Practice question

Polarity arises because actin protomers are asymmetric and assemble head to tail orienting ATP hydrolysis vector. Myosin S1 decoration electron microscopy historically defined barbed and pointed ends based on arrowhead appearance. Kinetic measurements show association rate constant for ATP G actin about 11.6 per micromolar per second at plus end versus 1.3 at minus end, dissociation rate 1.4 per second versus 0.8. Net result polymerization velocity ten times faster at barbed end at any monomer level above Cc. This asymmetry explained by structural differences: barbed end exposes W loop and hydrophobic cleft accommodating incoming monomer, while pointed end requires subdomain rearrangement less favorable with ADP actin. In migrating cells free barbed ends concentrate at plasma membrane oriented outward producing pushing force for lamellipodia elongation. Minus ends face cell interior often capped by tropomodulin or embedded in Arp2/3 branch. Myosin V and most myosins move toward plus end guided by polarity, myosin VI moves opposite. This rapid growth bias enables directional protrusion and polarized transport.