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Myosin Light Chain Kinase

The contractile interaction between actin microfilaments and conventional smooth muscle and non-muscle myosin II is regulated by phosphorylation of the 20kDa regulatory light chains by myosin light chain kinase (MLCK).  MLCK binds both actin (Sellers & Pato, 1984; Smith et al, 1999; Smith & Stull, 2000; Hatch et al, 2001) and myosin  II.  The MLCK activity is Ca2+-calmodulin dependent. The Kd of MLCK for actin is 4mM (Sellers & Pato, 1984), through an N-terminus region that contains three repeats DFRxxL motifs (Smith et al, 1999).  Image reconstruction indicates that MLCK binds the actin filament in a manner that allows the simultaneous binding of the other major thin filament components, calponin, tropomyosin etc.  The binding site uniquely is suggested to be on the outside of sub-domain 1 and perhaps the 228-232 region (sub-domain 4) on another actin monomer.  Two MLCK genes are expressed one in muscle and the other in smooth muscle and non-muscle tissues.  This latter gene is expressed by differential splicing as a long and a shorter form (Herring et al, 2000).   There is approximately one MLCK molecule for every 100 actin molecules in smooth muscle (Hatch et al, 2001), each MLCK contains at least three of the DFRxxL motifs and so each molecule binds three actins (Smith et al, 1999; Smith & Stull, 2000).

References:-

Birukov, K. G., Csortos, C., Marzilli, L., Dudek, S., Ma, S.-F., Bresnick, A. R., Verin, A. D., Cotter, R. J. & Garcia, J. G. N. (2001) Differential regulation of alternatively spiced endothelial cell myosin light chain kinase isoforms by p60Src., J.Biol.Chem. 276, 8567-8573

Hatch, V., Zhi, G., Smith, L., Stull, J. T., Craig, R. & Lehman, W. (2001) Myosin light chain kinase binding to a unique site on F-actin revealed by three-dimensional image reconstruction., J. Cell Biol. 154, 611-617.

Herring, B. P., Dixon, S. & Gallagher, P. J. (2000) Smooth muscle myosin light chain kinase expression in cardiac and skeletal muscle, Am.J.Physiol. 279, C1656-C1664.

Sellers, J.R. & Pato, M.D. (1984). J.Biol.Chem. 259, 7740-7746.

Smith, L. &  Stull, J. T. (2000). “Myosin light chain kinase binding to actin filaments.” FEBS letters 480(2-3), 298-300.

Smith, L., Su, X., Lin, P.-j., Zhi, G. & Stull, J. T. (1999) Identification of a novel actin binding binding motif in smooth muscle myosin light chain kinase., J. Biol.Chem. 274, 29433-29438.

 
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