#Arp2/3

Cytoskeleton

In receptor-mediated endocytosis, which protein activates the Arp2/3 complex to drive actin assembly?

Endocytosis of cargo such as EGFR transferrin requires actin assistance in mammalian cells against high membrane tension especially under high membrane stiffness. Clathrin coated pit maturation involves transient actin patch assembly lasting 10 to 20 seconds around neck. Key signaling axis Cdc42 GTP recruits N WASP, which in resting state autoinhibited by intramolecular interaction between basic region GBD and VCA. Activation occurs when Cdc42 GTP binds GBD, PIP2 binds basic region, and SH3 proteins TOCA1 syndapin bind proline rich stretch, releasing VCA. Free VCA binds one G actin via WH2 and Arp2/3 via acidic region bringing complex to side of preexisting filament. Branch nucleation at 70 degrees generates pushing force assisting invagination and cooperates with dynamin GTPase for scission. Cells from Wiskott Aldrich syndrome patients lacking functional WASP show defective TCR endocytosis and actin foci. Formins create unbranched filaments elsewhere, myosin I provides tension but not activation, CapZ restricts growth. Thus WASP family as WASp functions as initiator activating Arp2/3 to drive actin assembly during receptor mediated endocytosis and vesicle formation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: WASP Arp2/3 Activation in Endocytosis.

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Cytoskeleton

Which of the following proteins acts as a nucleation promoting factor for the Arp2/3 complex?

Arp2/3 mediated branch formation depends on nucleation promoting factors bearing C central A acidic region plus WH2 actin binding motifs. Endogenous factors include N WASP activated by Cdc42 and PIP2 in endocytosis and filopodia, WAVE activated by Rac at lamellipodia, WHAMM at Golgi. Bacterial pathogens mimic motif to hijack host cytoskeleton. ActA surface protein of Listeria monocytogenes 639 amino acids contains N terminal polycationic stretch, central proline rich repeats binding Ena VASP and acidic actin binding motifs functioning as VCA equivalent recruiting Arp2/3 with affinity 0.3 micromolar. By polar localization achieved through Sec translocation and proteolytic processing leaving ActA concentrated at old pole, bacterium creates sustained local activation generating polarized comet tail propulsion at 0.05 to 0.3 micron per second enabling cell to cell spread without extracellular exposure. Rho GTP activates formin ROCK but not directly Arp2/3 as NPF, gelsolin caps and severs, vinculin links adhesion plaques. Thus ActA exemplary bacterial NPF for Arp2/3 dependent motility and spread.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: ActA as NPF for Arp2/3 Activation.

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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 force essential for protrusion. Imaging with photoactivatable actin shows retrograde flow balancing polymerization. Pharmacological inhibition CK666 blocking Arp2/3 activation site abrogates lamellipodia formation, cells instead form filopodia via formins, confirming Arp2/3 indispensability for lamellipodia. Myosin II provides retraction at rear, dynein microtubule motor for centrosome positioning, spectrin supporting membrane cortex, while Arp2/3 specifically drives lamellipodia protrusive network and motility. Continuous turnover of branched meshwork allows rapid adaptation to chemotactic cues and directional persistence during migration.

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 molecular weight of the Arp2/3 complex?

Arp2/3 complex purification and characterization resolved molecular composition and mass. Original isolation from Acanthamoeba by Pollard demonstrated seven polypeptides co fractionating through multiple chromatographic steps with actin binding activity. Subunit sequencing identified two actin related proteins Arp2 44 kilodalton and Arp3 50 kilodalton sharing actin fold, plus ARPC1 p40 WD repeat beta propeller 40 kilodalton, ARPC2 p34 34 kilodalton, ARPC3 p21 21 kilodalton, ARPC4 p20 20 kilodalton, ARPC5 p16 14 to 16 kilodalton. Sum calculated 224 kilodalton, sucrose gradient sedimentation and gel filtration elution volume around 200 to 220 kilodalton confirming monomeric assembly. Cryo electron microscopy volume consistent with 15 nanometer globular particle. Higher mass 400 to 600 kilodalton would imply dimer or association with nucleation promoting factor, lower 30 to 50 corresponds to actin monomer, 100 to 150 would be incomplete subcomplex lacking several subunits, insufficient for nucleation. Therefore assignment 200 to 220 kilodalton aligns with structural and biochemical consensus for intact active complex and functional unit.

Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Arp2/3 Complex Molecular Weight and Composition.

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Cytoskeleton

What is the main function of the Arp2/3 complex?

Branched actin organization essential for protrusion relies on Arp2/3 complex discovering daughter filaments on sides of mother filaments. Complex comprises Arp2, Arp3 actin related proteins plus ARPC1 to ARPC5 subunits forming 220 kilodalton assembly. Inactive state holds Arp2 and Arp3 apart preventing nucleation. Upon binding nucleation promoting factor such as WASP WAVE containing VCA acidic domain delivering monomer plus side binding to mother filament via ARPC2 ARPC4 clamp, complex undergoes short pitch activation bringing Arp2 Arp3 together resembling barbed end template. Daughter filament nucleates and elongates with its pointed end capped by Arp2/3 at branch junction, preserving 70 degree angle observed by rotational shadowing. Repeated rounds create dendritic meshwork where growing plus ends push membrane outward, generating force for lamellipodia advancement, phagocytic cup closure and Listeria comet propulsion. Arp2/3 does not depolymerize filaments like cofilin, does not stabilize microtubules which is MAP function, nor transport organelles as myosin does, its core function is branched nucleation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Arp2/3 Complex and Branched Nucleation.

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