Subunit arrangement in the human 20S proteasome

By means of immunoelectron microscopy and chemical crosslinking of neighboring subunits, the positions of the individual subunits in the proteasome are determined and it is shown that for the trypsin-like, the chymotrypsinlike, and the postglutamyl cleaving activities, the pairs of beta type subunits are nearest neighbors.

Abstract

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.