Skeletal muscle regulatory proteins enhance F-actin in vitro motility

Using an in vitro motility assay, we have investigated the effects of rabbit skeletal muscle regulatory proteins, troponin and tropomyosin, on the gliding of F-actin filaments or F-actin filaments containing these regulatory proteins. We demonstrate that Ca²⁺ does not affect the motility of F-actin gliding on HMM, but does in the presence of skeletal muscle tropomyosin and troponin. We conclude that Ca²⁺ affects motility through troponin because, like F-actin, F-actin-Tm filaments show no Ca²⁺-dependence to their gliding speeds. Furthermore, there is a large enhancement of the gliding speed (about 75%) in the presence of skeletal muscle tropomyosin, troponin + saturating Ca²⁺ over that seen with F-actin filaments. This enhancement is not due to the action of tropomyosin alone as skeletal muscle tropomyosin without troponin enhances the speed little (about 5%) over that of F-actin. Thus troponin confers Ca²⁺ sensitivity to the motility and, additionally, potentiates motility greatly along with tropomyosin in the presence of saturating Ca²⁺. When [HMM] is varied, the decline in speed of F-actin seen at low HMM density is changed little by tropomyosin in the F-actin-Tm filaments. These data show that the skeletal regulatory proteins interact with F-actin to enhance the interaction with HMM particularly in the presence of troponin and saturating Ca²⁺ and enhance the gliding speed in the in vitro motility assay as they potentiate the ATPase activity in the isolated proteins. This enhancement of speed in the motility assay cannot be ascribed to tropomyosin alone.