Figure 3.

Initiation site selection by the proteasome. (a–c) Requirements for effective initiation regions. The initiation region has to be long enough (a), with a suitable amino acid composition (b), and be located in a position accessible to the proteasome (c). (d) Initiation of proteasomal degradation in trans. The proteasome can recognize a protein complex through ubiquitinated subunits (yellow and blue) but degrades only subunits with accessible initiation regions (green and red). (e) Speculative models of substrate recognition via degradation adaptors. The proteasome may recognize substrates through adaptor proteins (pink and orange) that recognize the ubiquitin chain attached to a substrate or the substrate directly without ubiquitination.