Cryo-high-resolution scanning electron microscopy was used to analyze
the conformation of fibronectin fibrils formed in human skin fibroblast
cultures or in a cell-free system by treating soluble plasma fibronectin
with guanidine. Structurally, fibrils assembled in the cell-free system
and in culture were similar. Assembly of both fibrillar networks involves
interactions with the III1 and amino terminal repeats of
fibronectin; their conformations consist of either smooth surfaces or
patches of smooth surfaces and nodules randomly spaced along the fibril.
The random distribution of these two conformations in fibrils indicates
that fibronectin fibrils are capable of undergoing localized conformational
changes. The nodules may be discrete domains of 3 to 4 type III repeats,
as they can be labeled with the monoclonal antibody IST-2 to the
III13-14 repeats in fibronectin and are found in 160 kDa and
85 kDa fragments of fibronectin that only contain type III repeats. In our
study, smooth regions of fibrils were never recognized by the IST-2 antibody,
suggesting that the epitope in the III13-14 repeats is masked in
these regions. These results demonstrate that fibronectin fibrils are
flexible and certain epitopes of fibronectin may be buried, or exposed,
depending on the conformation of the fibril. They also show that fibrils
assembled in cell-free conditions can be a powerful tool for studying fibril
formation.
Biosynthesis of Bovine Plasma Proteins in a Cell-Free System. Amino-Terminal Sequence of Preproalbumin
