The interacting-heads motif (IHM) is a structure of myosin that has been proposed to
modulate cardiac output by occluding myosin molecules from undergoing the force-
generating cycle. It is hypothesized to be the structural basis for the super-relaxed state
(SRX), a low-ATPase kinetic state thought to be cardioprotective. The goal of the present
study was to test this hypothesis by determining directly and quantitatively the fractions of
myosin in the IHM and SRX under the same conditions in solution. To detect the structural …