Dependence of the shape of the plasma fibronectin molecule on solvent composition. Ionic strength and glycerol content.

Fibronectin has been purified to apparent homogeneity according to measurements of molecular weight and diffusion constant from light scattering and sedimentation in the analytical ultracentrifuge. This gives two estimates of molecular weight close to 500,000. (The difference with the gel electrophoretic value of 440,000 may indicate the presence of some material of molecular weight higher than that of the monomer.) The (corrected) diffusion constant is found to decrease with ionic strength, much more rapidly in 30% glycerol than in water. A corresponding change occurs when the glycerol content is varied from 0 to 30% at moderate ionic strength, but at very low ionic strength the diffusion constant does not depend on glycerol content. It is concluded that fibronectin can occur in two extreme conformations: the open form, in 30% glycerol at moderate ionic strength, should correspond to the extended shapes visualized by others by electron microscopy of samples prepared from solutions containing glycerol, while the closed form occurs under more physiological conditions. The Stokes radii of these forms, 14.5 and 9.6 nm, respectively, have been compared with calculated Stokes radii of chains of beads of overall length 140 nm having varying stiffness, simulated by a Monte Carlo procedure. These have a Stokes radius of 14.5 nm when made relatively extended to where the distance between chain ends averages 90 nm, as observed in the electron micrographs. When these chains are made very flexible, the Stokes radius reaches a limit of about 10 nm, apparently a lower limit to which a randomly coiled molecule of this size and volume may collapse in a disordered fashion. These results and electron micrographs obtained at low ionic strength suggest a tangled model of the compact form, rather than the ordered model recently proposed by others. Finally, some speculation is offered in regard to a possible physiological role of a conformation change of fibronectin.