Myosin refers to a family of
motor proteins (other motor proteins include
dynein and
kinesin). To date (Feb 2001) there are 16 known
proteins in this family.
All Myosin's posess a
heavy chain which binds
actin (which in the form of
F-actin forms filaments called
microfilaments that help cells move and function). While these heavy chains are not all identical, they retain a high degree of similarity across the Myosin family.
You may realize (from the Webster 1913 definition) that Myosins are particularly concentrated in cells that are specialized for motility. The myosin found in muscle cells (which actually come in three flavors of
skeletal muscle,
cardiac muscle, and
smooth muscle and here we are referring to skeletal, but other types are relatively analogous in nature) is for the most part
Myosin II.
Myosin I and II are the most abundant myosin proteins found in humans. Unlike
Myosin II, Myosin I also has an actin binding site in it's tail (as well as in its head (the heavy chain)).
Other interesting myosins are
Myosin 5, which is responsible for a good deal of intracellular motility and is especially essential (and highly concentrated) in the vertebrate brain ecause of it's role in transport of
vacuoles essential to
synaptic communication, and
Mysoin 6 and
Myosin 7 which are essential to the eyes an the tiny hairs in the inner ear (
mutations resulting no
expression or misexpression of these proteins often leads to
deaf blind syndrome).
Myosin is frequently used in experiments with actin, and can be used to determine the
polarity of an actin filament. By bindind
Myosin S1 to F-Actin (called "Myosin decorating" by cell biologists), and then adding more G-actin and allowing polymerization, the plus end can be discerned as the side of the "decorated" (initial bit of) actin filament with a much longer undecorated tail.
To learn more about the role and importance of
Myosin in muscle, you might like to read about
Myosin II.