Subunit arrangement in the human 20S proteasome

In human 20S proteasomes two copies of each of seven different alpha-type and seven different beta-type subunits are assembled to form a stack of four seven-membered rings, giving the general structure alpha(1-7), beta(1-7), beta(1-7), alpha(1-7). By means of immunoelectron microscopy and chemical crosslinking of neighboring subunits, we have determined the positions of the individual subunits in the proteasome. The topography shows that for the trypsin-like, the chymotrypsin-like, and the postglutamyl cleaving activities, the pairs of beta type subunits, which are thought to form active sites, are nearest neighbors.