Abstract
Bone morphogenetic protein-2 (BMP-2) induces bone formation and regeneration in adult vertebrates and regulates important developmental processes in all animals. BMP-2 is a homodimeric cysteine knot protein that, as a member of the transforming growth factor-beta (TGF-beta) superfamily, signals by oligomerizing type I and type II receptor serine-kinases in the cell membrane. The binding epitopes of BMP-2 for BMPR-IA (type I) and BMPR-II or ActR-II (type II) were characterized using BMP-2 mutant proteins for analysis of interactions with receptor ectodomains. A large epitope 1 for high-affinity BMPR-IA binding was detected spanning the interface of the BMP-2 dimer. A smaller epitope 2 for the low-affinity binding of BMPR-II was found to be assembled by determinants of a single monomer. Symmetry-related pairs of the two juxtaposed epitopes occur near the BMP-2 poles. Mutations in both epitopes yielded variants with reduced biological activity in C2C12 cells; however, only epitope 2 variants behaved as antagonists partially or completely inhibiting BMP-2 activity. These findings provide a framework for the molecular description of receptor recognition and activation in the BMP/TGF-beta superfamily.