Major histocompatibility complex (MHC) class I and II proteins have been in the focus of interest for immunologists, biochemists, cell biologists, and structural biologists for decades. With dozens of entries in the Protein Data Bank, their crystal structures are now sufficiently well understood, while their dynamic properties such as peptide binding and intracellular trafficking and their immunological (as well as non-immunological) functions are still being intensely investigated. In recent years, new methods and technologies have emerged to detect and characterize the conformational changes and intermediate states that accompany peptide binding and exchange by MHC proteins. These techniques have delivered more detailed information and allowed us to compare the molecular mechanisms of peptide selection between MHC class I and II proteins, suggesting both similarities and differences. Here, we review these recent achievements and suggest avenues for further work.
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