A SOLUBLE MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I PEPTIDE-BINDING PLATFORM UNDERGOES A CONFORMATIONAL CHANGE IN RESPONSE TO PEPTIDE EPITOPES

Class I major histocompatibility complexes (MHC) are heterotrimeric st ructures comprising heavy chains (HC), beta(2)-microglobulin (beta(2)- m), and short antigenic peptides of 8-10 amino acids. These components assemble in the endoplasmic reticulum and are released to the cell su rface only when a peptide of the appropriate length and sequence is in corporated into the structure. The binding of beta(2)-m and peptide to HC is cooperative, and there is indirect evidence that the formation of a stable heterotrimer from an unstable HC:beta(2)-m heterodimer inv olves a peptide-induced conformational change in the HC. Such a confor mational change could ensure both a strong interaction between the thr ee components and also signal the release of stably assembled class I MHC molecules from the endoplasmic reticulum. A peptide-induced confor mational change in HC has been demonstrated in cell lysates lacking be ta(2)-m to which synthetic peptides were added. Many features of this conformational change suggest that it may be physiologically relevant. In an attempt to study the peptide-induced conformational change in d etail we have expressed a soluble, truncated form of the mouse H-2D(b) HC that contains only the peptide binding domains of the class I mole cule. We have shown that this peptide-binding ''platform'' is relative ly stable in physiological buffers and undergoes a conformational chan ge that is detectable with antibodies, in response to synthetic peptid es. We also show that the structural features of peptides that induce this conformational change in the platform are the same as those requi red to observe the conformational change in full-length HC. In this re spect, therefore, the HC alpha(1), and alpha(2) domains, which togethe r form the peptide binding site of class I MHC, are able to act indepe ndently of the rest of the molecule.