A current popular model to explain phosphorylation of smooth muscle myosin (SMM) by smooth muscle 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 (up-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 μM and 0.2 μM and phosphorylated SMM with a Kd ~ 20 μM 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 Hill coefficient of 0.9. Similar properties were observed from reconstituted MLCK-CaM-SMM. Using motility assays, co-sedimentation assays, and on-coverslip ELISA 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.

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The June 29, 1992, Little Skull Mountain earthquake (M=5.6) was recorded at nine strong-motion stations in Las Vegas, Nevada. Ground motion on alluvial sites is much greater than on rock sites, especially at low frequencies, and had a longer duration than expected. Five of the seven stations on sediment have Fourier spectral amplifications that average at least a factor of five greater than the rock reference sites from 0.5 to 2 Hz. Thus, large earthquakes at moderate distances, that would excite strong long-period waves, are a concern. We generated synthetic seismograms, including the site amplification, for magnitude 7.4 earthquakes on the Death Valley fault system 150km from Las Vegas. The synthetics have average peak accelerations ranging from 0.058 to 0.13g at the rock and from 0.051 to 0.22g at the sediment sites. The average peak velocity and displacement is 49 cm/sec and 33 cm, respectively, on the larger component of a sediment site.

Background: SMM is activated by RLC phosphorylation in the lever arm. Results: Modifying RLC-ELC interaction hampers the ability of phosphorylation to activate motor functions. Conclusion: A major consequence of phosphorylation is to stabilize RLC-ELC interactions and associated conformations of the lever arm elbow. Significance: Learning how this myosin is regulated furthers the understanding of activation and relaxation of smooth muscle contraction.