#actin polymerization

Regeneration -ll

Actin polymerization in sperm cells occurs primarily during:

Actin polymerization in sperm is temporally restricted to acrosomal reaction. G-actin monomers stored bound to profilin in sperm head rapidly assemble into F-actin bundle forming slender acrosomal process that extends bindin to reach vitelline envelope. Calcium influx and pH rise activate actin nucleation via formin and depolymerization of capping proteins, generating force to protrude process through egg jelly remnants. Cytochalasin B or latrunculin blocking polymerization prevents process formation and fertilization fails. Cortical reaction involves secretory exocytosis in egg, fertilization

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Actin polymerization driving acrosomal process extension in sea urchin sperm.

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Sea urchin -l

Actin polymerization in sperm cells occurs primarily during:

During acrosomal reaction, sperm subacrosomal region contains concentrated pool of globular actin monomers maintained unpolymerized by profilin. Upon Ca2+ influx triggered by egg jelly contact, actin rapidly polymerizes into bundle of filamentous F-actin pushing acrosomal membrane forward forming slender elongated acrosomal process about one micrometer long, thrusting bindin-bearing tip through viscous egg jelly toward vitelline envelope receptor field. Cytochalasin D experiments blocking actin polymerization abolish process extension and prevent fertilization. Fertilization cone actin polymer

Ref: Tilney & Inoue, J Cell Biol 1985, Acrosomal process actin; Gilbert Chapter 7: Actin polymerization in sperm.

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Cytoskeleton

Which protein is responsible for the intracellular movement of Listeria by actin polymerization?

Intracellular pathogenesis of Listeria monocytogenes illustrates actin based propulsion mechanism. After internalization via E cadherin or Met receptor mediated endocytosis, bacterium escapes vacuole via pore forming toxin listeriolysin O and phospholipases PlcA PlcB, enters cytosol and replicates. It then expresses ActA protein anchored via C terminal transmembrane domain, retained at old pole after secretion. ActA N terminal domain mimics eukaryotic nucleation promoting factor recruiting and activating host Arp2/3 complex via acidic motifs and binding Ena VASP via proline rich repeats accele

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Listeria ActA and Actin-Based Motility.

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Cytoskeleton

The polymerization of actin filaments follows which sequence?

Actin assembly exhibits sigmoidal kinetics reflecting cooperative nucleation mechanism. Lag phase corresponds to nucleation where monomers collide forming unstable dimer with Kd micromolar and trimer tetramer nucleus, high free energy barrier due to entropy loss and weak contacts, hence extremely slow without nucleators. Over minutes trimer accumulates reaching critical nucleus size three to four subunits, providing template for rapid elongation phase where ATP G actin adds efficiently at barbed end with near diffusion limited rate, elongating filament linearly until monomer pool depleted to c

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: Actin Assembly Nuceleation Elongation Stages.

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Cytoskeleton

The rate of ATP-actin addition is __ times faster at the (+) end compared to the (-) end.

Elongation kinetics at filament ends measured using TIRF microscopy of single filaments and pyrene actin assembly shows strong asymmetry. Barbed plus end characterized by high on rate constant 11.6 per micromolar per second and off rate 1.4 per second, critical concentration 0.12 micromolar, while pointed minus end on rate 1.3 per micromolar per second off rate 0.8 per second Cc 0.6 micromolar. Ratio on rates roughly nine fold, often approximated as ten fold faster at plus end for ATP actin at physiological ionic strength 50 millimolar KCl 1 millimolar MgCl2. ATP actin cap at plus end stabiliz

Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Barbed End Kinetics and Actin Polarity.

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