MYOSIN LIGHT-CHAIN PHOSPHORYLATION CONTROLS INSULIN-SECRETION AT A PROXIMAL STEP IN THE SECRETORY CASCADE

The aim of this study was to investigate-how insulin secretion is cont rolled by phosphorylation of the myosin light chain (MLC). Ca2+-evoked insulin release from pancreatic islets permeabilized with streptolysi n O was inhibited by different monoclonal antibodies against myosin li ght-chain kinase (MLCK) to an extent parallel to their inhibition of p urified MLCK. Anti-MLCK antibody also inhibited insulin release caused by the stable GTP analog guanosine 5'-O-(3-thiodiphosphate), even at a substimulatory concentration (0.1 mu M) of Ca2+. Free Ca2+ increased MLC peptide phosphorylation by beta-cell extracts in vitro. In contra st to the phosphorylation by purified MLCK or by calmodulin (CaM) kina se II, the activity partially remained with the beta-cell under nonsti mulatory Ca2+ (0.1 mu M) conditions. The MLCK inhibitor ML-9 inhibited the activity in the beta-cell with both substimulatory and stimulator y Ca2+ whereas KN-62, an inhibitor of CaM kinase II, only exerted an i nfluence in the latter case. ML-9 decreased intracellular granule move ment in MIN6 cells under basal and acetylcholine-stimulated conditions . We propose that MLC phosphorylation may modulate translocation of se cretory granules, resulting in enhanced insulin secretion.