title: Nucleation of actin polymerization by villin and elongation at subcritical monomer concentration.
abstract: We have obtained a quantitative description of villin-nucleated actin polymerization in physiological salt by determining the concentrations of free villin (V), villin-actin monomer (VA), villin-actin dimer (VA2), and villin-actin oligomer (VAn). Over a range of actin-villin ratios from 0.1 to 20 we determined the concentration of actin-bound villin by measuring the low-intensity pyrenylactin fluorescence of the two terminal actins in each villin-actin polymer. (To this end we first showed that each villin-actin oligomer and polymer contains two low-intensity pyrenylactin molecules.) We determined the concentration of free villin using a calibrated cutting activity assay. The pattern of increase in bound villin together with the pattern of increase in high-intensity pyrenylactin fluorescence with increasing G-actin concentration indicated, first, that villin-actin monomers were not formed at detectable levels even at a 12-fold villin excess over actin. Second, there was no stoichiometric villin-actin dimer formation at actin-villin ratios of 2. Instead there was an equilibrium between free villin, VA2, and VAn. Defining K1 = [VA]/[V][A] and K2 = [VA2]/[VA][A], a good fit of the data was obtained with K1 much less than K2 and a value of K1K2 = Kv = 10(12)-10(13) M-2 = [VA2]/[V][A]2, i.e., 1/Kv1/2 = (0.3-1) X 10(-6) M. We have assumed here that the monomer binding constant of VA2 to form VA3 was equal to the monomer binding constant of pointed filament ends, K infinity = 1/c infinity, obtained as described below.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000199:Actins / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D007700:Kinetics; D046911:Macromolecular Substances; D008433:Mathematics; D008840:Microfilament Proteins / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
chemical_list: D000199:Actins; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D046911:Macromolecular Substances; D008840:Microfilament Proteins; C026615:villin; D005033:Ethylmaleimide