Structure and flexibility of plasma fibronectin in solution: electron spin resonance spin-label, circular dichroism and sedimentation studies

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Abstract
Human plasma fibronectin was investigated by ESR spin-label methods in conjunction with circular dichroism (CD) and sedimentation techniques to investigate its structure and flexibility in solution. The buried SH groups of fibronectin were modified with a maleimide spin-label. Both conventional and saturation transfer ESR spectra give a rotational correlation time of about (2-3) .times. 10-8 s for plasma fibronectin, a value that is at least 40 times faster than the rotational correlation time calculated from the minimal molecular dimensions. This argues that plasma fibronectin is not a compact, globular protein and suggests that the regions of ordered structural domains have a relatively high degree of independent mobility. ESR, CD and sedimentation measurements showed that many structural features of plasma fibronectin remain unchanged when the pH is decreased from 7.4 to 3.0. ESR results indicate an unfolding of the protein molecule either at pH 11 or in 4 M urea solution. Similarly, the sedimentation coefficient decreases from .apprx. 13 to 8.4 S when the pH is raised to 10.8. At pH values above 11, the CD spectrum resembles a random coil; however, some ordered structure is retained either at pH 11 or in 4 M urea. It is likely that the sulfhydryl-containing regions of the molecule are more sensitive to urea or alkali than are portions of the molecule stabilized by intrachain disulfide bonds. Data obtained from all 3 experimental methods indicate that the protein is more expanded at physiological pH and high ionic strength (.gtoreq. 0.5) but retains its major regions of tertiary structure. The correlation of ESR, CD and ultracentrifugal methods provides an effective way to probe the structure, shape and flexibility of protein molecules.