Structural basis for postfusion-specific binding to Respiratory Syncytial Virus F protein by the antigenic site-I antibody 131-2a

Marta Šiborová¹, Weiwei Peng¹, Xuesheng Wu², Wenjuan Du², Douwe Schulte¹, Matti F. Pronker¹, Cornelis A. M. de Haan², Joost Snijder¹

¹ Biomolecular Mass Spectrometry and Proteomics, Bijvoet Center for Biomolecular Research and Utrecht Institute of Pharmaceutical Sciences, Utrecht University, Padualaan 8, 3584CH Utrecht, The Netherlands

² Virology Group, Division of Infectious Diseases and Immunology, Department of Biomolecular Health Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, Yalelaan 1, 3584CL, the Netherlands

The Respiratory Syncytial Virus (RSV) Fusion (F) protein is a major target of antiviral antibodies following natural infection and vaccination and responsible for mediating fusion between the viral envelope and the host membrane. The fusion process is driven by a large-scale conformational change in F, switching irreversibly from the metastable prefusion state to the stable postfusion conformation. Previous research has identified six distinct antigenic sites in RSV-F, termed sites Ř, I, II, III, IV, and V. Of these, only antigenic site I is fully specific to the postfusion conformation of F. A monoclonal antibody 131-2a that targets postfusion F specifically has been widely employed as a research tool to probe for postfusion F and to define antigenic site I in serological studies, yet the sequence and epitope of the antibody remained unknown.
We used mass spectrometry-based de novo sequencing to reverse engineer 131-2a. Reverse engineered 131-2a was then used to investigate 131-2a  epitope and to define antigenic site I by single particle cryo-electron microscopy. This elucidated the structural basis for the antibody binding to the postfusion RSV-F.