FIBROBLAST CONTRACTILITY WITHOUT AN INCREASE IN BASAL MYOSIN LIGHT-CHAIN PHOSPHORYLATION IN WILD-TYPE CELLS AND CELLS EXPRESSING THE CATALYTIC DOMAIN OF MYOSIN LIGHT-CHAIN KINASE

We investigated the role of myosin light chain (MLC(20)) phosphorylati on (MLC-P) in non-muscle contractility by comparing MLC-P and the cont ractile properties of wild type 3T3 fibroblasts and 3T3 fibroblasts ex pressing the catalytic domain of myosin light chain kinase (tMK), MLC- P is 0.96 mol of PO4/mol of MLC(20) in cells expressing tMK compared t o 0.20 mol of PO4/mol of MLC(20) in control cells, Expressing tMK also results in a 2-fold increase in cortical stiffness compared to contro l cells. Contractile properties were quantified by growing wild type a nd transfected fibroblasts in collagen and attaching the ensuing fiber s to an apparatus for performing mechanical measurements, Serum stimul ation resulted in a dose-dependent increase in force with maximal forc e generated in the presence of 30% (v/v) serum. Surprisingly, MLC-P di d not increase in wild type cells following stimulation with 30% serum , and tMK expression did not affect the contractile properties of fibe rs made from these cells. Moreover, the dose responses to serum, maxim al force, force-Velocity relationships, and dynamic stiffness were sim ilar in the wild type cells and fibroblasts expressing tMK. These data demonstrate that non-muscle cells can generate force without an incre ase in MLC-P, and that an increase in MLC-P does not affect the contra ctile properties of fibroblast fibers.