Quantitative measurements of Ca2+/calmodulin binding and activation of myosin light chain kinase in cells

Abstract Myosin II regulatory light chain (RLC) phosphorylation by Ca 2+ /calmodulin (CaM)-dependent myosin light chain kinase (MLCK) is implicated in many cellular actin cytoskeletal functions. We examined MLCK activation quantitatively with a fluorescent biosensor MLCK where Ca 2+ -dependent increases in kinase activity were coincident with decreases in fluorescence resonance energy transfer (FRET) in vitro. In cells stably transfected with CaM sensor MLCK, increasing [Ca 2+ ] i increased MLCK activation and RLC phosphorylation coincidently. There was no evidence for CaM binding but not activating MLCK at low [Ca 2+ ] i . At saturating [Ca 2+ ] i MLCK was not fully activated probably due to limited availability of cellular Ca 2+ /CaM.