Prolyl oligopeptidase is implicated in the metabolism of
neuropeptides and is involved in
amnesia and depression. It contains a
peptidase and an unusual beta-propeller domain that excludes large
peptides and
proteins from the active site. The propeller consists of seven blades not closed by a "Velcro" between the first and last blades. The propeller domain was expressed as a stable, soluble
protein, P(7). Its conformational identity with that of the native propeller was verified by circular dichroism and digestion with
trypsin. Differential scanning calorimetry, kinetic denaturation with
urea and equilibrium denaturation with
guanidinium chloride have shown that the propeller is more stable than the parent
prolyl oligopeptidase. The deletion of the seventh blade of P(7) led to a stable structure, a six-bladed propeller, P(6), which immediately dimerized, in contrast with the monomeric P(7). Addition of an 11
amino acid residue extension to the C terminus of P(6) also produced a dimer, whereas the P(6) labelled with a His-tag at the N terminus displayed a monomer structure. The stability of P(6) and its variants was lower than that of P(7). The denatured propellers refolded readily. This study shows that the unclosed P(7) is a stable structure, and suggests that an opening between the
peptidase and the propeller domains is more important for the substrate entry than is the putative opening between the first and seventh blades. Our results suggest that the propellers are simple, versatile structures, which can be prepared artificially.