Actin, Myosin, and Cell Movement 1) Structure of skeletal muscle cells

a) Muscle fiber:

(i) muscle cell (50 μm in diameter, several cm in length)

(ii) consists of bundles of myofibril

(iii) desmin (intermediate filament) binds myofibrils one another side by side

b) each myofibril is organized a chain of sarcomere

c) sarcomere: a contractile unit (2.3 μm long) of a myofibril

2) Skeletal muscle contraction

a) Micro-structure of sarcomere

• A band: dark band

• I band: light band

• Actin: thin filament (7 nm in diameter)

• Myosin: thick filament (15 nm in diameter)

• H zone: middle region of A band

containing only myosin

Striated

Nebulin: *associated with actin filaments

*acts as rulers that determine the length of actin filament

Titin: * extends from the M line to Z disk

*spring-like function

- keep myosin molecules centered

- maintain resting tension to allow the muscle

snap back if overextended

b) Myosin

(i) hydrolyze ATP to move along actin filament from (-) end toward (+) end.

• myosin I:

one globular head

move vesicles on filaments

• myosin II:

two globular head

slide actin filaments during muscle contraction.

Myosin II binding sites

- actin binding sites

- ATPase activity

2 light chains: Essential light chains, regulatory light chain

2 Heavy chains: Globular heads and long α-helical tails, which coil around each

other to form dimmers.

* Driving force: myosin II’s ATP hydrolysis

(ii) Organization of myosin thick filaments

(iii) Sliding-filament model

Myosin action

c) Ca++ regulation of muscle contraction

Ca++ release

• signal from nerve → Ca2+ release from sarcoplasmic reticulum

(10-7 → 10-5 M in cytosol) → contraction

• released Ca++ are pumped back into sarcoplasmic reticulum by Ca2+ ATPase.

Tropomyosin

• A fibrous protein that binds along the actin filaments

• In striated muscle, each tropomyosin is bound to troponin complex

Troponin complex

• T: responsible for positioning the complex on the actin filament.

• I: inhibits interaction between actin & myosin.

• C: Specialized form of calmodulin

When [Ca2+] is high, Ca++ binds to TnC

→ TnC cause TnI to release fromTn complex

→ tropomyosin shift its position

→ myosin head can bind to actin filament

→ contraction

When [Ca2+] is low, TnI moves

tropomyosin to a position where

myosin head and actin interact

→ no muscle contraction

3) Contraction of heart (cardiac) muscle

a) Contractile machinery of cardiac muscle is the same as that of skeletal muscle.

b) Like skeletal muscle, heart muscle is striated.

c) A single nucleus/cell.

d) Intercalated disk:

(i) attaches one cell to the next by means of desmosome.

(ii) connects actin filaments of myofibrils of adjacent cells.

(iii) contain gap junctions, which allow an action potential to spread rapidly

from one cell to the next → action potential generated in one part of heart

will spread to all cardiac muscle cells, and the whole heart will contract.

4) Smooth muscle

a) Not striated.

b) A single nucleus per cell.

c) Highly elongated spindle-shaped cells.

d) Compared to striated muscle, contraction of smooth muscle is slow, but

can produce large movement even though it lacks the leverage provided

by attachments to bones.

5) Nonmuscle cells (in stress fibers, adhesion juction, cytokinesis following mitosis)

a) Assembly (15-20 myosin II)

b) Regulation of myosin by phosphorylation

- Unlikely to skeletal muscle, cytosolic

Ca2+ affects contraction via activation

of MLCK (myosin light-chain kinase)

- Contraction smooth muscle is also

done through this mechanism

4) Unconventional myosin

- Myosin I; more studied than the 12 other classes of unconventional myosin (III ~ XIV)

(i) Cannot induce contraction

(ii) Involved in carrying cargo by moving along the actin filament

7) Cell crawling

- Polymerization & crosslinking of actin filaments are required,

because cytochalasin blocks the formation of surface protrusion

- Myosin II actions are required.

Examples:

• Movement of amoeba

• Migration of embryonic cells during development

• Invasion tissues by WBC to fight infection

• Migration of cells involved in wound healing

• Spread of cancer cells during the metastasis

• Phgocytosis

• Extension of nerve cells during development

Review - Actin, Myosin, and Cell Movement

- Cytoskeleton II

1. Intermediate filaments

2. Microtubules

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