Background: In contrast to the myosins of vertebrate skeletal muscle, molluscan myosins are regulated molecules whose enzymatic activity is switched on by the direct binding of Ca2+. The head portion (S1) of the molecule consists of a motor domain and a regulatory domain (RD) containing a 'regulatory' and an 'essential' light chain (RLC and ELC, respectively). The structures of scallop myosin RD with bound Ca2+, as well as the S1 fragment of chicken skeletal muscle myosin, have been determined previously to 2.8 A resolution.
Results: We have determined the structure at 2.0 A resolution of scallop myosin RD with bound Ca2+. The unusual coordination at the specific Ca(2+)-binding site in the ELC has now been clarified, as has the structural basis for Mg2+ binding to the RLC. A comparison of the scallop RD structure with that in the chicken S1 structure shows differences in the bending of the two RDs in two different places.
Conclusions: Based on these structural results, a model for regulation is proposed in which the Ca(2+)-bound RD is a rigid structure, and transient flexibility of the Ca(2+)-free RD allows the myosin heads to make stabilizing intramolecular linkage which shut off the motor.