The pro-form of BMP-2 interferes with BMP-2 signalling by competing with BMP-2 for IA receptor binding

Pro-forms of growth factors have received increasing attention since it was shown that they can affect both the maturation and functions of mature growth factors. Here, we assessed the biological function of the pro-form of bone morphogenetic protein-2 (BMP-2), a member of the transforming growth factor beta (TGFbeta)/BMP superfamily. The role of the 263 amino acids of the pro-peptide is currently unclear. In order to obtain an insight into the function of the pro-form (proBMP-2), the ability of proBMP-2 to induce alkaline phosphatase (AP), a marker enzyme for cells differentiating into osteoblasts, was tested. Interestingly, in contrast to mature BMP-2, proBMP-2 did not lead to induction of AP. Instead, proBMP-2 inhibited the induction of AP by BMP-2. This result raised the question of whether proBMP-2 may compete with mature BMP-2 for receptor binding. ProBMP-2 was found to bind to the purified extracellular ligand binding domain (ECD) of BMPR-IA, a high-affinity receptor for mature BMP-2, with a similar affinity as mature BMP-2. Binding of proBMP-2 to BMPR-IA was confirmed in cell culture by cross-linking proBMP-2 to BMPR-IA presented on the cell surface. In contrast to this finding, proBMP-2 did not bind to the ECD of BMPR-II. ProBMP-2 also differed from BMP-2 in its capacity to induce p38 and Smad phosphorylation. The data presented here suggest that the pro-domain of BMP-2 can alter the signalling properties of the growth factor by modulating the ability of the mature part to interact with the receptors.