Ca2+-independent Smooth Muscle Contraction

Smooth muscle contraction follows an increase in cytosolic Ca(2+) concentration, activation of myosin light chain kinase, and phosphorylation of the 20-kDa light chain of myosin at Ser(19). Several agonists acting via G protein-coupled receptors elicit a contraction without a change in Ca(2+)(i) via inhibition of myosin light chain phosphatase and increased myosin phosphorylation. We showed that microcystin (phosphatase inhibitor)-induced contraction of skinned smooth muscle occurred in the absence of Ca(2+) and correlated with phosphorylation of myosin light chain at Ser(19) and Thr(18) by a kinase distinct from myosin light chain kinase. In this study, we identify this kinase as integrin-linked kinase. Chicken gizzard integrin-linked kinase cDNA was cloned, sequenced, expressed in E. coli, and shown to phosphorylate myosin light chain in the absence of Ca(2+) at Ser(19) and Thr(18). Subcellular fractionation revealed two distinct populations of integrin-linked kinase, including a Triton X-100-insoluble component that phosphorylates myosin in a Ca(2+)-independent manner. These results suggest a novel function for integrin-linked kinase in the regulation of smooth muscle contraction via Ca(2+)-independent phosphorylation of myosin, raise the possibility that integrin-linked kinase may also play a role in regulation of nonmuscle motility, and confirm that integrin-linked kinase is indeed a functional protein-serine/threonine kinase.