molecular motors: proteins involved in muscle contraction and cell movement

skeletal muscle contraction

• Composition of skeletal muscle

• Structure of the 2 key proteins, namely myosin and actin, involved in skeletal muscle contraction

• The sliding filament model

• Triggering the contraction response

cellular motility

actin treadmilling

muscle fibres

• Very long, multinucleated cells

• Produced by  cell fusion

• Consist of myofibrils (~1000)

microfibrils 

these consist of many thin and thick filaments, surrounded by flattened vesicles containing Ca2+ called sarcoplasmic reticula 

sarcomere

the function unit of contraction

myofibril contraction

during contraction, the length of each filament does not change but the sarcomere reduces in length due to overlapping/sliding of the filament

key proteins involved in muscle contraction

Myosin - found in the thick filaments

Actin - found in the thin filaments

Tropomyosin and troponin - found in thin filaments, inhibitory role

myosin structure

6 polypeptide chains

2 identical heavy chains: each with an N-terminal globular domain (myosin head) and an alpha-helical tail 

function of each myosin head

functions as an ATPase and has an ATP binding site

thick filament structure

several hundred myosin molecules form a staggered array with their globular heads pointing away from the filament 

G-actin structure

Actin is the most abundant cytosolic proteins in eukaryotes. The monomeric form is known as G-actin (g for globular). There are approximately 375 residues. There are 4 protein domains. The binding sites for ATP in the cleft.

actin filament structure (F-actin)

300-400 globular G-actin monomers polymerise (attach to each other) to form a fibrous polymer of actin known as F-actin. F-actin polymers form double-stranded helices.

myosin-actin interaction

The actin filaments is at the top. Myosin head globular regions are coloured blue and green. In a myofibril every actin monomer has the potential to bind one myosin head. The thick filament has many myosin heads projecting from it.

mechanism of force generation in muscle 

The process is unidirectional. The two heads of a myosin molecule function independently of each other. Contraction cycles must be repeated many times to contract muscle. All this happens very fast - in a strong contraction, over 500 myosin heads per thick filament complete this cycle approximately 5 times per second.

tropomyosin and troponin

These are components of the thin filament.

Tropomyosin: dimer of a-helical subunits that form a coiled coil. The coils join to form cables that runs the length of the F-actin filaments.

Troponin consists of 3 subunits:

• Troponin C (binds Ca2+)

• Troponin I (binds actin, prevents binding of the myosin head)

• Troponin T (binds tropomyosin)

In relaxed muscle, the tropomyosin-troponin complex blocks the myosin-binding sites on actin.

Ca2+ triggers muscle contractions

At its resting conformation, tropomyosin-troponin blocks myosin binding to actin (muscle is relaxed).

Nerve impulse -> Ca2+ released from sarcoplasmic reticulum -> intracellular Ca2+ increases

Ca2+ binds troponin C

Triggers conformational change in tropomyosin-troponin to remove the inhibition by troponin I and expose the myosin binding on actin 

why do muscles get thicker after exercise?

Increased muscle fibre diameter known as skeletal muscle hypertrophy. More myofibrils (myosin and actin filaments) are added to each muscle fibre (cell) to increase muscle mass.

What happens when you damage a muscle?

new muscle fibres (cells) are generated

satellite cells

Muscle stem cells. They are located between the sarcolemma and basement membrane of muscle fibres (quiescent). They are involved in normal growth of muscle, muscle plasticity (remodelling) and regeneration after injury/disease. They are the main source of new myonuclei in postnatal skeletal muscle.

does smooth muscle contract the same way as skeletal muscle? not quite… 

Elongated, spindle-shaped cells with a single centrally located nucleus. Contraction is involuntary, control by the autonomic nervous system (ANS).

Smooth is not straited - filaments are not arranged into sarcomeres

• Myosin and actin filaments

• Intermediate fibres

• Dense bodies (analogous to z-line in skeletal muscle)

Ca2+ from extracellular fluid binds calmodulin.

roles of actin in non-muscle cells

Actin is the most abundant cytosolic protein in eukaryotes. G-actin forms polymers of fibrous actin (F-actin) known as microfilaments.

actin microfilaments play a role in

• Changes in cell shape

• Cell division

• Cellular locomotion

• Endocytosis

• Organelle transport

how do cells move? microfilament treadmilling

Actin monomers continually add up to the (+) end of the filament - but dissociate at the same rate from the (-) end. It exerts a force of plasma membrane. This allows cytoplasm to move in one direction.

Actin-mediated cell locomotion is driven by ATP hydrolysis.