Keepers at the final gates: regulatory complexes and gating of the proteasome channel.

The proteolytic active sites of the 26S proteasome are sequestered within the central chamber of its 20S catalytic core particle. Access to this chamber is through a narrow channel defined by the outer alpha subunits. Free proteasome 20S core particles are found in an autoinhibited state in which the N-termini of neighboring alpha subunits are anchored by an intricate lattice of interactions blocking access to the channel. Entry of substrates into proteasomes can be enhanced by attachment of activators or regulatory particles. An important part of this activation is channel gating; regulatory particles rearrange the blocking residues to form an open pore and promote substrate entry into the proteolytic chamber. Interestingly, some substrates can open the entrance themselves and thus facilitate their own destruction. In this review, we will discuss the mechanisms proposed for channel gating and the interactions required to maintain stable closed and open conformations.