Eukaryotic Test 3 Lecture 5

All skeletal muscles operate by shortening

Shortening of individual sarcomeres result from the sliding of filaments one over another

Sliding of thin filaments toward the center result in increased overlap of filaments, and decreased width of I and H bands

Actin: Actin filaments of each sarcomere are aligned w/barbed end linked to the Z line

Tropomyosin: Elongated molecule fitting securely into grooves within the thin filament. Each tropomyosin molecule is associated w/ 7 actin subunits

Troponin: Globular protein complex composed of 3 subunits, contacting both actin and tropomyosin

Each thick filament is composed of several hundred myosin II molecules together w/small amount of other proteins. Polarity of thick filaments is reversed at center of the sarcomere

• Titin:
• 38,000 AAs, 3.5 million Daltons.
• Originate at M line and extend along myosin filament, continuing past A band and terminating at Z line

Each myosin head extends outward & binds tightly to thin filament, forming cross-bridges

While bound to actin filament, myosin head undergoes conformational change which moves thin actin filament ~10nm toward center of sarcomere

Muscle myosin is non-processive motor. It remains in contact w/ thin filament for only a small fraction of the overall cycle

1. Molecule of ATP binds to myosin head, inducing disassociation of cross-bridge from actin filament

2. ATP is hydrolyzed. ADP & P remain bound to active site while energy is being absorbed by protein

3. Energized myosin attaches the actin molecule

4. bound P is released triggering large conformational change, shiftin actin filament toward center of sarcomere

5. bound ADP is released

Process in which arrival of nerve impulse at the muscle PM is converted to the shortening of the sarcomeres

Sarcoplasmic Reticulum: Membranous sleeve around the myofibril

About 80% of integral protein of the SR (sarcoplasmic Reticulum) membrane consists of Ca2+ ATPase molecules

Relaxed State: Ca2+ level w/in cytoplasm of muscle cells are very low, below required threshold for contaction

At arrival of action potential by way of T tuules, Ca2+ channels in SR are opened & CA2+ diffuses into myofibrils, triggering muscle contraction

After stimulation CA2+ channels close. Ca2+ is pumped out of cytosol into the lumen of SR

When Ca2+ increase, the interaction between calcium and troponin leads to a movement of tropomyosin from position b to a, exposing the binding site on thin filament to the myosin head

1. Critical components are present in less ordered, labile, transient arrangements

2. They are typically restricted to a thin cortex just beneath the PM

1. Nucleating Proteins: Arp2/3 complex, after activation functions as a template to which actin monomers can be added

2. Monomer-Sequestering Proteins (Thymosins): Bind to ATP monomers & prevent them from polymerizing

3. End-blocking (Capping) Proteins: Binds to one or the other end of the filaments, forming a cap

4. Monomer-Polymerizing Proteins (Profilin): Binds to the same site as thymosin but promotes growth of actin filaments

5. Actin Filament-Depolymerizing Proteins (Cofilin Family): Bind to acting-ADP subunits & the pointed end of actin filaments

6. Cross-linking Proteins: Cross-link 2 or more separated actin filaments to loose networks.

7. Filament-Severing Proteins: Bind to side of existing filament and break it in 2

8. Membrane-Binding Proteins: Link membranes to actin filaments

• 1. Protruding a part of the cell in moving direction
• 2. Form temporary sites of anchorage at lower surface of protrusion
• 3. Move forward
• 4. Breaking rear contacts w/ the substratum

Crawling cells have a broadened frontal end and a narrow tail

Leading edge is extended out from the cell as a broad, flattened, veil-like protrustion called a lamellipodium

• 1. Stimulus is received at one of the cell
• 2. Activation of the Arp2/3 complexes by member of WASP family
• 3. Activated Arp2/3 complexes serve as nucleating sites for actin filaments
• 4. Arp2/3 complexes bind to the sides of new actin filaments
• 5. New actin filaments are nucleated which form as branches and the growth of barbed ends of older filaments are blocked by addition of capping protein
• 6. Addition of acitn subunits to barbed ends of more recently formed filaments pushes membranes of lamellipodium outward. Old capped filaments undergo disassembly from pointed ends