Structural basis for monobody OP-4 binding to open-form adenylate kinase

Monobodies are fibronectin type-III-based binding proteins that specifically bind target proteins and regulate their functions. We previously identified monobodies that selectively recognize either the OPEN or CLOSED conformation of adenylate kinase (Adk), revealing that monobodies can discriminate distinct conformational states of a target protein. However, the molecular basis of OPEN-form recognition has remained unclear because the structure of the complex between an OPEN-form-specific monobody and Adk had not been determined. To address this issue, we determined the crystal structure of the complex between Adk and the OPEN-form-specific monobody OP-4, employing hierarchical clustering analysis of X-ray diffraction datasets. The structure revealed that OP-4 binds to the surface formed by the expanded LID and CORE domains of Adk. Mutations in the interface residues reduced the OP- 4-binding affinity, indicating that the crystallographically identified interface is also relevant in solution. In particular, R123 mutations markedly impaired OP-4 binding. Molecular dynamics simulations further suggested that the R123–D159–R156 hydrogen-bond network is retained in solution and may contribute to efficient complex formation.

These findings establish the structural basis for monobody OP-4 binding to open-form Adk and identify the R123-centered interaction network as a key determinant of complex formation.