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Thin-filament linked regulation of smooth muscle myosin

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Abstract

Phosphorylation of the regulatory light chain subunit of smooth muscle myosin is sufficient, but not necessary for muscle contraction. It has been suggested that thin-filament regulation may also contribute to the regulation of contraction. A hallmark feature of regulated thin filaments, previously described for vertebrate skeletal muscle, is the capacity of strong-binding or rigor-like cross bridges to “turn-on” the actin filament. Turned-on thin filaments stimulate cross-bridge attachment even in the absence of calcium. The present study utilized an in vitro sliding-filament motility assay to test for thin-filament regulation of both unphosphorylated and phosphorylated smooth muscle myosins. Regulated thin-filaments were reconstituted from skeletal muscle actin and chicken gizzard smooth muscle tropomyosin (TmCG), and then turned-on either (1) by rigor cross bridges at low concentrations of MgATP, or (2) by adding N-ethyl-maleimide-modified skeletal subfragment S1(NEM-S1), which forms rigor-like bonds in the presence of MgATP. For control actin·TmCG filaments, force production by unphosphorylated myosin was 0.5% of that produced by thiophosphorylated myosin. The force exerted on actin·Tm filaments by both unphosphorylated and phosphorylated myosins was increased by reducing the [MgATP] to 10–100 μM MgATP (rigor-dependent activation). Force was also increased by actin·TmCG filaments that had been turned-on by NEM-S1 binding, with force production by unphosphorylated myosin increased 80-fold vs. 2.3-fold for thiophosphorylated myosin. TmCG was required for increased force production with both low MgATP and NEM-S1. Unloaded filament velocity for NEM-S1-activated thin filaments was 0.72 μm/sec with unphosphorylated myosin compared to 1.24 μm/sec with thiophosphorylated myosin. Taken together, these results suggest that thin-filament regulation may play a role in the activation of both unphosphorylated and phosphorylated smooth muscle myosins and suggest a possible mechanism for activation of slowly cycling unphosphorylated cross bridges (i.e. latch-state) during tonic contractions of smooth muscle.

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  1. Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, 05405-0068, Vermont

    J. R. Haeberle

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Haeberle, J.R. Thin-filament linked regulation of smooth muscle myosin. J Muscle Res Cell Motil 20, 363–370 (1999). https://doi.org/10.1023/A:1005408402323

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