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

Regulatory light chain of myosin II (MRLC) was identified as a novel substrate of p90 ribosomal S6 kinase (RSE)-2, a Ser/Thr protein kinase which is phosphorylated and acti-vated 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 gluta-thione S-transferase (GST) fusion proteins of HeLa MRLC2 revealed that RSK-2 phos-phorylated wild-type MRLC2 (GST-wtMRLC2) but not its mutants GST-MRLC2S19A or GST-MRLC2T18ASiaA (alanine substituted for Serl9 or both Serl9 and ThrlS). These results revealed that RSK-2 phosphorylates MRLC at Serl9 as did MLCK. Phosphorylation of myosin II by RSK-2 resulted in activation of actin-activated MgATPase activity of myo-sin II. Interestingly, RSK-2 activity to phosphorylate MRLC was suppressed by phos-phorylation with MAPK. RSK-2 might be a mediator that regulates myosin II activity through the MAPK cascade