Smooth muscle myosin: a high force-generating molecular motor

Smooth muscle generates as much force per cross sectional area of muscle as skeletal muscle with only one-fifth the myosin content. Although this apparent difference could be explained at the tissue or cellular level, it is possible that at the molecular level smooth muscle cross-bridges generate greater average force than skeletal muscle cross-bridges. To test this hypothesis, we used an in vitro motility assay (VanBuren et al., 1994) in which either chicken thiophosphorylated gizzard smooth or pectoralis skeletal muscle monomeric myosin is adhered to a nitrocellulose surface. A fluorescently labeled actin filament, attached to an ultracompliant (50-200 nm/pN) glass microneedle, is brought in contact with the myosin surface. Isometric force, being generated by myosin cross-bridges pulling on the attached actin filament, is calculated from the extent to which the calibrated microneedle is deflected. By measuring the density of myosin adhered to the surface, we estimated the number of myosin cross-bridges that are able to interact with a length of actin filament in contact with the myosin surface. In a direct comparison between smooth and skeletal muscle myosin, the average force per cross-bridge was 0.8 and 0.2 pN, respectively. Surprisingly, smooth muscle myosin generates approximately 4 times greater average force per cross-bridge head than skeletal muscle myosin. Because average isometric force is the product of the cross-bridge unitary force and duty cycle, we are presently using a laser optical trap in an attempt to measure unitary events from single myosin molecules. This approach should allow us to determine whether an increase in unitary force, duty cycle, or both contribute to smooth muscle myosin's enhanced force-generating capacity compared with skeletal muscle myosin.