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#actin

22 public questions tagged with this topic.

Cytoskeleton

Which protein links intermediate filaments to actin filaments and microtubules?

Plectin is a giant cytolinker of the plakin family that simultaneously recognizes actin, microtubules and intermediate filaments, integrating cytoskeletal networks into a cohesive mechanical continuum. Its structure includes an N-terminal actin-binding domain composed of two calponin-homology motifs, a central 200-nm coiled-coil rod dimerization domain, and a C-terminal repeat domain that binds vimentin, keratin, desmin and glial fibrillary acidic protein with distinct isoform specificity. Alternative splicing of first exons targets isoforms to hemidesmosomes via integrin beta4, to focal adhes

Ref: Alberts Ch 16; Fuchs & Cleveland, Science 1998 plakin plectin crosslinks IF, actin, microtubules.

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Cytoskeleton

Myosin II is involved in:

Myosin II is the principal motor generating contractile force for cell division in animal cells. It assembles into bipolar thick filaments through coiled coil tail dimerization, each filament containing about 30 dimers with heads oriented opposite at ends. In anaphase, equatorial cortex enriched in active RhoA GTP recruits anillin, septins and centralspindlin, concentrating myosin II and actin into contractile ring. Phosphorylation of regulatory light chain by ROCK and citron kinase activates myosin II ATPase increasing ten fold, driving filament sliding that constricts ring from 30 micrometer

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Cytokinesis and Contractile Ring Myosin II.

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Cytoskeleton

Which troponin subunit is responsible for calcium binding?

Troponin heterotrimer every 38 nanometer along thin filament alongside tropomyosin provides calcium dependent switch. Subunits: TnT 30 to 35 kilodalton tropomyosin binding anchoring complex, TnI 21 to 24 kilodalton inhibitory binding actin blocking myosin in diastole, TnC 18 kilodalton calcium binding sensor homologous to calmodulin containing four EF hand helix loop helix motifs. N terminal low affinity regulatory sites I II bind calcium transiently during systole with Kd 5 micromolar triggering activation, affinity modulated by magnesium competition, C terminal high affinity structural sites

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: Troponin C as Calcium-Binding Subunit.

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Cytoskeleton

Which protein is responsible for anchoring actin filaments to the Z-disc in muscle cells?

Organization of sarcomere depends on anchoring thin filaments at Z disc providing mechanical integrity during repetitive contraction. Z disc 100 nanometer thick electron dense structure contains alpha actinin antiparallel dimer crosslinking antiparallel barbed ends from adjacent sarcomeres into orthogonal lattice spaced 20 nanometer, plus titin Z repeats, telethonin, myotilin, FATZ. Nebulin huge filamentous protein 600 to 900 kilodalton with 185 repeats of 35 residue nebulin motif SDXXYK each binding one actin monomer, plus N terminus interacting with tropomodulin capping pointed end and C ter

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Nebulin Anchoring Actin at Z-Disc.

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Cytoskeleton

Which of the following determines the speed of myosin movement along actin filaments?

Mechanistic model of myosin step size explains velocity differences among isoforms. Lever arm hypothesis states small conformational change of converter domain upon phosphate release rotation about 10 degrees amplified by rigid lever arm helical neck decorated with IQ motifs bound to light chains acting as fulcrum. Step size roughly equals lever length times angular change, so each IQ motif about 5 to 6 nanometer contributes. Constructing chimeras fusing artificial alpha actinin repeats or duplicating IQ motifs increased sliding speed linearly in motility assays without altering ATPase kcat, c

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Lever Arm Length and Myosin Speed.

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Cytoskeleton

The power stroke in the actin-myosin cycle occurs due to:

Cross bridge cycle parsed by transient kinetics reveals distinct substeps responsible for detachment priming and force generation. Myosin rigor complex actin myosin no nucleotide strongly bound. ATP binding rate 1 to 3 per micromolar per second causes rapid dissociation 500 to 1000 per second. Detached myosin hydrolyzes ATP to ADP plus inorganic phosphate rate about 50 per second inducing recovery stroke moving lever arm into pre power stroke high energy conformation approximately 90 degree rotation storing strain. Myosin ADP Pi weakly binds actin via electrostatic contacts. Isomerization into

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: Power Stroke and Phosphate Release Mechanism.

