Smooth muscle myosin: regulation and properties

Abstract

The relationship of the biochemical states to the mechanical events in contraction of smooth muscle crossbridges is reviewed. These studies use direct measurements of the kinetics of P i and ADP release. The rate of release of P i from thiophosphorylated cycling cross-bridges held isometric was biphasic with turnovers of 1.8 s À1 and 0.3 s À1 , reflecting properties and forces directly acting on cross-bridges through mechanisms such as positive strain and inhibition by high-affinity MgADP binding. Fluorescent transients reporting release of an ADP analogue 3 0 -deac-edaADP were significantly faster in phasic than in tonic smooth muscles. Thiophosphorylation of myosin regulatory light chains (RLCs) increased and positive strain decreased the release rate around twofold. The rates of ADP release from rigor cross-bridges and the steady-state P i release from cycling isometric cross-bridges are similar, indicating that the ADP-release step or an isomerization preceding it may limit the ATPase rate. Thus ADP release in phasic and tonic smooth muscles is a regulated step with strain-and dephosphorylation-dependence. High affinity of cross-bridges for ADP and slow ADP release prolong the fraction of the duty cycle occupied by strongly bound AMÁADP state(s) and contribute to the high economy of force that is characteristic of smooth muscle. RLC thiophosphorylation led to structural changes in smooth muscle cross-bridges consistent with our findings that thiophosphorylation and strain modulate product release.

Thick filament mechano-sensing is a calcium-independent regulatory mechanism in skeletal muscle

“…This sequestered, ‘super-relaxed' or ‘OFF' state of the thick filament also inhibits the ATP-ase activity of resting skeletal muscle910. Exit from the structural and functional OFF state is controlled by phosphorylation of the myosin regulatory light chain in vertebrate smooth muscle21112, and by calcium binding to myosin in some invertebrate striated muscles13. In vertebrate skeletal muscle, recent X-ray diffraction studies on isolated single cells led to the proposal of a novel signalling pathway controlling release of the myosin motors from the OFF state: that the thick filament is switched ON by mechanical stress14.…”

Regulation of Gβγi-Dependent PLC-β3 Activity in Smooth Muscle: Inhibitory Phosphorylation of PLC-β3 by PKA and PKG and Stimulatory Phosphorylation of Gαi-GTPase-Activating Protein RGS2 by PKG

“…Activation of PLC-β1 via Gα q or PLC-β3 via Gβ i in response to contractile agonists results in the generation of inositol 1,4,5-trisphosphate (IP 3 ), IP 3 -dependent release of Ca 2+ , activation of Ca 2+ /calmodulin-dependent activation of MLC kinase and phosphorylation of MLC 20 , an essential step in smooth muscle contraction [1–4]. Activation of cAMP-dependent protein kinase (PKA) or cGMP-dependent protein kinase (PKG) in response to relaxant agonists results in the inhibition of Ca 2+ mobilization leading to inhibition of MLC kinase activity and MLC 20 dephosphorylation, an essential step in smooth muscle relaxation [4, 45].…”

“…Contraction of smooth muscle is mediated by Ca 2+ /calmodulin-dependent activation of myosin light chain (MLC) kinase and phosphorylation of MLC 20 , a prerequisite in acto-myosin interaction [1–4]. Mobilization of intracellular Ca 2+ release by main contractile agonists such as acetylcholine, which activate G q -coupled m3 receptors is mediated via Gα q -dependent activation of PLC-β1 isoform, whereas agonists such as adenosine, which activate G i -coupled A 1 receptors is mediated via Gβ i -dependent activation of PLC-β3 isoform [5–9].…”

Tonic and Phasic Smooth Muscle Contraction Is Not Regulated by the PKCα - CPI-17 Pathway in Swine Stomach Antrum and Fundus

“…Diverse innervation and circulating signaling molecules in addition to unique receptor and protein (isoform) expression and intracellular signaling pathways complicate the issue. Not unexpectedly, there are multiple proposed hypotheses to explain tonic force maintenance in SM including: slowly- or non-cycling smooth muscle myosin cross-bridges called latch bridges [1], [2], [3], [4]; recruitment of non-muscle myosin II [5], [6], [7]; formation of caldesmon or calponin dependent actin-to-myosin cross-links [8], [9]; formation of cytoskeletal force-bearing structures [10], [11], [12]; and regulation of myosin LC 20 phosphorylation via second messenger pathways affecting myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activity [13], [14], [15], [16], [17], [18]. This last category is of interest because of the reported differential expression, localization and regulation of these second messenger proteins in various SM tissues.…”

“…The expression of PKCα and CPI-17 and their spatial-temporal re-distribution upon tissue activation have been proposed in Ca 2+ sensitization of smooth muscle tissues (for review [14], [15], [16], [17]). Thus it is possible that differences in their expression and cellular localization/translocation could be involved in determining tonic vs. phasic contractile responses.…”