Bone morphogenetic protein-2 and -6 heterodimer illustrates the nature of ligand-receptor assembly.

TGF-beta superfamily ligands are homo- or heterodimeric protein molecules in the body and they recruit two membrane surface receptors known as type I and two type II kinase receptors. By binding to these receptors, the ligand proteins initiate a signaling cascade across the cell membrane. Even with the known three-dimensional atomic structure of the ligands in complex with cell-surface domains of these type I and type II receptors, it was unclear why the binding to these two types of receptors is sequential, meaning binding of one type (e.g. type II) is a prerequisite to recruit and bind the other type (e.g. type I). We generated a bone morphogenetic protein known as BMP-2/6 heterodimer. BMP-2/6 carries two asymmetric interfaces for each receptor type (type I and type II). We demonstrate that the BMP-2/6 possesses high affinity to both receptor types and increased cellular signaling activity when we characterized its property by cell-based chondrogenesis assays. Furthermore, we find that the minimal signaling complex actually consists of two type II receptors and one type I receptor bound to each ligand (e.g. BMP-2/6). Our study reveals how the protein-engineered BMP-2/6 may use their independent binding interfaces to differentially recruit the two different receptors. As a result, BMP-2/6 may generate new biological properties.