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Cytoskeleton
Actin filaments
Myosin
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Cards (23)
Conventional myosin (type
2
) are involved in muscle
contraction
, splitting the cell during cell
division
, and cell migration and
motility
Parts of conventional myosin?
Motor
(head)
Binds to the
actin
filament
binds and hydrolyzes
ATP
2.
Neck
(lever arm)
Moves during power stroke
3.
Tail
Intertwining of the two
heavy
chains
Allows the formation of filaments of
myosin
*S1 fragment includes the
head
and
neck
All myosins except for type VI move towards the
+
(barbed) end of actin
All machinery required for motor activity is contained in the
S1
fragment
Myosin II tails allow the protein to form filaments. In the myosin filament,
tails
point towards the
center
and heads point towards the
outside.
Filaments are:
Bipolar
: reversal of direction at the filament's
center
(i.e., heads are orientated in different directions)
Thick, since they are composed of the myosin
Skeletal muscle organization:
Skeletal muscles are usually anchored to
bones
Muscle fibres
: the cell unit of skeletal muscles that is composed of hundreds of
myofibrils
Myofibrils
: contain both actin and
myosin
and are composed of repeating contractile units called
sarcomeres
Sarcomeres
: The unit of
contraction
that has a characteristic banding pattern
Sarcomere organization:
A-band
: Dark staining pattern due to
overlap
of thick and thin filaments; includes
H-zone.
Every thick filament is surrounded by
6
thin filaments.
H-zone
: only contains thick filaments
I-band
(light staining): only contains thin filaments
Z-line
: contains proteins important for sarcomere's structure and stability
M-line
: dark staining in center of sarcomere, contains anchoring proteins.
Sarcomere visualization
A)
M-line
B)
A-band
C)
I-band
D)
I-band
E)
Z
F)
Z
6
Which zones decrease in length during contraction?
I-band
and
H-zone
Which zones do not decrease in length during contraction?
A-band
;
A-band
is the
length
of myosin filament which doesn't change in length but pulls on actin filaments to
shorten
the sarcomere.
Parts of myosin II...
A)
S1 fragment
B)
head
C)
neck
D)
light
E)
light
F)
tail
6
Molecular basis for contraction:
Myosin II heads in
thick
filaments binds to the six surrounding
thin
filaments
Myosin II is
non-processive
: each head is only in contact with actin for a fraction of the time
Myosin heads are not
synchronized
so the myosin heads act at
different
times, thus allowing smooth continuous movement
Single myosin power stroke moves an actin filament ⁓10nm
Neck acts a
lever
to move actin a
large
distance
Actin myosin contraction cycle:
ATP
binds to myosin head causing myosin to dissociate from
actin
ATP hydrolysis →
ADP
and inorganic
phosphate
remain bound to myosin
Energized
myosin binds to
actin
Release of
phosphate
triggers conformational change:
power stroke
: actin moves towards the center of the sarcomere
ADP
is released,
actin
is still attached
What is a neuromuscular junction?
Point of
contraction
between
motor
neuron and
muscle
fiber; site of transmission of the
nerve
impulse. Stimulated by
neurotransmitters
Motor fibers within a motor unit are stimulated simultaneously by a single motor neuron
How does the nerve impulse cause contraction (excitation-contraction coupling)?
transverse
tubules (T tubules) are
membrane
folds that propagate an impulse to the
interior
of the muscle cell
Sarcoplasmic reticulum is special smooth ER in muscle cells; stores
Ca2+
in lumen (pumped from cytosol)
Arrival of
action potential
at the SR opens
Ca2+
channels, releasing
Ca2+
into the cytoplasm
Excitation-contraction coupling muscle cell diagram:
A)
Neuromuscular junction
B)
Transverse tubules
C)
Myofibril
D)
SR
4
What does Ca2+ control in excitation-contraction coupling?
Controls the interactions between
myosin heads
and
actin.
In the absence of Ca2+:
troponin
controls position of
tropomyosin
which blocks
myosin-binding
sites on
actin
In the presence of Ca2+: Ca2+ binds to
troponin
which moves
tropomyosin
, exposing the
myosin-binding
site on
actin
Ca2+ role in excitation-contraction coupling:
A)
Actin
B)
Troponin
C)
tropomyosin
D)
Ca2+
E)
presence of Ca2+
F)
Absence of Ca2+
G)
troponin
H)
troponin
8
Thin filaments contain:
actin
tropomyosin-
rod
shaped
troponin-
globular
Unconventional myosin (type V):
moves processively along
actin
filaments and with hand-over-hand movement
long
necks
acts as swinging arms
can take very
large
steps (36nm or 13 subunits of G-actin)
Move towards + end
can associate with
vesicles
and
organelles
Act similarly to
kinesin's
role in microtubules
Transport by Unconventional Myosins:
Some vesicles contain both microtubule motors and actin filament motors
Movement over long distances occurs mostlyon microtubules
Local movement in the outskirts of the cell: actin filaments
Practice Q
A)
treadmilling
1