Calcium control of actin-myosin based contraction in triton models of mouse 3T3 fibroblasts is mediated by the myosin light chain kinase (MLCK)-calmodulin complex.

Triton extraction of mouse 3T3 cells provides cellular models able to contract in a Ca2+-dependent manner upon exposure to MgATP. Such models reveal myosin kinase (MLCK) along their microfilaments and contraction is inhibited by antigen affinity-purified antibodies to gizzard MLCK. When extraction is performed at higher salt concentrations the models become non-contractile and immunofluorescence microscopy fails to detect the kinase. Inactive models can be functionally reactivated by exposure to exogenously provided purified kinase plus calmodulin present in the normal buffer. Reconstituted models again reveal microfilament-associated kinase. Functional reconstitution requires both the addition of kinase and calmodulin and does not occur in the presence of trifluoperazine. The combined results indicate that contractility of non-muscle cellular models is controlled by calmodulin-dependent MLCK acting on the actin-myosin-containing microfilaments with a calcium control exerted in the phosphorylation step of myosin.