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

Myosin II regulatory light chain (RLC) phosphorylation by Ca2+/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 Ca2+-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 [Ca2+](i) increased MLCK activation and RLC phosphorylation coincidently. There was no evidence for CaM binding but not activating MLCK at low [Ca2+](i). At saturating [Ca2+](i), MLCK was not fully activated probably due to limited availability of cellular Ca2+/CaM. (C) 2003 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.