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Volume 271, Number 27, Issue of July 5, 1996 pp. 15849-15849
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

MINIREVIEW:
Myosin Minireview Series*

James T. Stull

From the Department of Physiology, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9040

INTRODUCTION
FOOTNOTES
REFERENCES

INTRODUCTION

In 1988, the Journal of Biological Chemistry initiated publication of minireviews to inform readers of recent advances in biochemistry and molecular biology outside their own areas of expertise. These short reviews have been very successful and most valuable as a reference and teaching resource in the form of the annual Minireview Compendium. They will continue to be an important feature of the Journal. We are now adding a new component in the form of thematic minireviews with three or more related minireviews published in sequential issues of the Journal. This first series focuses on myosin and includes ``The Structural Basis of the Myosin ATPase Activity'' by Ivan Rayment, ``Vertebrate Unconventional Myosins'' by Tama Hasson and Mark S. Mooseker, and ``Regulation of Class I and Class II Myosins by Heavy Chain Phosphorylation'' by Hanna Brzeska and Edward D. Korn.

The voluntary contraction of muscle on its scaffolding of bone provides movements for diverse functions. How skeletal muscles pull is one of the best understood of all kinds of cell movements due to the intense efforts of many individuals. The primary contractile protein myosin was discovered in 1859 (1) and identified as an actin-activated ATPase 80 years later (2, 3). It was then discovered that myosin and actin were differently localized in thick and thin filaments, respectively, and that the shortening of a muscle cell takes place by the sliding of these filaments past each other in the highly organized muscle sarcomere (4, 5). Sarcomeric or conventional myosin is a large hexameric protein with two globular heads and a long coiled-coil alpha -helical rod. Part of the rod aggregates to form bipolar thick filaments with protruding globular heads containing binding sites for actin thin filaments and ATP.

Remarkable advancements in new technologies are providing basic insights into how conventional myosin motors transduce the chemical energy of ATP into mechanical work. In vitro motility assays allow measurements of maximal sliding velocities of fluorescently labeled actin filaments over coverslips coated with different myosins (6). Force measurements are made with a single actin filament binding to myosin (7, 8). Myosins expressed in Dictyostelium and baculoviral vector systems provide powerful recombinant DNA approaches for analyzing the structural basis of myosin's motor functions (9, 10, 11). Essential information came from the high resolution x-ray determinations of the structures of actin (12) and the myosin head domain from skeletal muscle (13).

It was initially thought that detailed studies of contractile proteins from striated muscles would provide a basis for understanding many nonmuscle contractile and motility events because conventional myosins were found in most eukaryotic cells (14). Motility, cell shape changes, cytokinesis, and secretion were among the numerous functions proposed to be driven by a conventional myosin motor. However, clues for significant differences emerged with the discovery that conventional myosins from platelets and smooth muscle were regulated by a Ca2+-dependent, reversible phosphorylation mechanism (15, 16, 17). Initial studies on Ca2+ as an intracellular messenger molecule were almost exclusively confined to skeletal muscle and troponin as a thin filament regulatory Ca2+-binding protein (18). However, troponin is not found in non-striated muscle cells. The primary mechanism for Ca2+ regulation of these conventional myosins is phosphorylation of the regulatory light chain subunit by a dedicated Ca2+/calmodulin-dependent myosin light chain kinase, which markedly increases actin-activated myosin ATPase activity (19, 20).

Another hint of differences in the myosin motor family came from the observation that Acanthamoeba contained a small, single-headed myosin (21). Many initially considered this unconventional Acanthamoeba myosin a proteolytic fragment of its conventional myosin. However, this is not so. Molecular biology and genetics have revealed that small as well as large unconventional myosins are commonplace. The myosin head or motor domain is highly conserved, which contributed to the successful hunt for new myosin motors by the polymerase chain reaction. Additionally, phenotypic defects in a variety of organisms were found to be due to mutations in an unconventional myosin motor.

Members of the myosin superfamily have grown dramatically in the past few years. The identity of an unconventional myosin is primarily based on sequence similarities in the motor domain. Because of this conservation of structure, it is expected that the mechanochemical transduction mechanism will be similar in all myosins. Insight into the structural basis of this process is a topic of the minireview by Ivan Rayment (first in the series). Sequence information is being complemented by biochemical and cellular characterizations of unconventional myosins to uncover biological functions of newly discovered motor molecules. There are sufficient differences in myosin structures to allow grouping into 1 conventional and at least 10 unconventional classes based upon differences in the tail and motor domains. In the second minireview in the series Tama Hansson and Mark S. Mooseker discuss recent advances in studies on vertebrate unconventional myosins.

