The murine liver-specific nonclassical MHC class I molecule Q10 binds a classical peptide repertoire

The biological properties of the nonclassical class I MHC molecules secrete d into blood and tissue fluids are not currently understood. To address thi s issue, we studied the murine Q10 molecule, one of the most abundant, solu ble class Ib molecules. Mass spectrometry analyses of hybrid Q10 polypeptid es revealed that alpha 1 alpha 2 domains of Q10 associate with 8-9 long pep tides similar to the classical class I MHC ligands, Several of the sequence d peptides matched intracellularly synthesized murine proteins, This findin g and the observation that the Q10 hybrid assembly is TAP2-dependent suppor ts the notion that Q10 groove is loaded by the classical class I Ag present ation pathway. Peptides eluted from Q10 displayed a binding motif typical o f H-2K, D, and L ligands, They carried conserved residues at P2 (Gly), P6 ( Leu), and P omega (Phe/Leu), The role of these residues as anchors/auxiliar y anchors was confirmed by Ala substitution experiments. The Q10 peptide re pertoire was heterogeneous, with 75% of the groove occupied by a multitude of diverse peptides; however, 25% of the molecules bound a single peptide i dentical to a region of a TCR V beta-chain. Since this peptide did not disp lay enhanced binding affinity for Q10 nor does its origin and sequence sugg est that it is functionally significant, we propose that the nonclassical c lass I groove of Q10 resembles H-2K, D, and L grooves more than the highly specialized clefts of nonclassical class I Ags such as Qa-1, HLA-E, and M3.