Aspartyl Dipeptidase, S. tymphimurium (1fye)	Sedolisin, Pseudomonas (1ga4)
Ser120, His157, Glu192	Ser287, Glu80, Asp84
Carboxypeptidase B, Streptomyces (3pte)	Signal Peptidase I, E. coli (1b12)
Ser62, Lys65, Tyr159	Ser90, Lys145

Since the role played by the serine is using the OH as a nucleophile, there is no reason why threonine could not serve just as well.

The proteasome in archaea and eukaryotes is a threonine protease:

• The proteasome forms a 20S structure of four seven-membered rings

• The two outer rings are homoheptamers

• The two inner rings are homoheptamers containing the catalytic sites

• In eukaryotes, the 20S unit associates with a 6S unit which uses ATP
  1. Open the gate at the end of the outer ring

  2. Unfold the target protein

  3. Translocate it into the degradation chamber

• The cap unit from any species works with the core unit from any other species

Side View of the 20S Proteasome (1pma)
End View, Closed	End View, Open
The 20S Plus Two 6S Caps
One of the Catalytic Subunits
Catalytic Residues: Thr1, Glu17, Lys33