Muscle contraction is the result of tiny myosin molecular motors that are assembled into filaments and convert the chemical energy stored in ATP into the power muscle generates. However, in relaxed muscle, these myosins continue to slowly hydrolyze ATP, with some of the chemical energy released as heat—analogous to the engine of an idling automobile. However, not all relaxed myosin molecules are the same! By some estimates, 50% or more of the myosin are folded into a sequestered state known as the “super relaxed” state of myosin. This sequestration is thought to regulate the basal metabolic rate of muscle, as well as the availability of myosin heads to generate power. However, the details of how these super relaxed myosin are regulated and whether they are spatially distributed throughout the myosin filament have remained elusive. Our study addresses these details using single-molecule biophysical techniques.
The cover image for the September 15 issue of Biophysical Journal combines Guy Kennedy’s rendering of skeletal muscle myosin motors assembled into a single filament, hydrolyzing molecules of fluorescently tagged ATP, with an experimental image of single molecules of ATP within a myofibril—the subcellular organelle comprised of numerous myosin filaments.
The upper portion illustrates a myosin filament, as inspired by structural data from Cryo EM and X-ray crystallography. Protruding from the filament are the two-headed myosin molecules (purple), with the core of the filament formed from the myosins’ slender tails. Fluorescently tagged ATP molecules (glowing green balls) are freely diffusing (squiggly green lines) in solution until bound to the catalytic site in the myosin head. Some of the myosin are folded into the super relaxed state (pink), predominantly within a select region of the filament that is also populated by the regulatory protein Myosin-Binding Protein C (MyBP-C), which is shown in yellow.
The lower portion of the cover is a high-resolution microscopic image of a skeletal muscle myofibril comprised of many myosin filaments. Fluorescently labeled myomesin (red), a protein only found at the center of the myosin filament and spaced 2.2 microns apart, provides a landmark to map the binding location of individual fluorescent ATP molecules (green). These data provide spatial and temporal information necessary to describe the rate at which ATP is hydrolyzed and where the super relaxed myosin exist along the filament.
Findings from this study implicate MyBP-C in stabilizing myosin motors into the super relaxed state and, consequently, the potential for regulating resting muscle metabolism and how many myosin motors can be recruited to generate force upon activation. Since mutations to MyBP-C are a leading cause of cardiac and skeletal muscle myopathies that are marked by hypercontractility, the inability of mutant MyBP-C to sequester myosin in the super relaxed state might be a potential mechanism for the disease state.
For more information about our research, please visit http://physioweb.uvm.edu/warshaw-lab/.
- Shane R. Nelson, Amy Li, Samantha Beck-Previs, Guy G. Kennedy, David M. Warshaw