Kinetic scheme of myosin phosphorylation by ZIP kinase

Zipper-interacting protein kinase (ZIPK) is a ubiquitous serine/threonine protein kinase that plays pivotal roles in regulating cell motility, division, and smooth muscle contractility through phosphorylation of myosin. In this study, we systematically investigated the phosphorylation reactions of smooth muscle myosin (SMM) by ZIPK. We found that ZIPK phosphorylates MRLC sequentially, first at Ser ¹⁹ and then at Thr ¹⁸ , determined by quantitative mass spectrometry analysis on wild-type MRLC. Analysis on phosphomimic and unphosphorylatable MRLC mutants indicates that the phosphorylation rate at Ser ¹⁹ on unphosphorylated MRLC is 1.5 times faster than that at Thr ¹⁸ on Ser ¹⁹ -phosphorylated MRLC. Comparison between SMM and isolated MRLC revealed that the phosphorylation rate of SMM is slower than that of isolated MRLC. To dissect the molecular mechanism responsible for this difference, we measured interactions between ZIPK and SMM by co-sedimentation assay. The result suggests that the C-terminal domain of ZIPK interacts with the heavy chain of SMM, and as a result, competitive binding of ZIPK to MRLC and the myosin heavy chain suppresses phosphorylation of SMM compared to isolated MRLC. By incorporating the kinetic and dissociation constants obtained from mutant analysis and co-sedimentation assays, respectively, a simple kinetic model accurately reproduced the time courses of phosphorylation for both isolated MRLC and SMM. This provides systemic insight into the regulatory mechanism of myosin contractility by ZIPK.