Institut fur Biochemie, 0-8033 Martinsried bei Miinchen, German Federal Republic
Synopsis
Fibronectin, consisting of two threadlike subunits connected close to their Gterminal ends, exists in a soluble and a fibrous form. Models are presented that propose distinct foldings of the rodlike subunits in the soluble dimer and an extended arrangement in the aggregates. The proposed conformations are based on the analysis of electrostatic attractions and noncovalent affinities between domains. Electrofocusing of proteolytic fragments revealed a sequence of four domains with alternating charges in the Nterminal half of each subunit and two domains with opposite charges close to the G terminus. Complementary sites with affinity to each other were localized by radioimmuno-binding assay in the gelatin-binding and in the subsequent DNA-binding domain of the N-terminal tetradomain sequence. Claiming that in soluble fibronectin electrostatic attractions and noncovalent affinities should be neutralized within the molecule resulted in the construction of a conformation with backfolded subunits, each containing an extra loop in which domains with complementary affiinity sites are saturated by each other. The model is in accord with hydrodynamic and electronmicroscopic data. There is, however, a n alternative folding in which electrostatic and noncovalent affinity sites in the N-terminal half of each subunit are saturated by an interchain interaction within the molecule. Consequently, a rearrangement of the molecule without significant shape change cannot be excluded. In the aggregated form, the N-terminal tetradomain sequence gives rise to a n intermolecular interaction while the Gterminal domains become available for binding ligands.