There is little similarity outside the motor domain in unconventional myosins. Nonmotor domains bind to different macromolecules with cellular targeting for different movement tasks. They are also important sites of regulation. Besides phosphorylation of the regulatory light chain, phosphorylation of the C-terminal tail of nonmuscle conventional myosin regulates actin-activated myosin ATPase activity. Additionally, the motor domain of unconventional myosin (class I) may be phosphorylated. The regulation of myosins by heavy chain phosphorylation is a topic of the minireview by Hanna Brzeska and Edward D. Korn (third in the series).

FOOTNOTES

*   These minireviews will be reprinted in the 1996 Minireview Compendium, which will be available in December, 1996. 

REFERENCES

  1. Kühne, W. (1859) Arch. f. Anat. Physiol. u. wissensch. Med. 748
  2. Engelhardt, W. A., Ljubimowa, M. N. (1939) Nature 144, 668-669
  3. Straub, F. B. (1943) Stud. Inst. Med. Chem. Univ. Szeged 2, 3-15
  4. Huxley, A. F., Niedergerke, R. (1954) Nature 173, 971-973 [CrossRef][Medline] [Order article via Infotrieve]
  5. Huxley, H. E., Hanson, J. (1954) Nature 173, 973-976 [CrossRef][Medline] [Order article via Infotrieve]
  6. Kron, S. J., Spudich, J. A. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 6272-6276 [Abstract/Free Full Text]
  7. Kishino, A., Yanagida, T. (1988) Nature 334, 74-76 [CrossRef][Medline] [Order article via Infotrieve]
  8. Finer, J. T., Simmons, R. M., Spudich, J. A. (1994) Nature 368, 113-119 [CrossRef][Medline] [Order article via Infotrieve]
  9. De Lozanne, A., Spudich, J. A. (1987) Science 236, 1086-1091 [Abstract/Free Full Text]
  10. Sweeney, H. L., Yang, Z., Zhi, G., Stull, J. T., Trybus, K. M. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 1490-1494 [Abstract/Free Full Text]
  11. Trybus, K. M. (1994) J. Muscle Res. Cell Motil. 15, 587-594 [CrossRef][Medline] [Order article via Infotrieve]
  12. Kabsch, W., Mannherz, H. G., Suck, D., Pai, E. F., Holmes, K. C. (1990) Nature 347, 37-44 [CrossRef][Medline] [Order article via Infotrieve]
  13. Rayment, I., Rypniewski, W. R., Schmidt-Bäse, K., Smith, R., Tomchick, D. R., Benning, M. M., Winkelmann, D. A., Wesenberg, G., Holden, H. M. (1993) Science 261, 50-58 [Abstract/Free Full Text]
  14. Clarke, M., Spudich, J. A. (1977) Annu. Rev. Biochem. 46, 797-822 [CrossRef][Medline] [Order article via Infotrieve]
  15. Adelstein, R. S., Conti, M. A. (1975) Nature 256, 597-598 [CrossRef][Medline] [Order article via Infotrieve]
  16. Gorecka, A., Aksoy, M. O., Hartshorne, D. J. (1976) Biochem. Biophys. Res. Commun. 71, 325-331 [CrossRef][Medline] [Order article via Infotrieve]
  17. Small, J. V., Sobieszek, A. (1977) Eur. J. Biochem. 76, 521-530 [Medline] [Order article via Infotrieve]
  18. Ebashi, S., Kodama, A. (1965) J. Biochem. (Tokyo) 58, 107-108 [Free Full Text]
  19. Kamm, K. E., Grange, R. W. (1996) Biochemistry of Smooth Muscle Contraction (Barany, M., eds) , p. 355, Academic Press, Orlando, FL
  20. Stull, J. T., Krueger, J. K., Kamm, K. E., Gao, Z.-H., Zhi, G., Padre, R. (1996) Biochemistry of Smooth Muscle Contraction (Barany, M., eds) , p. 119, Academic Press, Orlando, FL
  21. Pollard, T. D., Korn, E. D. (1973) J. Biol. Chem. 248, 4682-4690 [Abstract/Free Full Text]
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Copyright © 1996 by the American Society for Biochemistry and Molecular Biology.