The living state of biological cells manifests itself through mechanical motion on many length scales. Behind this motion are processes that generate and transform mechanical forces of various types. As with other cell functions, the machinery for cellular mechanics involves proteins. Their flexible structures can be deformed and restored, and are often essential for handling, transforming, and using mechanical force. For instance, proteins of muscle and the extracellular matrix exhibit salient elasticity upon stretching, mechanosensory proteins transduce weak mechanical stimuli into electrical signals, and so-called regulatory proteins force DNA into loops controlling, thereby, gene expression. In a recent review, the structure-function relationship of four protein complexes with well defined and representative mechanical functions has been described. The first protein system reviewed is titin, a protein that confers passive elasticity on muscle. The second system reviewed is the elastic extracellular matrix protein fibronectin and its cellular receptor integrin. The third protein system covered are the proteins cadherin and ankyrin involved in the transduction apparatus of mechanical senses and hearing. The last system surveyed is the lac repressor,  a protein which regulates gene expression by looping DNA. In each case, molecular dynamics simulations using NAMD provided insights into the physical mechanisms underlying the associated mechanical functions of living cells. (more on our mechanobiology web site).