ABSTRACT The conversion of mechanical stress into a biochemical signal in a muscle cell requires a force sensor. Titin
kinase, the catalytic domain of the elastic muscle protein titin, has been suggested as a candidate. Its activation requires major
conformational changes resulting in the exposure of its active site. Here, force-probe molecular dynamics simulations were
used to obtain insight into the tension-induced activation mechanism. We find evidence for a sequential mechanically induced
opening of the catalytic site without complete domain unfolding. Our results suggest the rupture of two terminal b-sheets as the
primary unfolding steps. The low force resistance of the C-terminal relative to the N-terminal b-sheet is attributed to their
different geometry. A subsequent rearrangement of the autoinhibitory tail is seen to lead to the exposure of the active site, as is
required for titin kinase activity. These results support the hypothesis of titin kinase as a force sensor.