Myosin Light Chain Phosphatase

Although several myofilament proteins are modified by protein phosphorylation, the 18-kDa myosin light chain 2 (MLC2) has special significance. In striated muscle, after some experimental struggles, MLC2 phosphorylation was shown to modulate myofilament activation by Ca2+.1 However, in the case of smooth muscle, phosphorylation of the MLC2 by a Ca2+ calmodulin–dependent kinase triggers contraction.2 The state of MLC2 phosphorylation in smooth muscle determines whether crossbridges are turned off, cycling, or in a latch or catch-like state. Understandably, the initial focus of experiments was on Ca2+ and the regulation of MLC2 kinase (MLCK) activity. It was always understood that control of the level of MLC phosphorylation requires some balance of activities and separation of powers between an MLCK and MLC2 phosphatase (MLCP),3 yet little attention was given to the possibility that the activity of MLCP could be modulated. Now MLCP seems to be the Cinderella of phosphoryl group transfer enzymes.This mindset concerning regulation of MLC2 phosphorylation changed dramatically with two observations. One was evidence that addition of GTP-γ-S to permeable preparations of smooth muscle was able to sensitize the myofilaments to Ca2+ and slow down relaxation, alterations suggesting modulation of MLCP.4 A second was the elucidation of the functional domains of MLCP,5 which consist of a 37-kDa catalytic subunit, a 20-kDa subunit of unknown function, and a 110- to 130-kDa subunit that targets MLCP to myosin. This targeting domain of MLCP, which is termed myosin binding site (MBS) or myosin phosphatase targeting peptide, binds to myosin or MLC2 and promotes catalytic activity of the 37-kDa domain. MBS was also shown to contain Ser and Thr residues, which, when phosphorylated, inhibit the ability of MBS to activate the catalytic domain, thereby reducing activity of the MLCP holoenzyme.5 A significant pathway …