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Cytoskeleton

Which myosin type is responsible for intracellular cargo transport?

Intracellular cargo trafficking covers both microtubule and actin routes. Long range 10 to 100 micron interphase transport uses kinesin dynein on microtubules, while short range local transport within cortical actin rich zones 1 to 10 micron uses myosin. Processive myosin capable of multiple steps without dissociation required for efficient transport. Myosin V class features high duty ratio spending about 70 percent cycle strongly attached, dimeric coiled coil tail, six IQ motifs with calmodulin light chains per head forming long 24 nanometer lever arm achieving 36 nanometer step matching heli

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Myosin V and Cargo Transport.

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Cytoskeleton

The rapid turnover of actin filaments in vivo compared to in vitro is primarily due to:

Pure actin solution polymerized exhibits slow turnover due to low off rates 0.2 to 1 per second and minimal nucleation. In living cell turnover accelerated 50 to 100 fold evidenced by FRAP recovery half time 10 to 30 seconds for cortical actin. Acceleration attributed to collaborative action of cofilin and profilin. Cofilin severs ADP actin filaments increasing number of pointed ends depolymerizing at 10 per second vs 0.8 spontaneous and also increases off rate at pointed ends. Profilin ADP G actin complex undergoes rapid nucleotide exchange rate constant increase 1000 fold from 0.02 to 20 per

Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Rapid Actin Turnover by Cofilin Profilin In Vivo.

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Cytoskeleton

Which of the following proteins is required for the formation of lamellipodia?

Lamellipodium broad veil like extension 1 to 5 micron wide 0.1 to 0.2 micron thick driving mesenchymal migration requires continuous branched actin assembly at leading edge. Key organizer is Rac1 GTP small GTPase activating pentameric WAVE regulatory complex comprising WAVE, Abi, Nap1, Sra1 and HSPC300. Upon activation Rac1 binding to Sra1 releases VCA domain of WAVE which binds G actin and Arp2/3 complex at membrane, inducing nucleation of daughter filaments at 70 degree angle. Iterative branching creates dense network with barbed ends abutting membrane, polymerizing and generating pushing fo

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Lamellipodia Formation and Arp2/3 Requirement.

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Cytoskeleton

What is the function of CapZ in actin regulation?

Barbed end availability determines where new polymerization occurs in lamellipodia and filopodia. CapZ heterodimer alpha beta forms cap with extremely low dissociation rate half life 30 minutes in vitro. Binding interface extensive, burying 2000 square angstroms, alpha C terminal tentacle interacting with hydrophobic cleft between actin subdomains 1 and 3 of ultimate subunit, beta tentacle contacting penultimate subunit groove, physically blocking monomer docking. Quantitative fluorescence speckle microscopy shows most barbed ends in resting cells are capped, only about 5 percent free, but sec

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: CapZ Barbed End Capping Function.

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Cytoskeleton

Which of the following proteins is a major crosslinker of actin filaments in the cytoskeleton?

Actin filaments crosslinked into distinct architectures produce diverse mechanical properties. Short crosslinkers like fimbrin fascin 8 to 12 nanometer spacing generate tight parallel bundles in microvilli and filopodia for protrusion, long crosslinkers alpha actinin filamin 30 to 40 nanometer create loose networks and antiparallel bundles allowing myosin II insertion for contraction. Alpha actinin antiparallel homodimer 35 nanometer rod contains N terminal actin binding domain with paired calponin homology CH1 CH2, four spectrin repeats dimerization core and C terminal calmodulin like EF hand

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Alpha-Actinin Crosslinking Actin Filaments.

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Cytoskeleton

Which of the following proteins stabilizes actin filaments by binding along their length?

Skeletal muscle thin filaments extend from Z disc to near M line, length precisely regulated 1.0 micrometer in cardiac, 1.1 to 1.3 in skeletal for optimal overlap with thick filaments for force generation. Nebulin, 600 to 900 kilodalton protein encoded by NEB gene, composed of 185 tandem nebulin repeats of 35 amino acids each predicted to contact single actin monomer plus C terminal SH3 domain anchoring at Z disc via interactions with CapZ, titin Z repeats and desmin intermediate filaments. By spanning entire thin filament, nebulin acts as molecular ruler templating polymerization length durin

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Nebulin and Thin Filament Stabilization.

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