BUNDLING OF ACTIN-FILAMENTS BY MYOSIN LIGHT-CHAIN KINASE FROM SMOOTH-MUSCLE

Myosin light chain kinase has an inhibitory effect on the interaction of actin filaments with phosphorylated smooth muscle myosin. Myosin li ght chain kinase binds to actin filaments, and the inhibition is attri butable to the actin-binding activity and not the kinase activity of m yosin light chain kinase [Kohama et al. (1992) Biochem. Biophys. Res. Commun. 184, 1204-1211]. We now report that myosin light chain kinase is able to assemble actin filaments into thick bundles, which can be v isualized by optical and electron microscopy and can be monitored by m easuring the sedimentation and flow birefringence of actin filaments. The bundling activity of myosin light chain kinase is abolished by cal modulin in the presence of Ca2+. The possibility is discussed that myo sin light chain kinase has multiple actin-binding sites through which it can cross-link actin filaments. (C) 1994 Academic Press, Inc.