Characterization of Tightly Associated Smooth Muscle Myosin–Myosin Light-Chain Kinase–Calmodulin Complexes

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A current popular model to explain phosphorylation of smooth muscle myosin (SMM) by myosin light-chain kinase (MLCK) proposes that MLCK is bound tightly to actin but weakly to SMM. We found that MLCK and calmodulin (CaM) co-purify with unphosphorylated SMM from chicken gizzard, suggesting that they are tightly bound. Although the MLCK:SMM molar ratio in SMM preparations was well below stoichiometric (1:73±9), the ratio was ∼23–37% of that in gizzard tissue. Fifteen to 30% of MLCK was associated with CaM at ∼1 nM free [Ca2+]. There were two MLCK pools that bound unphosphorylated SMM with Kd∼10 and 0.2 μM and phosphorylated SMM with Kd∼20 and 0.2 μM. Using an in vitro motility assay to measure actin sliding velocities, we showed that the co-purifying MLCK–CaM was activated by Ca2+ and phosphorylation of SMM occurred at a pCa50 of 6.1 and at a Hill coefficient of 0.9. Similar properties were observed from reconstituted MLCK–CaM–SMM. Using motility assays, co-sedimentation assays, and on-coverslip enzyme-linked immunosorbent assays to quantify proteins on the motility assay coverslip, we provide strong evidence that most of the MLCK is bound directly to SMM through the telokin domain and some may also be bound to both SMM and to co-purifying actin through the N-terminal actin-binding domain. These results suggest that this MLCK may play a role in the initiation of contraction.