Kringles: modules specialized for protein binding

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Abstract
Prothrombin, plasminogen, urokinase- and tissue-type plasminogen activators contain homologous structures known as kringles. The kringles correspond to autonomous structural and folding domains which mediate the binding of these multidomain proteins to other proteins. During evolution the different kringles retained the same gross architecture, the kringle-fold, yet diverged to bind different proteins. We show that the amino acid sequences of the type II structures of the gelatin-binding region of fibronectin are homologous with those of the protease-kringles. Prediction of secondary structures revealed a remarkable agreement in the positions of predicted β-sheets, suggesting that the folding of kringles and type II structures may also be similar. As a corollary of this finding, the disulphide-bridge pattern of type II structures is shown to be homologous to that in kringles. It is noteworthy that protease-kringles and fibronectin type II structures have similar functions inasmuch as they mediate the binding of multidomain proteins to other proteins. It is proposed that the kringles of proteases and type II structures of fibronectin evolved from a common ancestral protein binding module.