Subversion of the host immune system by viruses is often mediated by molecular decoys that sequester host proteins pivotal to mounting effective immune responses. The widespread mammalian pathogen parapox Orf virus deploys GIF, a member of the poxvirus immune evasion superfamily, to antagonize GM-CSF (granulocyte macrophage colony-stimulating factor) and IL-2 (interleukin-2), two pleiotropic cytokines of the mammalian immune system. However, structural and mechanistic insights into the unprecedented functional duality of GIF have remained elusive. Here we reveal that GIF employs a dimeric binding platform that sequesters two copies of its target cytokines with high affinity and slow dissociation kinetics to yield distinct complexes featuring mutually exclusive interaction footprints. We illustrate how GIF serves as a competitive decoy receptor by leveraging binding hotspots underlying the cognate receptor interactions of GM-CSF and IL-2, without sharing any structural similarity with the cytokine receptors. Our findings contribute to the tracing of novel molecular mimicry mechanisms employed by pathogenic viruses.

Bijvoet Center for Biomolecular Research Faculty of Science Utrecht University 3584CH Utrecht The Netherlands

Department of Haematology University of Cambridge School of Clinical Medicine Cambridge Institute for Medical Research Wellcome Trust MRC Building Cambridge Biomedical Campus Box 139 Cambridge CB2 0XY UK

Laboratory for Protein Biochemistry and Biomolecular Engineering Department of Biochemistry and Microbiology Ghent University Technologiepark 927 9052 Ghent Belgium

Masaryk University and CEITEC 62500 Brno Czech Republic

University Grenoble Alpes CNRS CEA IBS F 38044 Grenoble France

VIB Inflammation Research Center Technologiepark 927 9052 Ghent Belgium

VIB Structural Biology Research Center Vrije Universiteit Brussel Pleinlaan 2 1040 Brussels Belgium

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