Primary structure of a DNA- and heparin-binding domain (Domain III) in XXXX plasma fibronectin.

@entity1 @entity1522 @entity1472 @entity3208 @entity535 @entity794 @entity291
The complete @entity535 sequence of a DNA- and heparin-binding domain isolated by limited thermolysin digestion of @entity1 plasma fibronectin has been obtained. The domain contains 90 @entity1522 with a calculated molecular weight of 10,225. The apparent molecular mass of this domain is 14 kDa when analyzed by @entity3208 -gel electrophoresis. The anomalously high molecular size estimation may be due to the inaccuracy of this method in the low range. The structure was established from microsequence analysis of the chymotryptic, tryptic, and @entity794 protease peptides. The molecular ion of each of the chymotryptic peptides was obtained by fast atom bombardment mass spectrometry. The domain has a preponderance of basic residues with a net charge of +5 at neutral pH. The basic nature of the domain may account for its affinity for the polyanions, DNA and heparin. The predicted secondary structure is beta-sheet, in common with all of the type III internal sequence homology structures obtained for fibronectin so far. The location of the domain in fibronectin was made possible by limited thermolysin digestion and identification of the fragments and by comparison of the sequence of the 14-kDa fragment with the partial structure of @entity291 plasma fibronectin. The domain comprises residues 585-675 and defines a region immediately adjacent to the collagen-binding domain. Numbering domains beginning at the @entity1472 terminus, this domain is Domain III after the fibrin/heparin/actin/ @entity794 binding Domain I and the collagen-binding Domain II. The domain was obtained from a larger precursor (56 kDa) which bound heparin, DNA, and gelatin. Further digestion of the 56-kDa fragment gave rise to a 40-kDa fragment which only bound gelatin, and a 14-kDa fragment which only bound heparin or DNA. The 14-kDa fragment (Domain III) marks the beginning of the type III homology region in fibronectin, for there may be up to 15 repeats of 90 @entity1522 . The size of this domain corresponds to one repeat of 90 @entity1522 and it has some sequence homology to the other type III sequences found thus far in fibronectin.
