Download Cytoskeleton and Cell Motility

Cell Behaviour 3 – Cytoskeleton and Cell Motility
Anil Chopra
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Describe the types of movements that cells undergo.
Explain treadmilling in the context of filament polymerisation
Describe what is meant by molecular motors
Describe the movement of vesicles along microtubules, with reference to the
molecular motors.
5. Describe myosin molecules in general, and the molecular organisation of myosin
II.
6. Describe the formation of myosin filaments.
7. Describe the structure of a skeletal muscle cell, and the arrangement of filaments
and membranes within it.
8. Summarise the sliding filament mechanism of contraction of striated muscle.
9. Describe the sequence of enzymatic changes making up the cross-bridge cycle of
muscle contraction.
10. Explain the molecular basis for muscle stiffness (rigor mortis) after death.
11. Describe the genetic of myosin heavy chains and the relationship between fibre
type and myosin heavy chain expression
12. Describe the functional difference between muscle cells expressing myosin heavy
chains I and II.
13. Describe the distribution of fibre types in human skeletal muscle, and the effect of
sporting activity on fibre type distribution.
14. Compare the mechanisms of control of the cross-bridge cycle in striated and
smooth muscle.
15. Give examples of the different classes of myosin that have been identified, and
correlate their structure with their different functions.
Examples of cell movement
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Migration of phagocytic cells towards site of infection
Migration of cancerous cells away from site of primary tumour - invasion
Migration of cells during embryological development
Cytoplasmic streaming
Muscle contraction
Swimming: waving of cilia/flagellae; movement of liquids
Transport of organelles, Movement of vesicles
Phagocytosis
Mechanisms of Cellular Movement
• Involves coordinated shape changes due to cytoskeleton (actin mainly)
• Needs appropriate signalling to coordinate parts of cell and control direction
• May depend on extracellular signals and receptor pathways
• Cells become polarised - line up with a thin actin-containing extension
(lamellipodium) forming the leading edge
• Microtubule system also is aligned - MTOC (microtubule organising centre) is
forward of the nucleus, as is the Golgi
Cellular Motors
Treadmilling
This is where polymers move by polymerising at one end and depolymerising at the
other.
Kinesins
Kinesins move vesicles along microtubules by
“walking” along the microtubules toward the “plus”
end. They hydrolyse ATP to ADP and Pi.
Dynein and NCD
Dynein and NCD (nonclaret disjunctional) move vesicles towards the minus end of
microtubules. These also hydrolyse ATP to ADP and Pi.
Myosin
There are a number of different myosin classes each with their own function. The one
in muscle is sarcomeric (class II).
It is a heterohexamer: two identical heavy chains and two pairs of non-identical light
chains, the essential and the regulatory light chains.
Its head contains an actin binding domain and an ATPase. Its neck is helical and
contains a sharp bend at the LMM-rod junction. Its tail consists of an α-helix 1096
dimerised residues.
The myosin filaments in skeletal muscle are packed in such a way that
the amount of contractile material in a given volume is increased to give
maximal power. This makes it fast, powerful, efficient and able to be
adapted by training. They are
packed in a helical structure.
Each bundle of filaments is a myofibril, of which there may be hundreds in each
skeletal muscle cell.
 T-cells are invaginations in the sarcoplasmic membrane.
 The sarcoplasmic reticulum is a specialised endoplasmic reticulum, storing
calcium
 A sarcomere is the repeating distance between
adjacent Z-lines.
 H-band consists of myosin filaments (thick
filaments)
 I-band consists of actin filaments (thin filaments)
 A-band consists of myosin filaments and thin
filaments in the overlap region.
 Actin filaments are joined together at the Z-line.
 Myosin filaments are joined together at the M-line
Sarcomeres get shorter when muscles contract. In this process they use ATP. It is
brought about by the “cross-bridges” that form between the myosin and actin
molecules. The heads of the myosin molecule bind to sites on the actin molecule
when activated. This results in the filaments sliding over one another.
In the absence of ATP, Actin and myosin are tightly bound, conferring rigidity and
stiffness to the muscles. This is what happens after death and is called rigor mortis.
The state is reversible upon addition of ATP into the myofilament space.
Myosin Genes
There are 8 heavy chain genes,
2 cardiac genes :
 MyHC-a – chromosome 14
 MyHC-b also called MyHC type I or b-slow, chromosome 17
6 skeletal genes
 MyHC-embryonic – chromosome 17
 MyHC-perinatal – chromosome 17
 MyHC-IIa – chromosome 17
 MyHC-IIb – chromosome 17
 MyHC-IIx/d – chromosome 17
 MyHC-extraocular – chromosome 14
Embryonic and perinatal mysosin heavy chains predominate during early skeletal
muscle development and persists in some muscles such as extra-ocular, laryngeal and
masseter.
The cardiac MyHCs (of which there are 2 – MyHC α and β) are mainly expressed in
the heart, however, MyHCα is also expressed in the masseter and extraocular muscles,
and MyHCβ is expressed in skeletal muscle “slow type I” and “fast type IIa, IIb and
IIx”.
The relative types of myosin between species and within a species is constantly
evolving slowly with different evolutionary advantages. Some muscles in the body are
tonic (i.e. they have more than 50% MyHC I / MyHCβ fibres) or phasic (more than
50% MyHC II). The relative amounts of slow type I and fast type II fibres in muscles
can be changed by training.
Skeletal & Cardiac Muscle
Contraction
 At the neuromuscular junction,
the action potential crosses the
synapse.
 The depolarisation of the muscle
cells results in the release of Ca2+
from the sarcoplasmic reticulum.
 Ca2+ binds to troponin on
tropomyosin causing it to change
shape and expose the myosin
binding sites on the actin.
 Myosin heads bind to the sites
and cause the sliding chain.
Smooth Muscle Contraction
 Controlled by the thick filaments, not the thin filaments as in skeletal or cardiac
muscle.
 Calcium released into the cell as a result of electrical or hormonal activity at the
cell surface binds to a soluble protein, calmodulin.
 The Ca-activated calmodulin binds to and activates myosin light chain kinase
(MLCK) which phosphorylates a small protein that binds around the tail of the
myosin head, myosin light chain 2 (Myosin-LC2).
 Phosphorylation of M-LC2 allows myosin binding to actin and contraction.