Not all empty MHC class I molecules are molten globules: Tryptophan fluorescence reveals a two-step mechanism of thermal denaturation

When major histocompatibility complex (MHC) class I molecules bind peptide, they change their conformation and their dynamics. The structure and properties of the peptide-empty class I are still largely unknown. We have investigated the thermal denaturation of the murine class I allotypes H-2Db and H-2Kb through the fluorescence of their intrinsic tryptophans, and we find that it occurs via an empty form that can also be produced by folding denatured recombinant class I molecules. It rapidly binds exogenous peptides. Our data demonstrate that the empty form of class I is a distinct conformational state with at least transient stability.

► Tryptophan fluorescence measures thermal denaturation of MHC class I proteins.
► Thermal denaturation occurs via an empty peptide-receptive intermediate.
► Such empty forms of class I can be also produced in vitro from recombinant protein.
► Empty class I shows fast peptide on-rates. They are folded and structured.