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Cell and Organelle Movement
I. Motor Proteins
A. Myosin II, first identified in skeletal muscle, fig. 16-51
1. Responsible for generating force for muscle contraction and cytokenesis
2. Elongated protein composed of two heavy chains and two light chains
a. Heavy chain composed of a globular head domain followed by
an extended long amino acid sequence which forms a coiled coil.
b. The globular head domain contains binding sites for actin and
ATP and is responsible for generating the motor force. Most highly
conserved domain.
c. The light chains bind adjacent to the globular head domain in the
“neck” region. These chains serve to regulate the head domain in a
calcium dependent manner.
d. The coiled coil domain bundles together with other myosin
molecules forming bipolar filaments, fig 16-52.
B. Multiple forms of Myosin, fig 16-54.
1. All myosins contain a highly conserved amino terminal motor domain
in the heavy chain with widely divergent carboxy terminal domains.
2. All myosins are regulated in a calcium dependent manner via the light
chains. Differences in light chains result in different responses to cellular
calcium signals.
3. Found in all eukaryotes, arose early in evolution.
C. Myosin activity
1. Except for one form, all myosins move toward the plus end of actin
filaments.
2. Motor activity entirely within the head domain, fig 16-53.
3. ATP dependent
4. Mechano-chemical cycle, dependent upon altered actin affinity, fig. 1658.
D. Kinesin, structurally similar motor protein to Myosin, fig 16-57, both contain a
ras fold.
1. Moves along microtubules.
2. First identified in the squid giant axon.
3. Most family members move towards the plus ends of microtubules.
4. Structurally similar to Myosin II
a. Two heavy chains containing globular head motor domains and
an elongated coiled coil and two light chains, fig 16-55.
b. Large protein superfamily.
5. Most members are involved in organelle and vesicle movement but
some also function in mitotic and meiotic spindle formation and
chromosome separation.
E. Dyneins, figure 16-56.
1. Also move along microtubules, towards minus ends.
2. Exceptionally large, multimeric proteins with MW exceeding
1,000,000. Composed of two to three heavy chains with an undefined
number of light chains.
3. Like myosin and kinesin, two headed motor protein
4. Multiple forms
a. Cytoplasmic, contain two motor heads, involved in vesicular
traffic.
b. Axonemal, contain two to three motor heads, involved in cilliar
and flagellar beating.
c. Minor branch with greater homology to cytoplasmic forms, but
with axonemal functions.
5. Among the fastest motor proteins.
F. Rate of activity is related to function.
1. Processivity, fig. 16-59.
2. Speed, fig 16-61.
G. Regulation, resulting from signal transduction cascades that results in
phosphorylation of relevant chains.
1. Fish Pigment Cells, kinesin v. dynein, fig 16-65, 16-66.
2. Myosin II in non-muscle cells, fig 16-67.
II. Intracellular transport
A. Coordinated function of kinesin and dynein maintain ER and Golgi structure
and localization, fig. 16-62.
B. Vesicles and organelles associate via the long tails and light chains in a
specific manner, fig. 16-63.
C. Alzheimer’s disease.
III. Muscle contraction
A. Dependent upon ATP driven sliding of highly organized actin filaments
against myosin II filament arrays.
B. Specialized, multi-nucleated cell type with cytoplasm dominantly composed of
myofibrils, fig 16-68.
C. Myofibrils
1. 1-2 μm in diameter, often as long as the muscle cell
2. Divided into alternating light and dark bands, aligned along the length
of the muscle cell, fig. 16-69a. The dark bands are bisected by a dark
region, the H zone, while the light bands are bisected by the Z disk or Z
line, fig 16-69b.
3. Composed of sacromeres, a contractile unit, about 2.2 μm. The
sarcomeres are the segment from one Z line to the next.
4. Each sarcomere is composed of an ordered array of thin filaments of
actin and actin associated proteins and thick filaments of bipolar
assembilies of actin.
5. In skeletal muscle, the myosin filaments form a regular hexagonal array
with the actin regularly spaced between, fig. 16-70. Cardiac and smooth
muscle contain the same arrays, but not so regularly ordered.
6. Uniformity of filament structure maintained by accessory proteins, fig
16-72.
a. The actin filament plus ends are anchored into the Z disc which
is composed of CapZ and α-actinin, capping the filament and
preventing actin depolymerization.
b. The over all length is determined by a template protein nebulin
which consists of a repeating 35- amino acid – actin binding
domain.
c. Tropomodulin caps and stabilizes the minus ends of actin.
d. The thick filaments of myosin are positioned uniformly between
the Z line by a long template protein known as titin.
1. Titin acts a molecular spring and contains a long series
of immunoglobulin like domains.
D. Contraction accomplished by the sliding of myosin filaments passed the actin
filaments, fig. 16-71.
1. No change in the length of either filament occurs.
2. Each myosin thick filament contains approximately 300 heads which do
not function in a coordinated manner. Each head cycles approximately
five times per second, allowing the sacromere to shorten its length by 10%
in less than 1/50 th of a second.
3. Rapid synchronized shortening by thousands of adjacent sarcomeres
allows for muscle to contract rapidly enough to accomplish motor
activities such as running and flying.
4. Initiated by a sudden rise in cytosolic calcium.
a. Calcium release from sarcoplasmic reticulum stimulated by
signal transmitted across transverse (T) tubules from nerve signal,
fig 16-73.
b. Calcium signal rapidly down regulated by calcium uptake back
to the sarcoplasmic reticulum by calcium ATPase
c. High ATP expense
d. Regulated by tropomyosin and troponin complex, fig 16-74.
E. Heart Muscle
1. Most heavily worked muscle
2. Multiple myosin isoforms, minor changes in expression can cause
serious heart disease, figure 16-75.
3. Cardiomyopathy
III. Cilia and Flagella
A. Hair-like cellular appendages composed of a microtubule core surrounded by
dynein.
B. Flagella are found on sperm and many protozoa. Allowing for swimming
through liquids. They move in an undulating fashion, fig. 16-76a.
C. Cilia tend to be shorter than flagella but are organized in a similar manner.
They produce a whip like motion, fig. 16-76b.
1. Move almost but not quite in synchrony resulting in a wave like pattern.
2. Can move a single celled organism such as Paramecium or move fluid
over a tissue such as in the respiratory tract or in the oviduct.
D. Movement is the result of the bending of the core, known as the axoneme.
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1. Composed of microtubules and associated proteins in a special pattern,
fig 16-77. Consistent in most eukaryotes.
2. Microtubules extend continuously along the length of the axoneme, 10
– 200 μm.
3. Crosslinked at regular intervals
4. Ciliary dynein forms bridges between neighboring doublets, fig 16-78,
16-79.
a. When the motor dynein is stimulated it attempts to walk along
the adjacent tubule but is prevented by links between tubules
converting what would be a sliding force into a bending force.
5. Basal bodies same form as centrioles, fig 16-80.
6. Kartagener’s syndrome – defect in ciliary dynein
a. immotile sperm leading to male sterility.
b. high susceptibility to lung infections due to paralyzed cilia
c. defects in development of left – right axis during embryonic
development.