Definition and transfer of a serological epitope specific for peptide-empty forms of MHC class I

Nascent class I molecules have been hypothesized to undergo a conformationa l change when they bind peptide based on the observation that most availabl e antibodies only detect peptide-loaded class I. Furthermore recent evidenc e suggests that this peptide-facilitated conformational change induces the release of class I from association with transporter associated with antige n processing (TAP)/tapasin and other endoplasmic reticulum proteins facilit ating class I assembly. To learn more about the structure of peptide-empty class I, we have studied mAb 64-3-7 that is specific for peptide-empty form s of L-d, We show here that mAb 64-3-7 detects a linear stretch of amino ac ids including principally residues 48Q and 50P, Furthermore, we demonstrate that the 64-3-7 epitope can be transferred to other class I molecules with limited mutagenesis. Interestingly, in the folded class I molecule residue s 48 and 50 are on a loop connecting a beta strand (under the bound peptide ) with the alpha(1) helix (rising above the ligand binding site). Thus it i s attractive to propose that this loop is a hinge region. Importantly, the three-dimensional structure of this loop is strikingly conserved among clas s I molecules. Thus our findings suggest that all class I molecules undergo a similar conformational change in the loop around residues 48 and 50 when they associate with peptide.