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Updated: May 22, 2026
Spatiotemporal Control of Protein Activity through Optogenetic Allosteric Regulation
Published on: October 4, 2024
Modification of interface between regulatory and essential light chains hampers phosphorylation-dependent activation
Shaowei Ni1, Feng Hong, Brian D Haldeman
1Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, Nevada 89557, USA.
Insights
Phosphorylation of the regulatory light chain (RLC) in smooth muscle myosin is crucial for maintaining its active state by stabilizing the interaction with the essential light chain (ELC). Disrupting this RLC/ELC interface prevents myosin activation, even when phosphorylated.
Area of Science:
- Molecular and Cellular Biology
- Biochemistry
- Muscle Physiology
Background:
- Smooth muscle contraction is regulated by myosin II, a motor protein whose activity is modulated by phosphorylation of its regulatory light chain (RLC).
- Interactions between the RLC, essential light chain (ELC), and heavy chain (HC) are critical for myosin function, but the precise role of these interactions in regulation remains incompletely understood.
Purpose of the Study:
- To investigate the regulatory significance of interactions between the RLC, ELC, and HC in smooth muscle heavy meromyosin.
- To determine how disrupting these specific protein-protein interactions affects myosin's biochemical and biophysical properties, particularly in response to RLC phosphorylation.
Main Methods:
- Site-directed mutagenesis of RLC, ELC, and HC based on scallop myosin coordinates to disrupt predicted interaction interfaces.
- Biochemical assays measuring basal ATPase, actin-activated ATPase (Vmax, KATPase), actin-sliding velocities, rigor binding to actin, and kinetics of ATP binding and ADP release.
- Molecular dynamics simulations to analyze the structural consequences of disrupting the RLC/ELC interface.
Main Results:
- Mutants mimicking wild-type behavior when unphosphorylated, indicating proper 'off-state' characteristics.
- Phosphorylation of RLC mutants (smM129Q/smG130C) that disrupt RLC/ELC interaction abolished motility and reduced ATPase activity, while other parameters remained unchanged.
- Disruption of the RLC/ELC interface led to increased flexibility and a bias towards the inhibited structural state, even after RLC phosphorylation.
Conclusions:
- The interaction between the RLC and ELC is essential for smooth muscle myosin activation.
- RLC phosphorylation's primary role is to stabilize the RLC/ELC interface, thereby promoting the active myosin state.
- Breaking the RLC/ELC interface prevents phosphorylation from overcoming the inhibited state, highlighting the importance of this interaction for allosteric regulation.
Abstract:
We examined the regulatory importance of interactions between regulatory light chain (RLC), essential light chain (ELC), and adjacent heavy chain (HC) in the regulatory domain of smooth muscle heavy meromyosin. After mutating the HC, RLC, and/or ELC to disrupt their predicted interactions (using scallop myosin coordinates), we measured basal ATPase, V(max), and K(ATPase) of actin-activated ATPase, actin-sliding velocities, rigor binding to actin, and kinetics of ATP binding and ADP release. If unphosphorylated, all mutants were similar to wild type showing turned-off behaviors. In contrast, if phosphorylated, mutation of RLC residues smM129Q and smG130C in the F-G helix linker, which interact with the ELC (Ca(2+) binding in scallop), was sufficient to abolish motility and diminish ATPase activity, without altering other parameters. ELC mutations within this interacting ELC loop (smR20M and smK25A) were normal, but smM129Q/G130C-R20M or -K25A showed a partially recovered phenotype suggesting that interaction between the RLC and ELC is important. A molecular dynamics study suggested that breaking the RLC/ELC interface leads to increased flexibility at the interface and ELC-binding site of the HC. We hypothesize that this leads to hampered activation by allowing a pre-existing equilibrium between activated and inhibited structural distributions (Vileno, B., Chamoun, J., Liang, H., Brewer, P., Haldeman, B. D., Facemyer, K. C., Salzameda, B., Song, L., Li, H. C., Cremo, C. R., and Fajer, P. G. (2011) Broad disorder and the allosteric mechanism of myosin II regulation by phosphorylation. Proc. Natl. Acad. Sci. U.S.A. 108, 8218-8223) to be biased strongly toward the inhibited distribution even when the RLC is phosphorylated. We propose that an important structural function of RLC phosphorylation is to promote or assist in the maintenance of an intact RLC/ELC interface. If the RLC/ELC interface is broken, the off-state structures are no longer destabilized by phosphorylation.
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