Primary structure of a glycosylated DNA-binding domain in human plasma fibronectin

The complete amino acid sequence of a DNA-binding domain isolated from human plasma fibronectin after limited trypsin digestion has been obtained. It contains 132 amino acids and one biantennary glycosyl unit at residue 104, for an estimated Mr of 16,931. The fragment can be purified by a two-step procedure consisting of DNA-affinity chromatography and reverse-phase high performance liquid chromatography. It can also be purified by heparin-affinity chromatography. The domain is unusual in its susceptibility to tryptic-like cleavages even by neutral or aromatic residue-specific proteases. It has no cysteine residues and is predicted to favor a beta-sheet structure by Chou and Fasman analysis. Based on this analysis we have proposed a model which exhibits a clustering of aromatic and basic residues, consistent with similar involvement of basic and aromatic residues in other DNA-binding proteins. The net charge of the domain at neutral pH (+1, without sialic acid) argues against a nonspecific charge interaction with polyanionic macromolecules such as DNA and heparin. Internal sequence repeats occur at intervals of 30, 60, and 90 residues, thus suggesting a maximum size for a repetitive building block which gave rise to this domain.