MHC Class 1 Protein

Class I major histocompatibility complex proteins display short
peptides, or antigens, derived from normal cell proteins. Peptide-
loaded MHC proteins are located on the cell surface where they can be
examined by passing T cells of the immune system. The MHC complex has
two subunits. The smaller subunit, b2 microglobulin, resembles an
immunoglobulin domain. The larger a subunit also has an immunoglobulin-
like domain which is linked to a head domain containing the antigen-
binding groove. The antigen-binding groove in the MHC head domain is
built from two walls composed of long alpha helices that rest on a
floor composed of an eight stranded beta ***. The peptide on display
fits snugly between the helices in the groove.The peptide backbone is
bound at both ends by highly conserved regions of the MHC protein.
Some peptide side chains extend downwards into specific binding
pockets in the groove, while other peptide side chains project upwards
where they can be recognized by T cells. MHC class I proteins display
their bound peptides on the cell surface for immune surveillance.
Immune cells, called cytotoxic or killer T cells, for example, express
T-cell receptors that bind to the MHC head domain and the bound
peptide. If the cell expressing the MHC protein displays a peptide
foreign to the immune system, the T cell is activated by this receptor-
MHC interaction. The activated T cell then proceeds to destroy the
abnormal cell. Cut-away views of this peptide-bound MHC protein
complexed with a T-cell receptor reveal the exquisite precision with
which the interacting surfaces fit together."


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