D Mizuno, C Tardin, CF Schmidt, FC MacKintosh, 
Science, 315:370 (2007). (full text)

Abstract: Cells both actively generate and sensitively react to forces through their mechanical framework, the cytoskeleton, which is a nonequilibrium composite material including polymers and motor proteins. We measured the dynamics and mechanical properties of a simple three-component model system consisting of myosin II, actin filaments, and cross-linkers. In this system, stresses arising from motor activity control the network mechanics, increasing stiffness by a factor of nearly 100 and qualitatively changing the viscoelastic response of the network. We present a quantitative theoretical model connecting the large-scale properties of this active gel to molecular force generation.