Myosin


Myosins are a family of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin to be discovered was in 1864 by Wilhelm Kühne. Kühne had extracted a viscous protein from skeletal muscle that he held responsible for keeping the tension state in muscle. He called this protein myosin. The term has been extended to include a group of similar ATPases found in the cells of both striated muscle tissue and smooth muscle tissue.
Following the discovery in 1973 of enzymes with myosin-like function in Acanthamoeba castellanii, a global range of divergent myosin genes have been discovered throughout the realm of eukaryotes.
Although myosin was originally thought to be restricted to muscle cells, there is no single "myosin"; rather it is a very large superfamily of genes whose protein products share the basic properties of actin binding, ATP hydrolysis, and force transduction. Virtually all eukaryotic cells contain myosin isoforms. Some isoforms have specialized functions in certain cell types, while other isoforms are ubiquitous. The structure and function of myosin is globally conserved across species, to the extent that rabbit muscle myosin II will bind to actin from an amoeba.

Structure and functions

Domains

Most myosin molecules are composed of a head, neck, and tail domain.
  • The head domain binds the filamentous actin, and uses ATP hydrolysis to generate force and to "walk" along the filament towards the barbed end.
  • the neck domain acts as a linker and as a lever arm for transducing force generated by the catalytic motor domain. The neck domain can also serve as a binding site for myosin light chains which are distinct proteins that form part of a macromolecular complex and generally have regulatory functions.
  • The tail domain generally mediates interaction with cargo molecules and/or other myosin subunits. In some cases, the tail domain may play a role in regulating motor activity.

    Power stroke

Multiple myosin II molecules generate force in skeletal muscle through a power stroke mechanism fuelled by the energy released from ATP hydrolysis. The power stroke occurs at the release of phosphate from the myosin molecule after the ATP hydrolysis while myosin is tightly bound to actin. The effect of this release is a conformational change in the molecule that pulls against the actin. The release of the ADP molecule leads to the so-called rigor state of myosin. The binding of a new ATP molecule will release myosin from actin. ATP hydrolysis within the myosin will cause it to bind to actin again to repeat the cycle. The combined effect of the myriad power strokes causes the muscle to contract.

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 nonmuscle cells. However, beginning in the 1970s, researchers began to discover new myosin genes in simple eukaryotes encoding proteins that acted as monomers and were therefore entitled Class I myosins. These new myosins were collectively termed "unconventional myosins" 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. The unconventional myosins also have divergent tail domains, suggesting unique functions. The now diverse array of myosins likely evolved from an ancestral precursor.
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 the displacement of the cargo relative to the actin filament. 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. The velocity of a myosin motor depends upon the rate at which it passes through a complete kinetic cycle of ATP binding to the release of ADP.

Myosin classes

Myosin I

Myosin I, a ubiquitous cellular protein, functions as monomer and functions in vesicle transport. It has a step size of 10 nm and has been implicated as being responsible for the adaptation response of the stereocilia in the inner ear.

Myosin II

is the myosin type responsible for producing muscle contraction in muscle cells in most animal cell types. It is also found in non-muscle cells in contractile bundles called stress fibers.
  • Myosin II contains two heavy chains, each about 2000 amino acids in length, which constitute the head and tail domains. Each of these heavy chains contains the N-terminal head domain, while the C-terminal tails take on a coiled-coil morphology, holding the two heavy chains together. Thus, myosin II has two heads. The intermediate neck domain is the region creating the angle between the head and tail. In smooth muscle, a single gene ) codes for the heavy chains myosin II, but splice variants of this gene result in four distinct isoforms.
  • It also contains 4 myosin light chains, resulting in 2 per head, weighing 20 and 17 kDa. These bind the heavy chains in the "neck" region between the head and tail.
  • *The MLC20 is also known as the regulatory light chain and actively participates in muscle contraction.
  • *The MLC17 is also known as the essential light chain. Its exact function is unclear, but is believed to contribute to the structural stability of the myosin head along with MLC20. Two variants of MLC17 exist as a result of alternative splicing at the MLC17 gene.
In muscle cells, the long coiled-coil tails of the individual myosin molecules can auto-inhibit active function in the 10S conformation or upon phosphorylation, change to the 6S conformation and join, 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 and may be in either auto-inhibited or active conformation. The balance/transition between active and inactive states is subject to extensive chemical regulation.

Myosin III

Myosin III is a poorly understood member of the myosin family. It has been studied in vivo in the eyes of Drosophila, where it is thought to play a role in phototransduction. A human homologue gene for myosin III, MYO3A, has been uncovered through the Human Genome Project and is expressed in the retina and cochlea.

Myosin IV

Myosin IV has a single IQ motif and a tail that lacks any coiled-coil forming sequence. It has homology similar to the tail domains of Myosin VII and XV.

Myosin V

Myosin V is an unconventional myosin motor, which is processive as a dimer and has a step size of 36 nm. It translocates along actin filaments traveling towards the barbed end of the filaments. Myosin V is involved in the transport of cargo from the center of the cell to the periphery, but has been furthermore shown to act like a dynamic tether, retaining vesicles and organelles in the actin-rich periphery of cells. A recent single molecule in vitro reconstitution study on assembling actin filaments suggests that Myosin V travels farther on newly assembling F-actin, while processive runlengths are shorter on older F-actin.
The Myosin V motor head can be subdivided into the following functional regions:
  • Nucleotide-binding site - These elements together coordinate di-valent metal cations and catalyze hydrolysis:
  • * Switch I - This contains a highly conserved SSR motif. Isomerizes in the presence of ATP.
  • * Switch II - This is the Kinase-GTPase version of the Walker B motif DxxG. Isomerizes in the presence of ATP.
  • * P-loop - This contains the Walker A motif GxxxxGK. This is the primary ATP binding site.
  • Transducer - The seven β-strands that underpin the motor head's structure.
  • U50 and L50 - The Upper and Lower domains are each around 50kDa. Their spatial separation forms a cleft critical for binding to actin and some regulatory compounds.
  • SH1 helix and Relay - These elements together provide an essential mechanism for coupling the enzymatic state of the motor domain to the powerstroke-producing region.
  • Converter - This converts a change of conformation in the motor head to an angular displacement of the lever arm.