Ganong's Review of Medical Physiology, 23rd Edition

eceptors and form focal adhesion complexes, which serve as
points of traction with the surface over which the cell pulls itself.
In addition, some molecular motors use microfilaments as tracks.
MOLECULAR MOTORS
The molecular motors that move proteins, organelles, and
other cell parts (collectively referred to as “cargo”) to all parts
of the cell are 100 to 500 kDa ATPases. They attach to their
cargo at one end of the molecule and to microtubules or actin
polymers with the other end, sometimes referred to as the
“head.” They convert the energy of ATP into movement along
the cytoskeleton, taking their cargo with them. There are three
super families of molecular motors: kinesin, dynein, and myosin.
Examples of individual proteins from each superfamily
are shown in Figure 2–7. It is important to note that there is
extensive variation among superfamily members, allowing for
specialization of function (eg, choice of cargo, cytoskeletal filament
type, and/or direction of movement).
The conventional form of kinesin is a doubleheaded molecule
that tends to move its cargo toward the “+” ends of
microtubules. One head binds to the microtubule and then
bends its neck while the other head swings forward and binds,
producing almost continuous movement. Some kinesins are
associated with mitosis and meiosis. Other kinesins perform
different functions, including, in some instances, moving cargo
to the “–” end of microtubules. Dyneins have two heads, with
their neck pieces embedded in a complex of proteins. Cytoplasmic
dyneins have a function like that of conventional
kinesin, except they tend to move particles and membranes to
the “–” end of the microtubules. The multiple forms of myosin
in the body are divided into 18 classes. The heads of myosin
molecules bind to actin and produce motion by bending
their neck regions (myosin II) or walking along microfilaments,
one head after the other (myosin V). In these ways,
Conventional kinesin
80 nm
Head 1 Head 2
ADP
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology 37
Cargo
they perform functions as diverse as contraction of muscle
and cell migration.
CENTROSOMES
Near the nucleus in the cytoplasm of eukaryotic animal cells is a
centrosome. The centrosome is made up of two centrioles and
surrounding amorphous pericentriolar material. The centrioles
are short cylinders arranged so that they are at right angles
to each other. Microtubules in groups of three run longitudinally
in the walls of each centriole (Figure 2–1). Nine of these triplets
are spaced at regular intervals around the circumference.
The centrosomes are microtubule-organizing centers
(MTOCs) that contain γ-tubulin. The microtubules grow out
of this γ-tubulin in the pericentriolar material. When a cell
divides, the centrosomes duplicate themselves, and the pairs
move apart to the poles of the mitotic spindle, where they
monitor the steps in cell division. In multinucleate cells, a
centrosome is near each nucleus.
CILIA
Cargo-binding domain
Cilia are specialized cellular projections that are used by unicellular
organisms to propel themselves through liquid and by multicellular
organisms to propel mucus and other substances over the
surface of various epithelia. Cilia are functionally indistinct from
the eukaryotic flagella of sperm cells. Within the cilium there is an
axoneme that comprises a unique arrangement of nine outer microtubule
doublets and two inner microtubules (“9+2” arrangement).
Along this cytoskeleton is axonemal dynein. Coordinated
dynein-microtubule interactions within the axoneme are the basis
of ciliary and sperm movement. At the base of the axoneme
and just inside lies the basal body. It has nine circumferential
triplet microtubules, like a centriole, and there is evidence that
basal bodies and centrioles are interconvertible.
Light
chains
Myosin V
Cytoplasmic dynein
FIGURE 2–7 Three examples of molecular motors. Conventional kinesin is shown attached to cargo, in this case a membrane-bound organelle.
The way that myosin V “walks” along a microtubule is also shown. Note that the heads of the motors hydrolyze ATP and use the energy to produce motion.
4 nm
Head 2
ADP ATP
Head 1
Actin