Types
of motor proteins
Myosin
Myosins
are actin motors that form a superfamily of more than 15 members. One such
member, myosin V has been shown to be involved in the transport of membranous
cargo such as yeast vacuoles, secretory vesicles and melanosomes. The overall
structure of myosin V is depicted here. It is a dimeric molecule with each heavy
chain consisting of a head, neck and tail.
http://physiology.med.uvm.edu/jmoore/m5background.html
The
head:
The myosin V head domain, sometimes referred to as the motor domain is ~80 kD
and contains both the actin binding and ATP hydrolytic activities. Sequence
comparison of the motor domains from members of the myosin super-family reveal 3
primary regions of variability which have been proposed to be related to the
functional and enzymatic differences between myosins.
The
neck:
The neck domain of all myosins consists of repeating copies of the calmodulin-binding
IQ motif (IQXXXRGXXXR). However, variations on the sequence and number of motifs
are found among all myosins. Myosin V has
a neck that contains 6 IQ motifs, each of which has the potential to bind to
calmodulin.
The
Tail:
The tail region of myosin V consists of regions of alpha helical coiled coil
that are interrupted by globular regions. The coiled coil regions play a role in
dimerization while the globular regions are involved in cargo binding.
Kinesin
Conventional
kinesin is a dimer consisting of two identical 120 to 130-kDa chains, commonly
known as heavy chains. When kinesin is purified from brain homogenates, two
light chains (60 to 70 kDa) were found to be associated with the dimers .These
light chains, which are involved in the kinesin binding to organelles, are not
essential for motility generation and seem to have regulatory functions. Each
heavy chain contains a N-terminal globular motor domain (approximately 10 nm x 9
nm) with both a microtubule-binding site and an ATPase-active centre, a stalk
region, which is responsible for heavy chain dimerization, and a C-terminal
fan-shaped tail domain, which is implicated in cargo binding.
The
kinesin head domain, which is known to be a highly conserved region
characteristic for very different members of the kinesin superfamily, contains
both the microtubule binding domain and the ATP hydrolyzing centre.
http://www.dentistry.leeds.ac.uk/biochem/MBWeb/mb2/part1/kinesin.htm
Cytoplasmic
dynein is a multi-subunit protein complex composed of two identical heavy chains
of about 530 kDa each (dark blue), two 74 kDa intermediate chains (magenta and
yellow) and about four 53-59 kDa intermediate chains (green). In addition, there
are several light chains. The heavy chains each contain four ATP-binding sites,
including the ATP-hydrolytic site that provides the energy for its movement
along microtubules, and a microtubule binding site.
