Myosin II isoforms in smooth muscle: heterogeneity and function

Am J Physiol Cell Physiol. 2007 Aug;293(2):C493-508. doi: 10.1152/ajpcell.00131.2007. Epub 2007 May 2.


Both smooth muscle (SM) and nonmuscle class II myosin molecules are expressed in SM tissues comprising hollow organ systems. Individual SM cells may express one or more of multiple myosin II isoforms that differ in myosin heavy chain (MHC) and myosin light chain (MLC) subunits. Although much has been learned, the expression profiles, organization within contractile filaments, localization within cells, and precise roles in various contractile functions of these different myosin molecules are still not well understood. However, data supporting unique physiological roles for certain isoforms continues to build. Isoform differences located in the S1 head region of the MHC can alter actin binding and rates of ATP hydrolysis. Differences located in the MHC tail can alter the formation, stability, and size of the myosin thick filament. In these distinct ways, both head and tail isoform differences can alter force generation and muscle shortening velocities. The MLCs that are associated with the lever arm of the S1 head can affect the flexibility and range of motion of this domain and possibly the motion of the S2 and motor domains. Phosphorylation of MLC(20) has been associated with conformational changes in the S1 and/or S2 fragments regulating enzymatic activity of the entire myosin molecule. A challenge for the future will be delineation of the physiological significance of the heterogeneous expression of these isoforms in developmental, tissue-specific, and species-specific patterns and or the intra- and intercellular heterogeneity of myosin isoform expression in SM cells of a given organ.

Publication types

  • Research Support, N.I.H., Extramural
  • Review

MeSH terms

  • Actins / metabolism
  • Adenosine Triphosphate / metabolism
  • Amino Acid Sequence
  • Animals
  • Humans
  • Hydrolysis
  • Molecular Sequence Data
  • Muscle Contraction*
  • Muscle Development
  • Muscle, Smooth / cytology
  • Muscle, Smooth / growth & development
  • Muscle, Smooth / metabolism*
  • Myocytes, Smooth Muscle / metabolism
  • Myosin Heavy Chains / metabolism
  • Myosin Light Chains / metabolism*
  • Myosin Type II / chemistry
  • Myosin Type II / metabolism*
  • Nonmuscle Myosin Type IIA / metabolism
  • Nonmuscle Myosin Type IIB / metabolism
  • Phosphorylation
  • Protein Isoforms / metabolism
  • Protein Structure, Tertiary
  • Smooth Muscle Myosins / chemistry
  • Smooth Muscle Myosins / metabolism*


  • Actins
  • Myosin Light Chains
  • Protein Isoforms
  • Adenosine Triphosphate
  • Myosin Type II
  • Nonmuscle Myosin Type IIA
  • Nonmuscle Myosin Type IIB
  • Smooth Muscle Myosins
  • Myosin Heavy Chains