Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

924 Chapter 16: The Cytoskeleton
motor domain
1000 amino acids
myosin
type
I
matrix through focal adhesions or by forming a circumferential belt in an epithelial
cell, connecting it to adjacent cells through adherens junctions (discussed in
Chapter 19). As described in Chapter 17, actin and myosin II in the contractile
ring generate the force for cytokinesis, the final stage in cell division. Finally, as
MBoC6 m16.57/16.40
discussed later, contractile bundles also contribute to the adhesion and forward
motion of migrating cells.
Non-muscle cells also express a large family of other myosin proteins, which
have diverse structures and functions in the cell. Following the discovery of conventional
muscle myosin, a second member of the family was found in the freshwater
amoeba Acanthamoeba castellanii. This protein had a different tail structure
and seemed to function as a monomer, and so it was named myosin I (for oneheaded).
Conventional muscle myosin was renamed myosin II (for two-headed).
Subsequently, many other myosin types were discovered. The heavy chains generally
start with a recognizable myosin motor domain at the N-terminus and
then diverge widely with a variety of C-terminal tail domains (Figure 16–40). The
myosin family includes a number of one-headed and two-headed varieties that
are about equally related to myosin I and myosin II, and the nomenclature now
reflects their approximate order of discovery (myosin III through at least myosin
XVIII). Sequence comparisons among diverse eukaryotes indicate that there are
at least 37 distinct myosin families in the superfamily. All of the myosins except
one move toward the plus end of an actin filament, although they do so at different
speeds. The exception is myosin VI, which moves toward the minus end. The
myosin tails (and the tails of motor proteins generally) have apparently diversified
during evolution to permit the proteins to bind other subunits and to interact with
different cargoes.
Some myosins are found only in plants, and some are found only in vertebrates.
Most, however, are found in all eukaryotes, suggesting that myosins arose early in
eukaryotic evolution. The human genome includes about 40 myosin genes. Nine
of the human myosins are expressed primarily or exclusively in the hair cells of
the inner ear, and mutations in five of them are known to cause hereditary deafness.
These extremely specialized myosins are important for the construction and
function of the complex and beautiful bundles of actin found in stereocilia that
project from the apical surface of these cells (see Figure 9–51); these cellular protrusions
tilt in response to sound and convert sound waves into electrical signals.
The functions of most of the myosins remain to be determined, but several are
well characterized. The myosin I proteins often contain either a second actin-binding
site or a membrane-binding site in their tails, and they are generally involved
in intracellular organization—including the protrusion of actin-rich structures at
the cell surface, such as microvilli (see Panel 16–1 and Figure 16–4), and endocytosis.
Myosin V is a two-headed myosin with a large step size (Figure 16–41A)
and is involved in organelle transport along actin filaments. In contrast to myosin
II motors, which work in ensembles and are attached only transiently to actin
filaments so as not to interfere with one another, myosin V moves continuously,
II
III
V
VI
VII
XI
XIV
overall
structure
Figure 16–40 Myosin superfamily
members. Comparison of the domain
structure of the heavy chains of some
myosin types. All myosins share similar
motor domains (shown in dark green),
but their C-terminal tails (light green) and
N-terminal extensions (light blue) are very
diverse. On the right are depictions of
the molecular structure for these family
members. Many myosins form dimers, with
two motor domains per molecule, but a
few (such as I, III, and XIV) seem to function
as monomers, with just one motor domain.
Myosin VI, despite its overall structural
similarity to other family members, is unique
in moving toward the minus end (instead of
the plus end) of an actin filament. The small
insertion within its motor head domain,
not found in other myosins, is probably
responsible for this change in direction.