Myosin

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Part of the myosin structure, atoms in the heavy chain are colored red on the left-hand side, and atoms in the light chains are colored orange and yellow.

Myosins are a large family of motor proteins found in eukaryotic tissues. They are responsible for actin-based motility.

"The term “myosin” was originally used to describe a group of similar, but nonidentical, ATPases found in striated and smooth muscle cells." From Pollard and Korn, 1973[1]

Contents

Structure and Function

Domains

Most myosin molecules are composed of a head, neck, and tail domain.

Power stroke

Multiple myosin II molecules generate force in skeletal muscle through a power stroke mechanism fuelled by the energy released from ATP hydrolysis.[2] The power stroke occurs at the release of the products of ATP hydrolysis - ADP and phosphate - when myosin is tightly bound to actin. The effect of this release is a conformational change in the molecule that pulls against the actin. The combined effect of the myriad power strokes causes the muscle to contract. See muscle contraction.

Nomenclature, evolution, and the family tree

The wide variety of myosin genes found throughout the eukaryotic phyla were named according to different schemes as they were discovered. The nomenclature can therefore be somewhat confusing when attempting to compare the functions of myosin proteins within and between organisms.

Skeletal muscle myosin, the most conspicuous of the myosin superfamily due to its abundance in muscle fibers, was the first to be discovered. This protein makes up part of the sarcomere and forms macromolecular filaments composed of multiple myosin subunits. Similar filament-forming myosin proteins were found in cardiac muscle, smooth muscle, and non-muscle cells. However, beginning in the 1970s researchers began to discover new myosin genes in simple eukaryotes [1] encoding proteins that acted as monomers and were therefore entitled Class I myosins. These new myosins were collectively termed "unconventional myosins" [3] and have been found in many tissues other than muscle. These new superfamily members have been grouped according to phylogenetic relationships derived from a comparison of the amino acid sequences of their head domains, with each class being assigned a Roman numeral [4][5][6][7](see phylogenetic tree). The unconventional myosins also have divergent tail domains, suggesting unique functions[8]. The now diverse array of myosins likely evolved from an ancestral precursor (see picture).

Myosin unrooted phylogenetic tree — click on image to see larger version (Source: http://www.mrc-lmb.cam.ac.uk/myosin/trees/trees.html)

Analysis of the amino acid sequences of different myosins shows great variability among the tail domains but strong conservation of head domain sequences. Presumably this is so the myosins may interact, via their tails, with a large number of different cargoes, while the goal in each case - to move along actin filaments - remains the same and therefore requires the same machinery in the motor. For example, the human genome contains over 40 different myosin genes.

These differences in shape also determine the speed at which myosins can move along actin filaments. The hydrolysis of ATP and the subsequent release of the phosphate group causes the "power stroke," in which the "lever arm" or "neck" region of the heavy chain is dragged forward. Since the power stroke always moves the lever arm by the same angle, the length of the lever arm determines how fast the cargo will move. A longer lever arm will cause the cargo to traverse a greater distance even though the lever arm undergoes the same angular displacement - just as a person with longer legs can move farther with each individual step. Myosin V, for example, has a much longer neck region than myosin II, and therefore moves 30-40 nanometers with each stroke as opposed to only 5-10.

Myosin Classes

Myosin I

Myosin I's function is unknown, but it is believed to be responsible for vesicle transport or the contraction vacuole of cells.[9]

Myosin II

Sliding filament model of muscle contraction.

Myosin II is the best-studied example of these properties.

In muscle cells, it is myosin II that is responsible for producing the contractile force. Here, the long coiled-coil tails of the individual myosin molecules join together, forming the thick filaments of the sarcomere. The force-producing head domains stick out from the side of the thick filament, ready to walk along the adjacent actin-based thin filaments in response to the proper chemical signals.

Genes in humans

Note that not all of these genes are active.

Myosin light chains are distinct and have their own properties. They are not considered "myosins" but are components of the macromolecular complexes that make up the functional myosin enzymes.

Paramyosin

Paramyosin is a large 93-115kDa muscle protein that has been described in a number of diverse invertebrate phyla[10]. It is though that invertebrate thick filaments are composted of an inner paramyosin core that is surrounded by myosin. The myosin interacts with actin resulting in fibre contraction[11]. Paramyosin is found in many different invertebrate species, for example, Brachiopoda, Sipunculidea, Nematoda, Annelida, Mollusca, Arachnida, Insecta[10]. Paramyosin is responsible for the "catch" mechanism that enables sustained contraction of muscles with very little energy expenditure, such that a clam can remain closed for extended periods.

Footnotes

  1. ^ a b Pollard and Korn, 1973
  2. ^ Tyska MJ, Warshaw DM (2002). "The myosin power stroke". Cell Motil Cytoskeleton 51 (1): 1-15. PMID 11810692. 
  3. ^ Cheney and Mooseker, 1992
  4. ^ Cheney et al., 1993
  5. ^ Goodson, 2004
  6. ^ Hodge and Cope, 2000
  7. ^ Berg et al., 2001
  8. ^ Oliver et al., 1999
  9. ^ Sutherland Macive (6/4/03). "Myosin I". Retrieved on 2007-05-23.
  10. ^ a b Winkelman, L. 1976
  11. ^ Twarog, B. M. 1976

References


Additional images

Phase 1

Phase 2

Phase 3

Phase 4

Myosin-painting

Myosin powerstroke

See also

External links

v • d • e
Proteins of the cytoskeleton
Microfilaments
Intermediate filaments
type 1 and 2 (Cytokeratin, type I, type II) - type 3 (Desmin, GFAP, Peripherin, Vimentin) - type 4 (Internexin, Nestin, Neurofilament, Synemin, Syncoilin) - type 5 (Lamin A, B)
Microtubules
Catenins
Nonhuman
Other
v • d • e
Acid anhydride hydrolases: ATPases (EC 3.6.3-3.6.4)
3.6.3
3.6.4
Dynein - Kinesin - Myosin
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Histology: muscle tissue
Striated muscle
General
Smooth muscle