title: Brownian motion of inert tracer macromolecules in polymerized and spontaneously bundled mixtures of actin and filamin.
abstract: By use of light microscopy and fluorescence photobleaching recovery, we have studied (a) structures that form in a system composed of copolymerized rabbit muscle actin and chicken gizzard filamin and (b) the Brownian motion of inert tracer macromolecules in this matrix. We have used as tracers size-fractionated fluorescein-labeled ficoll and submicron polystyrene latex particles. In F-actin solutions, the relative diffusion coefficient of the tracer was a decreasing function of both tracer size and actin concentration. Also, a percolation transition for latex particle mobility was found to follow a form suggested by Ogston (Ogston, A. G. 1958. Trans. Faraday Soc. 54:1754-1757) for random filament matrices. The inclusion of filamin before polymerization resulted in increased tracer mobility. Below a filamin dimer-to-actin monomer ratio of 1:140, no structural features were observed in the light microscope. At or above this ratio for all actin concentrations tested, a three-dimensional network of filament bundles was clearly discriminated. Latex particles were always excluded from the bundles. By use of a dialysis optical cell in which polymerization could be initiated with very little hydrodynamic stress, we found that filamin can spontaneously bundle F-actin. A simple physical picture explains how dynamics can affect the structural result of coassembly and provides a further hypothesis on the balance between random filament cross-linking and large-scale bundling. Control of this balance may be important in cytoplasmic motile events.
authors.affiliation: Center for Fluorescence Research in Biomedical Sciences, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213.
authors.forename: L
authors.initials: L
authors.lastname: Hou
authors.forename: K
authors.initials: K
authors.lastname: Luby-Phelps
authors.forename: F
authors.initials: F
authors.lastname: Lanni
mesh_terms: D000199:Actins / Q000502:physiology*; D001704:Biopolymers; D003285:Contractile Proteins / Q000502:physiology*; D004058:Diffusion; D005362:Ficoll; D064448:Filamins; D016650:Fluorescein-5-isothiocyanate; D005452:Fluoresceins; D005782:Gels; D008840:Microfilament Proteins / Q000502:physiology*; D008853:Microscopy; D009038:Motion; D055095:Optics and Photonics / Q000295:instrumentation; D013861:Thiocyanates; D014741:Video Recording
chemical_list: D000199:Actins; D001704:Biopolymers; D003285:Contractile Proteins; D064448:Filamins; D005452:Fluoresceins; D005782:Gels; D008840:Microfilament Proteins; D013861:Thiocyanates; D005362:Ficoll; D016650:Fluorescein-5-isothiocyanate