The Solution Structure of BMPR-IA Reveals a Local Disorder-to-Order Transition upon BMP-2 Binding

The structure of the extracellular domain of BMP receptor IA was determined in solution by NMR spectroscopy and compared to its structure when bound to its ligand BMP-2. While most parts of the secondary structure are highly conserved between the bound and unbound forms, large conformational rearrangements can be observed in the β4β5 loop of BMPR-IA, which is in contact with BMP-2 and harbors the main binding determinants for the BMPR-IA−BMP-2 interaction. In its unbound form, helix α1 in BMPR-IA, which is in the center of the binding epitope for BMP-2, is missing. Since BMP-2 also shows conformational changes in the type I receptor epitope upon binding to BMPR-IA, both binding partners pass through an induced fit mechanism to adapt their binding interfaces to a given interaction surface. The inherent flexibility of both partners possibly explains the promiscuous ligand−receptor interaction observed in the BMP protein superfamily.