Dynamics of free versus complexed β2-microglobulin and the evolution of interfaces in MHC class I molecules

In major histocompatibility complex (MHC) class I molecules, monomorphic β2-microglobulin (β2m) is non-covalently bound to a heavy chain (HC) exhibiting a variable degree of polymorphism. β2M can stabilize a wide variety of complexes ranging from classical peptide binding to nonclassical lipid presenting MHC class I molecules as well as to MHC class I-like molecules that do not bind small ligands. Here we aim to assess the dynamics of individual regions in free as well as complexed β2m and to understand the evolution of the interfaces between β2m and different HC. Using human β2m and the HLA–B*27:09 complex as a model system, a comparison of free and HC-bound β2m by nuclear magnetic resonance spectroscopy was initially carried out. Although some regions retain their flexibility also after complex formation, these studies reveal that most parts of β2m gain rigidity upon binding to the HC. Sequence analyses demonstrate that some of the residues exhibiting flexibility participate in evolutionarily conserved β2m–HC contacts which are detectable in diverse vertebrate species or characterize a particular group of MHC class I complexes such as peptide- or lipid-binding molecules. Therefore, the spectroscopic experiments and the interface analyses demonstrate that β2m fulfills its role of interacting with diverse MHC class I HC as well as effector cell receptors not only by engaging in conserved intermolecular contacts but also by falling back upon key interface residues that exhibit a high degree of flexibility.