Practice question

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.