Subunit arrangement in the human 20S proteasome

In human 20S proteasomes two copies of each of seven different α-type and seven different β-type subunits are assembled to form a stack of four seven-membered rings, giving the general structure α(1–7), β(1–7), β(1–7), α(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 β type subunits, which are thought to form active sites, are nearest neighbors.