The putative RGD-dependent cell adhesion activity of matrix Gla protein is due to higher molecular weight contaminants.

This study was carried out to further characterize the RGD-dependent cell adhesion activity which was previously observed in the vitamin K-dependent matrix Gla protein (MGP) (Loeser, R. F., and Wallin, R. (1992) J. Biol. Chem. 267, 9459-9462). We have found that this cell adhesion activity can be completely removed from the 10-kDa MGP by gel filtration over Sephacryl S-200-HR. The higher molecular weight contaminants removed by the gel filtration step display potent cell adhesion activity. The additional evidence previously adduced in support of the putative cell adhesion activity of MGP is that heat decarboxylation of the vitamin K-dependent gamma-carboxyglutamate residues in MGP abolished the adhesion activity. The heat decarboxylation conditions used, however, appear to cause other chemical changes in proteins in addition to decarboxylation, as evidenced by the fact that the cell adhesion activity of fibronectin, which is not a vitamin K-dependent protein, is also destroyed by this procedure. The present evidence that the putative cell adhesion activity of MGP is caused by contaminating higher molecular weight cell adhesion proteins accounts for two apparent anomalies in the previously reported cell adhesion activity of MGP, the failure of antibodies raised against a synthetic peptide corresponding to the C-terminal 19 residues of bovine MGP to inhibit the cell adhesion activity of the intact, 79-residue bovine protein, and the potent inhibition of the cell adhesion activity of MGP by a synthetic peptide containing an RGD sequence, even though MGP does not contain this sequence.