAMP action appears to be certain low molecular mass polypeptides that co-extract with the ciliary ATPase dynein (Stommel and Stephens, 1985). Phosphorylation of these polypeptides by the catalytic subunit of cAMP-dependent protein kinase is correlated with the acquisition of motility

In the gill of the blue mussel Mytilus edulis, the lateral cilia, which are responsible for producing feeding currents, are neuronally activated (Aiello, 1960). The neurotransmitter is serotonin (Aiello, 1962), known to raise cAMP levels in the gill (Malanga and Poll, 1979). The functional target of cAMP action appears to be certain low molecular mass polypeptides that co-extract with the ciliary ATPase dynein (Stommel and Stephens, 1985). Phosphorylation of these polypeptides by the catalytic subunit of cAMP-dependent protein kinase is correlated with the acquisition of motility in ATP-reactivated cell models (Stephens and Stommel, 1989). Mounting evidence points to these target polypeptides as being dynein light chains and, even though the labile nature of Mytilus ciliary dynein has left this conclusion somewhat equivocal (Stephens and Prior, 1990a, 1991c), preliminary evidence for Spisula ciliary dynein strongly supports it (Stephens and Prior, 1991b). Serotonin also has been implicated in both natural spawning and sperm motility of bivalve molluscs (Lubet, 1981; Gibbons and Castagna, 1984; Hirai et al., 1988; Kadam and Koide, 1990), and there is now substantial pharmacological evidence for the presence of serotonin receptors on sperm of the surf clam Spisula solidissima (Kadam and Koide, 1990; Bandivdekar et al., 1992). The target of serotonin action, if any, at the level of the motile machinery of the sperm has not been studied. Since Mytilus and Spisula have both gill cilia and sperm flagella that can be activated in situ by serotonin, we carried out a comparative study of ciliary and flagellar dynein from these species, with emphasis on the target polypeptides of cAMP-dependent phos999 Journal of Cell Science 103, 999-1012 (1992) Printed in Great Britain © The Company of Biologists Limited 1992