immunoproteasome §

A proteasome modified with β1i, β2i and β5i units at their respective positions in the 20S core. Present in cells stimulated by interferons.

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Proteolytic activity

Inner rings are β subunits and outer are α, with β1, β2 and β5 proteolytic subunits forming the catalytic chamber. Sometimes these proteolytic subunits can be replaced by three alternative subunits that are introduced by interferons. These induced subunits are called β1i (LMP2), β2i (MECL-1) and β5i (LMP7). Both β1i and β5i are encoded by PSMB9 and BSMB8 genes outside the MHC locus. This means the proteasome can exists as a constitutive proteasome present in all cells and as the immunoproteasome, which is in all cells stimulated with interferons.

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The replacement of the catalytic core in a immunoproteasome alters the enzymatic specificity of the proteasome such that there is increased cleavage after hydrophobic residues and decreased cleavage after acidic residues. This produces peptides with carboxy-terminal residues that are preferred anchor residues for binding to most MHC class I molecules and are also the preferred structures for transport by TAP.