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-2D(b) and H-2K(b) 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.