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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.

In addition to Pin1, higher eukaryotes contain an additional parvulin gene, whose core sequence is highly conserved in all multi-cellular organisms examined so far, but absent from yeast. In humans the locus of this parvulin resides on chromosome Xq13 and encodes two protein species, Par14 and Par17. Par17, which is exclusively expressed in hominids and humans, is an N-terminal extended version of Par14 and results from alternative transcription initiation.

References

1. Mueller, J.W., Link, N.M., Matena, A., Hoppstock, L., Ruppel, A., Bayer, P., Blankenfeldt, W. (2011).Crystallographic Proof for an Extended Hydrogen-Bonding Network in Small Prolyl Isomerases. J.Am.Chem.Soc. 133, 20096-20099.
2. Rahfeld JU, Schierhorn A, Mann KH. (1994). A novel peptidyl-prolyl cis/trans isomerase from Escherichia coli. FEBS Lett 343:65.
3. Balbach J, Schmid FX. (2000). Proline isomerizarion and its catalysis in protein folding. In Mechanisms of Protein Folding 2nd ed. Ed. RH Pain. Frontiers in Molecular Biology series. Oxford University Press: Oxford, UK.
4. Scholz C, Rahfeld J, Fischer G, Schmid FX. (1997). Catalysis of protein folding by parvulin. J Mol Biol 273(3):752-62.
5. Gothel SF, Marahiel MA. (1999). Peptidyl-prolyl cis-trans isomerases, a superfamily of ubiquitous folding catalysts. Cell Mol Life Sci 55(3):423-36.
6. Pastorino L, Sun A, Lu PJ, Zhou XZ, Balastik M, Finn G, Wulf G, Lim J, Li SH, Li X, Xia W, Nicholson LK, Lu KP. (2006). The prolyl isomerase Pin1 regulates amyloid precursor protein processing and amyloid-beta production. Nature 440(7083):528-34.
7. Mueller JW, Bayer P. (2008).Small Family with Key Contacts: Par14 and Par17 Parvulin Proteins, Relatives of Pin1, Now Emerge in Biomedical Research. Perspectives in Medicinal Chemistry 2, 11–20.
8. Mueller JW, Kessler D, Neumann D, Stratmann T, Papatheodorou P, Hartmann-Fatu C, Bayer P. (2006). Characterization of novel elongated Parvulin isoforms that are ubiquitously expressed in human tissues and originate from alternative transcription initiation. BMC Molecular Biology, 7:9.
9. Kessler D, Papatheodorou P, Stratmann T, Dian EA, Hartmann-Fatu C, Rassow J, Bayer P, Mueller JW. (2007) . The DNA Binding Parvulin Par17 is Targeted to the Mitochondrial Matrix by a Recently Evolved Prepeptide Uniquely Present in Hominidae. BMC Biology, 5:37

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