Activation of Actin-Activated MgATPase Activity of Myosin II by Phosphorylation with MAPK-Activated Protein Kinase-1 b (RSK-2)

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Regulatory light chain of myosin II (MRLC) was identified as a novel substrate of p 90 ribosomal S 6 kinase (RSK)-2, a Ser/Thr protein kinase which is phosphorylated and activated by mitogen-activated protein kinase (MAPK) in vitro and in vivo. Phosphopeptide map of MRLC phosphorylated by RSK-2 was identical to that by myosin light chain kinase (MLCK). Phosphoserine was recovered by the phosphoamino acid analysis of MRLC phosphorylated by RSK-2. Further, phosphorylation using recombinant glutathione S-transferase (GST) fusion proteins of HeLa MRLC 2 revealed that RSK-2 phosphorylated wild-type MRLC 2 (GST-wtMRLC 2) but not its mutants GST-MRLC 2^{S 19 A} or GST-MRLC 2^{T 18 A 19 A} (alanine substituted for Ser 19 or both Ser 19 and Thr 18). These results revealed that RSK-2 phosphorylates MRLC at Ser 19 as did MLCK. Phosphorylation of myosin II by RSK-2 resulted in activation of actin-activated MgATPase activity of myosin II. Interestingly, RSK-2 activity to phosphorylate MRLC was suppressed by phosphorylation with MAPK. RSK-2 might be a mediator that regulates myosin II activity through the MAPK cascade.