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.
