Since their discovery as pluripotent cytokines extractable from bone matrix, it has been speculated how bone morphogenetic proteins (BMPs) become released and activated from the extracellular matrix (ECM). In contrast to TGF‐βs, most investigated BMPs are secreted as bioactive prodomain (PD)–growth factor (GF) complexes (CPLXs). Recently, we demonstrated that PD‐dependent targeting of BMP‐7 CPLXs to the extracellular fibrillin microfibril (FMF) components fibrillin‐1 and ‐2 represents a BMP sequestration mechanism by rendering the GF latent. Understanding how BMPs become activated from ECM scaffolds such as FMF is crucial to elucidate pathomechanisms characterized by aberrant BMP activation and ECM destruction. Here, we describe a new MMP‐dependent BMP‐7 activation mechanism from ECM‐targeted pools via specific PD degradation. Using Edman sequencing and mutagenesis, we identified a new and conserved MMP‐13 cleavage site within the BMP‐7 PD. A degradation screen with different BMP family PDs and representative MMP family members suggested utilization of the identified site in a general MMP‐driven BMP activation mechanism. Furthermore, sandwich ELISA and solid phase cleavage studies in combination with bioactivity assays, single particle TEM, and in silico molecular docking experiments provided evidence that PD cleavage by MMP‐13 leads to BMP‐7 CPLX disintegration and bioactive GF release.

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Bone morphogenetic proteins (BMP) are powerful regulators of cellular processes such as proliferation, differentiation, and apoptosis. However, the specific molecular requirements controlling the bioavailability of BMPs in the extracellular matrix (ECM) are not yet fully understood. Our previous work showed that BMPs are targeted to the ECM as growth factor‐prodomain (GF‐PD) complexes (CPLXs) via specific interactions of their PDs. We showed that BMP‐7 PD binding to the extracellular microfibril component fibrillin‐1 renders the CPLXs from an open, bioactive V‐shape into a closed, latent ring shape. Here, we show that specific PD interactions with heparin/heparan sulfate glycosaminoglycans (GAGs) allow to target and spatially concentrate BMP‐7 and BMP‐9 CPLXs in bioactive V‐shape conformation. However, targeting to GAGs may be BMP specific, since BMP‐10 GF and CPLX do not interact with heparin. Bioactivity assays on solid phase in combination with interaction studies showed that the BMP‐7 PD protects the BMP‐7 GF from inactivation by heparin. By using transmission electron microscopy, molecular docking, and site‐directed mutagenesis, we determined the BMP‐7 PD‐binding site for heparin. Further, fine‐mapping of the fibrillin‐1‐binding site within the BMP‐7 PD and molecular modeling showed that both binding sites are mutually exclusive in the open V‐ versus closed ring‐shape conformation. Together, our data suggest that targeting exquisite BMP PD‐binding sites by extracellular protein and GAG scaffolds integrates BMP GF bioavailability in a contextual manner in development, postnatal life, and connective tissue disease.