If you arrive late at the theatre, getting to your seat can be a problem, especially if it's in the middle of a row that's already full. Wouldn't it be convenient if you could access your seat without having to make the arduous journey from the end of the row? In the present model for the action of the 26S proteasome, it makes a similar onerous journey, digesting unfolded substrates processively from their termini. However, this model cannot explain all proteasome-catalysed proteolytic events, and now, in Science Express, Liu and colleagues show that some proteasome substrates can be cut in the middle.

The authors began by comparing the degradation of two 'natively disordered', physiological substrates — cyclin-dependent kinase inhibitor p21cip1 (p21) and α-synuclein — and the stable green fluorescent protein (GFP) by the latent/closed 20S proteasome (the proteasome core) and the active/open 26S proteasome (the 20S core plus the PA700 regulatory cap).

They found that p21 and α-synuclein were efficiently degraded in the presence of either the 20S or 26S proteasome, but that GFP was not degraded in either case. The 20S proteasome cannot degrade short peptide substrates, so the fact that it can efficiently degrade p21 and α-synuclein indicates that “...unfolded proteins themselves could open the gate which controls access to the otherwise occluded catalytic sites, thereby initiating a process similar to that employed by the proteasome regulators PA700 and PA28”.

Next, the authors studied the degradation of fusions between GFP and p21 or α-synuclein. When GFP was fused to either the amino or the carboxy termini of p21 or α-synuclein, they found that the 20S and 26S proteasomes efficiently degraded the p21 or α-synuclein domains, but left the GFP domain intact. They obtained the same result when they fused GFP to both termini of p21 or α-synuclein, which indicates that the 20S and 26S proteasomes are capable of endoproteolytic cleavage.

Liu and co-workers concluded by showing that the 20S and 26S proteasomes can effectively degrade circular α-synuclein and GFP–p21 substrates (in the latter case, the GFP moiety was left intact). This work has therefore shown that proteasomes can endoproteolytically cleave proteins and has provided a mechanism for the “...regulated release of transcription factors from inactive precursors as well as a means of accessing internal folding defects of misfolded multi-domain proteins”. It has also highlighted a potential physiological role for the 20S proteasome in the absence of its regulatory proteins.