20S core §

Together with 19S caps, forms a proteasome

A large cylindrical complex of 28 subunits, arranged in four stacked rings around a hollow core. ![[CleanShot 2023-12-10 at 10.52.27@2x.png]]

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.