Abstract
In the super-relaxed state of myosin, ATPase activity is strongly inhibited by binding of the myosin heads to the core of the thick filament in a structure known as the interacting-heads motif. In the disordered relaxed state myosin heads are not bound to the core of the thick filament and have an ATPase rate that is 10 fold greater. In the interacting-heads motif the two regulatory light chains appear to bind to each other. We have made single cysteine mutants of the regulatory light chain, placed both paramagnetic and fluorescent probes on them, and exchanged them into skinned skeletal muscle fibers. Many of the labeled light chains tended to disrupt the stability of the super-relaxed state, and showed spectral changes in the transition from the disordered relaxed state to the super-relaxed state. These data support the putative interface between the two regulatory light chains identified by cryo electron microscopy and show that both the divalent cation bound to the regulatory light chain and the N-terminus of the regulatory light chain play a role in the stability of the super-relaxed state. One probe showed a shift to shorter wavelengths in the super-relaxed state such that a ratio of intensities at 440nm to that at 520nm provided a measure of the population of the super-relaxed state amenable for high throughput screens for finding potential pharmaceuticals. The results provide a proof of concept that small molecules that bind to this region can destabilize the super-relaxed state and provide a method to search for small molecules that do so leading to a potentially effective treatment for Type 2 diabetes and obesity.
Highlights
In resting skeletal muscle myosin is found in two states, one in which it is bound to the core of the thick filament with a highly inhibited ATPase activity, and one in which it is disordered, free to diffuse in the inter-filament space and has a higher ATPase activity
Correlations between the stability of the SRX, measured kinetically, and the stability of the array of myosin heads bound to the core of the thick filament as measured by x-ray diffraction or by EM, has suggested the hypothesis that the inhibited state is associated with an ordered structural state [3, 8]
A comparison between the sequences of tarantula exoskeletal muscle and vertebrate skeletal muscle (S1 Fig) shows that many of the conserved amino acids are clustered on the surface on the N-terminal lobe, close to the location of the regulatory light chain (RLC)-RLC interface in the model of the Interacting-Heads Motif (IHM) (Fig 3)
Summary
In resting skeletal muscle myosin is found in two states, one in which it is bound to the core of the thick filament with a highly inhibited ATPase activity, and one in which it is disordered, free to diffuse in the inter-filament space and has a higher ATPase activity. Correlations between the stability of the SRX, measured kinetically, and the stability of the array of myosin heads bound to the core of the thick filament as measured by x-ray diffraction or by EM, has suggested the hypothesis that the inhibited state is associated with an ordered structural state [3, 8]. Testing this hypothesis is one of the goals of the present work. These myosin heads have the faster ATPase activity observed for purified myosin
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