We cloned the full-length cDNA for the cytoplasmic myosin II regulatory light chain (RLC) from a stage 1-2 Xenopus oocyte library. The Xenopus RLC is 94% identical to the chicken smooth muscle myosin RLC. All of the protein kinase C and myosin light chain kinase phosphorylation sites are conserved. Using trifluoperazine [Trybus, K. M., Waller, G. S., & Chatman, T. A. (1994) J. Cell Biol. 124, 963-969], we removed the RLC of smooth muscle myosin and replaced it with recombinant Xenopus RLCs. The wild-type Xenopus RLC substitutes for the gizzard RLC in actin-activated ATPase and in vitro motility assays. We made alanine substitutions of the two residues phosphorylated by myosin light chain kinase, Ser-19 and Thr-18. All of the myosin hybrids, regardless of their mutations or phosphorylation, have similar K+EDTA ATPase activities. As expected, the T18A, S19A hybrid had no actin-activated ATPase, whereas the T18A hybrid phosphorylated on Ser-19 had an actin-activated ATPase similar to that of wild-type hybrids phosphorylated only on Ser-19. The actin-activated ATPase of myosin phosphorylated only on Thr-18 is approximately 15-fold lower than that of myosin phosphorylated on Ser-19. Phosphorylation of either Ser-19 or Thr-18 permits the formation of filaments. Remarkably, in the gliding filament assay, myosin phosphorylated only on Thr-18 moves actin filaments at velocities similar to myosin phosphorylated on Ser-19 or both Thr-18 and Ser-19.
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