Matrix GLA Protein, a Regulatory Protein for Bone Morphogenetic Protein-2

• Published in: The Journal of biological chemistry Vol. 277; no. 6; pp. 4388 - 4394
• Main Authors: Zebboudj, Amina F., Imura, Minori, Boström, Kristina
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 08.02.2002; American Society for Biochemistry and Molecular Biology
• Subjects: Animals; Bone Morphogenetic Protein 2; Bone Morphogenetic Proteins - metabolism; Bone Morphogenetic Proteins - physiology; c-Myc protein; Calcium-Binding Proteins - physiology; Cell Differentiation - physiology; Cell Line; Culture Media, Conditioned; DNA-Binding Proteins - metabolism; Extracellular Matrix Proteins; FLAG protein; Humans; Immunohistochemistry; Matrix Gla Protein; Mice; Phosphorylation; Precipitin Tests; Protein Binding; Signal Transduction - physiology; Smad Proteins; Smad1 Protein; Smad2 protein; Trans-Activators - metabolism; Transforming Growth Factor beta
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M109683200

Matrix GLA protein (MGP) has been identified as a calcification inhibitor in cartilage and vasculature. Part of this effect may be attributed to its influence on osteoinductive activity of bone morphogenetic protein-2 (BMP-2). To detect binding between MGP and BMP-2, we performed immunoprecipitation using MGP and BMP-2 tagged with FLAG and c-Myc. The results showed co-precipitation of BMP-2 with MGP. To quantify the effect of MGP on BMP-2 activity, we assayed for alkaline phosphatase activity and showed a dose-dependent effect. Low levels of MGP relative to BMP-2 (<1-fold excess) resulted in mild enhancement of osteoinduction, whereas intermediate levels (1–15-fold excess) resulted in strong inhibition. High levels of MGP (>15-fold excess), however, resulted in pronounced enhancement of the osteoinductive effect of BMP-2. Cross-linking studies showed that inhibitory levels of MGP abolished BMP-2 receptor binding. Immunoblotting showed a corresponding decrease in activation of Smad1, part of the BMP signaling system. Enhancing levels of MGP resulted in increased Smad1 activation. To determine the cellular localization of BMP-2 in the presence of MGP, binding assays were performed on whole cells and cell-synthesized matrix. Inhibitory levels of MGP yielded increased matrix binding of BMP-2, suggesting that MGP inhibits BMP-2 in part via matrix association. These results suggest that MGP is a BMP-2 regulatory protein.