期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2017
卷号：114
期号：15
页码：3897-3902
DOI：10.1073/pnas.1613477114
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe adaptive immune system forms our primary defense against bacteria and viruses. Key players of this system are antigen receptors, dimeric molecules formed by two different types of immunoglobulin domains. It is generally believed that these receptors evolved from an ancestral dimer formed by only a single type of immunoglobulin. Using laboratory evolution, we have recreated such homodimeric receptors and characterized their interactions by X-ray crystallography. Our findings provide molecular insights and support of long-held theories concerning the evolution of the adaptive immune system. They also provide a blueprint for the experimental reconstruction of ancestral proteins in the large number of cases in which evolution has obscured sequence similarities beyond recognition, and which cannot be analyzed using current sequence-based approaches. Ancestral protein reconstruction allows the resurrection and characterization of ancient proteins based on computational analyses of sequences of modern-day proteins. Unfortunately, many protein families are highly divergent and not suitable for sequence-based reconstruction approaches. This limitation is exemplified by the antigen receptors of jawed vertebrates (B- and T-cell receptors), heterodimers formed by pairs of Ig domains. These receptors are believed to have evolved from an extinct homodimeric ancestor through a process of gene duplication and diversification; however molecular evidence has so far remained elusive. Here, we use a structural approach and laboratory evolution to reconstruct such molecules and characterize their interaction with antigen. High-resolution crystal structures of reconstructed homodimeric receptors in complex with hen-egg white lysozyme demonstrate how nanomolar affinity binding of asymmetrical antigen is enabled through selective recruitment and structural plasticity within the receptor-binding site. Our results provide structural evidence in support of long-held theories concerning the evolution of antigen receptors, and provide a blueprint for the experimental reconstruction of protein ancestry in the absence of phylogenetic evidence.
