Differential distribution of cAMP-dependent protein kinase isoforms in various tissues of the bivalve mollusc Mytilus galloprovincialis

The cAMP signalling pathway is involved in the regulation of basic physiological processes in bivalve molluscs. We had previously identified and characterized two isoforms of cAMP-dependent protein kinase (PKA) from the sea mussel Mytilus galloprovincialis that differ at their regulatory (R) subunit, namely, Rmyt1 or Rmyt2. Here we investigated the immunohistochemical expression of both PKA isoforms in various mussel tissues. Rmyt1 and Rmyt2 displayed a complementary subcellular localization. In general, Rmyt1 was found to be uniformly distributed in the cytoplasm of most cell types, whereas Rmyt2 appears to be localized only in the cell periphery and associated with certain cellular structures, such as the cilia of labial palps and gill filaments. Thus, both PKA isoforms appear to be non-redundant, but they have specific functions. Rmyt1 was the main isoform present in catch muscle fibers, which suggests that PKAmyt1 may be the isoform involved in the regulation of the catch state. Conversely, Rmyt2 was the only isoform detected in the cilia of gill filaments, indicating that PKAmyt2 could mediate the effects of cAMP on the ciliary beat frequency.