Direct detection of the myosin super-relaxed state and interacting heads motif in solution

Abstract We have used time-resolved fluorescence resonance energy transfer (TR-FRET) to detect the interacting-heads motif (IHM) of β-cardiac myosin in solution. Evidence for the IHM has been observed by several structural techniques, and it has been proposed to be the structural basis for the super-relaxed state (SRX), a low-ATPase state of myosin that has been observed biochemically in skinned muscle fibers using fluorescent ATP. It has been proposed that the disruption of this state, by mutation or chemical modification, is a major cause of heart disease, so drugs are being developed to stabilize it. The goal of the present study is to determine directly and quantitatively the correlation between the measured fractions of myosin in the IHM state and the SRX state under the same conditions in solution. We used TR-FRET to measure the distance between the two heads of bovine cardiac myosin, and found that there are two distinct populations, one of which is observable by FRET at a center distance of 2.0 nm, and the other is not detected, implying a distance greater than 4 nm. Under the same conditions, we also measured the fraction of heads in the SRX state using fluorescent nucleotide and stopped-flow kinetics. We found that, in the absence of crosslinking, the population of SRX exceeded that of IHM. In particular, the stabilizing effect of mavacamten was much greater on SRX (55% increase) than on IHM (4% increase). We conclude that the SRX and IHM states are related, but they are not identical.