#actin filament

Cytoskeleton

Which statement is TRUE about myosin and muscle contraction?

Among eighteen myosin classes class II myosins dominate muscle mechanics due to capacity to assemble into higher order structures essential for force scaling. Each myosin II molecule comprises two heavy chains with N terminal motor domains hydrolyzing ATP IQ motifs binding essential and regulatory light chains forming lever and C terminal extended coiled coil driving dimerization plus further self association into filaments. In skeletal muscle dimers polymerize antiparallel into 1.6 micron long thick filaments containing about 300 molecules bare zone in middle heads projecting helically outward 14.3 nm periodicity. These interdigitate with thin actin filaments decorated with troponin tropomyosin facilitating sliding filament mechanism with ordered cross bridge cycling. Non muscle myosin II isoforms form smaller filaments for cortical tension and adhesion. Myosin V and VI transport cargo on actin toward plus and minus ends myosin I tethers membranes myosin X bundles filopodia none form stable contractile thick filaments in sarcomeres. Therefore skeletal and cardiac contractility fundamentally depends on myosin II filament assembly and ATP driven power stroke scaled by light chain phosphorylation.

Ref: Lodish et al., Molecular Cell Biology, Chapter 18 – Myosin II major myosin of muscle thick filaments.

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Cytoskeleton

What is the approximate step size of Myosin V during intracellular transport?

Myosin V stride size uniquely adapted to actin filament helical symmetry enabling straight long distance walks without spiraling around filament axis. The molecule possesses six IQ motifs per heavy chain creating extended lever about 24 nm much longer than myosin II lever. During processive movement heads alternate hand over hand trailing head detaches after ATP binding swings forward as lever rotates about 100 degrees. Actin filament repeats every 13 monomers corresponding to 36 nm axial distance where binding site orientation repeats facing same direction. Center of mass therefore advances 36 nm per ATP hydrolysis cycle matching repeat. Detailed single molecule tracking using optical trap and fluorescence showed individual head moves 72 nm per step while other remains attached ensuring dimer spans two helical repeats. This large step allows melanosomes vacuoles and mRNA to be carried efficiently through cortical actin and explains why artificial short lever mutants show reduced processivity and frequent detachment. Coordination via internal strain dependent gating prevents both heads detaching simultaneously maintaining high duty ratio.

Ref: Mehta et al., Nature 1999; Purcell et al., PNAS 2005 – Myosin V 36 nm center and 72 nm head step mechanism.

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Cytoskeleton

In muscle contraction, when does the myosin head detach from actin?

Force production by myosin II follows an ordered ATPase cycle that tightly couples nucleotide state to actin affinity ensuring efficient energy usage. In rigor nucleotide free myosin binds filamentous actin with high affinity angle about 45 degrees maintaining tension. Rapid binding of Mg ATP to nucleotide pocket causes allosteric opening of actin binding cleft dropping affinity thousand fold and triggering swift detachment within milliseconds even before hydrolysis occurs. While detached intrinsic ATPase hydrolyzes ATP to ADP plus inorganic phosphate providing free energy that reorients lever arm into cocked 90 degree pre stroke conformation. ADP Pi myosin then weakly attaches to new actin monomer Pi release seals cleft and drives power stroke returning lever to post stroke position dragging actin toward M line. Finally ADP dissociates leaving rigor again available for next ATP. Therefore detachment strictly requires ATP binding not hydrolysis; hydrolysis fuels repriming and Pi release triggers force ensuring unidirectional sliding of thin past thick filaments underlying shortening of sarcomeres during contraction.

Ref: Lodish et al., Molecular Cell Biology 9th ed., Chapter 18: Myosin cross-bridge cycle, ATP binding mediated detachment mechanism.

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Cytoskeleton

Tropomodulin prevents actin filament growth by:

Pointed end regulation ensures uniform thin filament length in sarcomeres and stability of short filaments in erythrocyte cytoskeleton. Tropomodulin Tmod family capping proteins bind with nanomolar affinity to pointed minus end, preventing subunit addition and dissociation. Domain structure includes two tropomyosin binding amphipathic helices at N terminus residues 1 to 135 that interact with N terminus of tropomyosin coating filament, and C terminal leucine rich repeat domain residues 160 to 359 forming horseshoe that caps terminal actin subunits contacting subdomain 1 and 3 interface. By clamping pointed end and anchoring tropomyosin, tropomodulin locks filament length after elongation by leiomodin during development, which acts as nucleator competing at same site. Knockout in cardiomyocytes results in elongated thin filaments and dilated cardiomyopathy. Protein does not sequester G actin like thymosin, does not accelerate barbed end depolymerization which is cofilin gelsolin action, and does not bundle filaments like fascin, its specific activity capping minus end and stabilizing length.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Tropomodulin and Pointed End Capping.

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