The Ups and Downs of Smooth Muscle Myosin Regulation

Abstract

Since discovery that regulatory light chain (RLC) phosphorylation was the primary method of regulation of smooth and non-muscle myosin II, the structure of the phosphorylated and dephosphorylated forms has been a goal only partially realized. Regulation in both smooth muscle and the related scallop striated muscle myosin requires a 2-headed myosin species; single headed species are unregulated. Thus head to head interactions are key to achieving the inhibited state. A folded conformation of full length myosin, generally referred to as the 10S conformation was discovered early by conventional electron microscopy and in 1999, a structural explanation for the head-head interactions was obtained from 2-D arrays of dephosphorylated smooth muscle heavy meromyosin formed on lipid monolayers and imaged in 3-D by cryoEM. The structure was later confirmed by 2-D arrays of the 10S conformation of whole myosin. While this structure explained inhibition of the solubilized form of regulated myosins, it had not been observed in filaments. Surprisingly, the first observation of smooth muscle myosin-like head-head interactions in a thick filament was obtained from tarantula striated muscle, not from smooth muscle myosin. Even more surprising was the observation of a similar conformation in relaxed cardiac muscle thick filaments. Thus, these head-head interactions observed first in smooth muscle HMM, appear to be ubiquitous in relaxed muscle although still not confirmed for thick filaments in relaxed smooth muscle. Still to be determined is an explanation of the factors that can lead to solubilization and the location of the N-terminus of the RLC, whose location and structure has yet to be revealed, in the phosphorylated and dephosphorylated state. Theoretical modeling has provided possible explanations for several factors that affect regulation but has not yet yielded a coherent theory. Supported by NIAMSD.