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 stabilizes, with ATP hydrolysis occurring after incorporation generating ADP Pi intermediate that still retains higher affinity than ADP. ADP actin at minus end less stable. Profilin ATP actin addition exclusive to plus end exaggerates bias in vivo. This kinetic difference drives treadmilling when free monomer between two Cc values, enables lamellipodial protrusion where plus ends face membrane, and underlies polarity dependent myosin directionality vital for cell migration and contractility.
Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Barbed End Kinetics and Actin Polarity.