Role of the N-terminal region of the regulatory light chain in the dephosphorylation of myosin by myosin light chain phosphatase

Myosin regulatory light chain (RLC) is phosphorylated at various sites at i ts N-terminal region, and heterotrimeric myosin light chain phosphatase (ML CP) has phosphorylates myosin in vivo. Specificity of MLCP toward the vario us phosphorylation sites of RLC was studied, as well as the role of the N-t erminal region of RLC in the dephosphorylation of myosin by MLCP. MLCP deph osphorylated phosphoserine 19, phosphothreonine 18, and phosphothreonine 9 efficiently with almost identical rates, whereas it failed to dephosphoryla te phosphorylated serine 1/serine 2. Deletion of the N-terminal seven amino acid residues of RLC markedly decreased the dephosphorylation rate of phos phoserine 19 of RLC incorporated in the myosin molecule, whereas this delet ion did not significantly affect the dephosphorylation rate of isolated RLC . On the other hand, deletion of only four N-terminal amino acid residues s howed no effect on dephosphorylation of phosphoserine 19 of incorporated RL C, The inhibition of dephosphorylation by deletion of the seven N-terminal residues was also found with the catalytic subunit of MLCP. Phosphorylation at serine 1/serine 2 and threonine 9 did not influence the dephosphorylati on rate of serine 19 and threonine 18 by MLCP. These results suggest that t he N-terminal region of RLC plays an important role in substrate recognitio n of MLCP.