Chicken (Gallus gallus) MHCY class I molecules are highly polymorphic yet substantially different from polymorphic MHC class I molecules that bind peptide Ags. The binding grooves in MHCY class I molecules are hydrophobic and too narrow to accommodate peptides. An earlier structural study suggested that ligands for MHCY class I might be lipids, but the contents of the groove were not clearly identified. In this study, lysophospholipids have been identified by mass spectrometry as bound in two MHCY class I isoforms that differ substantially in sequence. The two isoforms, YF1*7.1 and YF1*RJF34, differ by 35 aa in the α1 and α2 domains that form the MHC class I ligand binding groove. Lyso-phosphatidylethanolamine (lyso-PE) 18:1 was the dominant lipid identified in YF1*7.1 and YF1*RJF34 expressed as recombinant molecules and renatured with β2-microglobulin in the presence of a total lipid extract from Escherichia coli. Less frequently detected were lyso-PE 17:1, lyso-PE 16:1, and lysophosphatidylglycerols 17:1 and 16:0. These data provide evidence that lysophospholipids are candidate ligands for MHCY class I molecules. Finding that MHCY class I isoforms differing substantially in sequence bind the same array of lysophospholipids indicates that the amino acid polymorphism that distinguishes MHCY class I molecules is not key in defining ligand specificity. The polymorphic positions lie mostly away from the binding groove and might define specificity in interactions of MHCY class I molecules with receptors that are presently unidentified. MHCY class I molecules are distinctive in bound ligand and in display of polymorphic residues.
This work was supported in part by U.S. Department of Agriculture, National Institute of Food and Agriculture National Research Initiative Competitive Grant 2017-67017-26570 (to M.M.M.), National Institutes of Health, National Cancer Institute Award P30CA033572 (City of Hope Cancer Center), California Institute for Regenerative Medicine Bridges Internship Award TB1-01182 at the California State University, Long Beach (Delgado), a Caltech–City of Hope Biomedical Initiative Pilot Grant (to M.M.M. and P.J. Bjorkman), and by the Eugene and Ruth Roberts Summer Student Academy at City of Hope (to J.D. and V.B.).
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