27Binding affinity maturation without structure determination remains a difficult challenge in the 28 computer-aided protein engineering. Precise binding mode identification is a vital prerequisite for 29 the affinity maturation. However, pure computational methods have been unreliable in practice so 30 far and experimental structural biology techniques are generally too costly. Herein, we show that 31 computational epitope localization followed by the full-atom energy minimization with 32 intermediate experimental validation can yield precisely bound complex model structure, which 33 ultimately enables effective affinity maturation and redesign of binding specificity. As a proof-of-34 concept, we targeted a leucine-rich repeat (LRR) protein binder which specifically binds to the 35 human IgG1 (hIgG1). Based on the computationally predicted binding mode of the LRR protein 36 binder to hIgG1, the binding affinity of the protein binder was significantly increased and its 37 specificity was redesigned toward multiple IgGs from other species. Experimental determination 38 of the complex structure showed that the predicted model closely matched the X-ray crystal 39 structure. Through the benchmark of therapeutically relevant existing LRR protein complexes, we 40 demonstrate that the present approach can be broadly applicable to other proteins which undergo 41 small conformational changes upon binding. 42 43
Significance Statement 44 45Computer-aided design of binding affinity and specificity of two interacting proteins without 46 structural determination is a challenging problem in protein engineering. Despite recent advances 47 in the computational biology techniques, however, in silico evaluation of binding energies has 48 proven to be extremely difficult. We show that, in the cases of protein-protein interactions where 49 only small structural changes upon binding occur, partial experimental validation of binding can 50 greatly complement the computational energy. Using an LRR (leucine-rich repeat) protein binder 51 as a model system, the binding orientation of two binding partners was precisely dentified by the 52 hybrid approach. Based on the predicted model, we could successfully redesign the binding affinity 53 and specificity of the protein binder by the full-atom energy calculation. 54 55