Caldesmon inhibits nonmuscle cell contractility and interferes with the formation of focal adhesions

Caldesmon is known to inhibit the ATPase activity of actomyosin in a Ca2+-c almodulin-regulated manner. Although a nonmuscle isoform of caldesmon is wi dely expressed, its functional role has not yet been elucidated. We studied the effects of nonmuscle caldesmon on cellular contractility, actin cytosk eletal organization, and the formation of focal adhesions in fibroblasts. T ransient transfection of nonmuscle caldesmon prevents myosin II-dependent c ell contractility and induces a decrease in the number and size of tyrosine -phosphorylated focal adhesions. Expression of caldesmon interferes with Rh o A-V14-mediated formation of focal adhesions and stress fibers as well as with formation of focal adhesions induced by microtubule disruption. This i nhibitory effect depends on the actin- and myosin-binding regions of caldes mon, because a truncated variant lacking both of these regions is inactive. The effects of caldesmon are blocked by the ionophore A23187, thapsigargin , and membrane depolarization, presumably because of the ability of Ca2+-ca lmodulin or Ca2+-S100 proteins to antagonize the inhibitory function of cal desmon on actomyosin contraction. These results indicate a role for nonmusc le caldesmon in the physiological regulation of actomyosin contractility an d adhesion-dependent signaling and further demonstrate the involvement of c ontractility in focal adhesion formation.