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Proc Natl Acad Sci U S A. 2008 Jul 22;105(29):10095-100. doi: 10.1073/pnas.0804551105. Epub 2008 Jul 15.

Structure of UL18, a peptide-binding viral MHC mimic, bound to a host inhibitory receptor.

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Division of Biology 114-96 and Howard Hughes Medical Institute, California Institute of Technology, Pasadena, CA 91125, USA.


UL18 is a human cytomegalovirus class I MHC (MHCI) homolog that binds the host inhibitory receptor LIR-1 and the only known viral MHC homolog that presents peptides. The 2.2-A structure of a LIR-1/UL18/peptide complex reveals increased contacts and optimal surface complementarity in the LIR-1/UL18 interface compared with LIR/MHCI interfaces, resulting in a >1,000-fold higher affinity. Despite sharing only approximately 25% sequence identity, UL18's structure and peptide binding are surprisingly similar to host MHCI. The crystal structure suggests that most of the UL18 surface, except where LIR-1 and the host-derived light chain bind, is covered by carbohydrates attached to 13 potential N-glycosylation sites, thereby preventing access to bound peptide and association with most MHCI-binding proteins. The LIR-1/UL18 structure demonstrates how a viral protein evolves from its host ancestor to impede unwanted interactions while preserving and improving its receptor-binding site.

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