Calcium regulated thin filaments from molluscan catch muscles contain a caldesmon-like regulatory protein.

The thin filaments of the anterior byssus retractor muscle of the edible mussel Mytilus and the transluscent and opaque adductors of the oyster Crassostrea have been isolated and their properties investigated. We find that the thin filaments from all three muscles can activate skeletal muscle myosin ATPase in the presence of calcium but that the activity is inhibited in its absence. The filaments contain a protein which interacts with antibodies to vertebrate smooth muscle caldesmon on immunoblots. The antibodies relieve the inhibition of the thin-filament-activated myosin MgATPase. They can also bundle the thin filaments. We conclude that a caldesmon-like protein is present in molluscan muscle. As in the vertebrate smooth muscle, it could act as part of a control mechanism in addition to the myosin regulatory system. Vertebrate smooth muscle caldesmon can crosslink actin and myosin and it has been suggested that it may in this way contribute to the latch state. A similar interaction may be involved in the catch mechanism in molluscan muscle.