Solution and surface effects on plasma fibronectin structure.

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Abstract
As assessed by EM, the shape of the plasma fibronectin molecule ranges from that of a compact particle to an elongated rod-like structure. The effects of solution and surface conditions on fibronectin shape were investigated. Freeze-dried, unstained human plasma fibronectin molecules deposited at pH 7.0-7.4 onto C films and examined by scanning transmission electron microscopy appeared relatively compact and pleiomorphic, with approximate average dimensions of 24 nm .times. 16 nm. Negatively stained molecules also had a similar shape but revealed greater detail in irregular, yarn-like structures were observed. Glutaraldehyde-induced intramolecular cross-linking did not alter the appearance of plasma fibronectin. Molecules deposited at pH 2.8, pH 9.3 or after succinylation were less compact than those deposited at neutral pH. Fibronectin molecules sprayed onto mica surfaces at pH 7, rotary shadowed and examined by transmission electron microscopy were elongated and nodular with a contour length of 120-130 nm. Sedimentation velocity experiments and EM observations indicate that fibronectin unfolds when it is succinylated, when the ionic strength is raised at pH 7, or when the pH is adjusted to 9.3 or 2.8. Greater unfolding is observed at pH 2.8 at low ionic strength (< 0.01) compared with material at that pH in 0.15 M NaCl solution. Apparently, the shape assumed by the fibronectin molecule can be strongly affected by solution conditions and by deposition onto certain surfaces and the images of fibronectin seen by scanning transmission electron microscopy at neutral pH on carbon film are representative of molecules in physiologic solution.