Parvulin, a 92-amino acid protein discovered in E. coli in 1994, is the smallest known protein with prolyl isomerase activity, which catalyzes the cis-trans isomerization of proline peptide bonds. Although parvulin has no homology with larger prolyl isomerases such as cyclophilin and FKBP, it does share structural features with subdomains of other proteins involved in preparing secreted proteins for export from the cell. 
Although parvulin is as active as the larger prolyl isomerases against a short proline-containing test peptide, it has lower relative activity against biological substrates, possibly because the larger molecules have a higher ability to bind the substrate peptide. Parvulin itself contains proline residues and its folding can be accelerated by the presence of cyclophilin; parvulin folding can also be autocatalytic.
A eukaryotic homolog of parvulin known as Pin1 is required to execute the transition from G2 phase to M phase in the cell cycle. Absence of Pin1 activity in humans has also been implicated in the folding and processing of the amyloid precursor protein, whose degradation product is the cytotoxic peptide amyloid beta implicated in Alzheimer's disease.
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