Molluscan smooth catch muscle contains calponin but not caldesmon

We isolated Ca2+-regulated thin filaments from the smooth muscle of the mussel Crenomytilus grayanus and studied the protein composition of different preparations from this muscle: whole muscle, heat-stable extract, fractions from heat-stable extract, thin filaments and intermediate stages of thin filaments purification. Among the protein components of the above-listed preparations, we did not find caldesmon (CaD), although two isoforms of a calponin-like (CaP-like) protein, which along with CaD is characteristic of vertebrate smooth muscle, were present in thin filaments. Thus, CaD is not Ca2+-regulator of thin filaments of this muscle. On the other hand, the mussel CaP-like protein is also not such Ca2+-regulator since we have shown that this protein can be selectively removed from isolated mussel thin filaments without loss of their Ca2+-sensitivity. We suggest that thin filaments in the smooth catch muscle possess other type of Ca2+-regulation, different from that in vertebrate smooth muscles.