Cardiac F-Actin has Two Functional States

Abstract

Previously, in mouse cardiac myofibrils, we showed that (i) strong cross-bridges (XBs) prevent the exchange of cTn within the A-band and (ii) ATP-induced release of strong XB allowed nearly uniform cTn exchange within the thin filament. Here, we examined the mechanism for XB-dependent exchange of cTn. We hypothesized that the affinity of cTn for the thin filament depended on the XB-dependent changes in the structure and/or dynamics of actin. To test this hypothesis, we stained rigor and Mg2+ATP-saturated myofibrils with rhodamine-phalloidin. We found that strong XBs inhibit phalloidin from associating with F-actin. Furthermore, the distribution of rhodamine-phalloidin staining of actin coincided with the distribution of exchanged cTn. Our results suggest the presence of at least two distinct conformations of F-actin--- a phalloidin-binding (relaxed) conformation and phalloidin-nonbinding (strained) conformation. A strongly bound XB places the associated actin in a strained conformation. The strained and relaxed conformations have functional significance: the affinity of cTn for the thin filament is approximately 100-times higher when the associated actin is in the strained conformation. We suggest that dynamic conformational changes in actin may play a role in the activation of the myofilament.