Activation of actin-activated MgATPase activity of myosin II by phosphorylation with MAPK-activated protein kinase-1b (RSK-2)

Regulatory light chain of myosin II (MRLC) was identified as a novel substr ate of p90 ribosomal S6 kinase (RSK)-2, a Ser/Thr protein kinase which is p hosphorylated and activated by mitogen-activated protein kinase (MAPK) in v itro and in vivo. Phosphopeptide map of MRLC phosphorylated by RSK-2 was id entical to that by myosin light chain kinase (MLCK), Phosphoserine was reco vered by the phosphoamino acid analysis of MRLC phosphorylated by RSK-2, Fu rther, phosphorylation using recombinant glutathione S-transferase (GST) fu sion proteins of HeLa MRLC2 revealed that RSK-2 phosphorylated wild-type MR LC2 (GST-wtMRLC2) but not its mutants GST-MRLC2(S19A) or GST-MRLC2(T18AS19A ) (alanine substituted for Ser19 or both Ser19 and Thr18). These results re vealed that RSK-2 phosphorylates MRLC at Ser19 as did MLCK, Phosphorylation of myosin II by RSK-2 resulted in activation of actin-activated MgATPase a ctivity of myosin II, Interestingly, RSK-2 activity to phosphorylate MRLC w as suppressed by phosphorylation with MAPK, RSK-2 might be a mediator that regulates myosin II activity through the MAPK cascade.