Nucleic Acids Research, 2003, Vol. 31, No. 16 4791-4796
© 2003 Oxford University Press
Two protein–protein interaction sites on the spliceosome-associated human cyclophilin CypH
1 Abteilung für Zelluläre Biochemie, Max Planck-Institut für Biophysikalische Chemie, D-37077 Göttingen, Germany, 2 Institut für Biochemie, Fachbereich Chemie, Philipps-Universität Marburg, Meerweinstraße, D-35039 Marburg, Germany, 3 Abteilung für Molekulare Strukturbiologie, Institut für Mikrobiologie und Genetik, Georg August-Universität, D-37077 Göttingen, Germany and 4 Laboratorio di Neurobiochimica, Neuroscience Sperimentali, Fondazione Santa Lucia, Via Ardeatina 306, 00179 Roma, Italy
*To whom correspondence should be addressed at Laboratorio di Neurobiochimica, Fondazione Santa Lucia, Via Ardeatina, 306, 00179 Roma, Italy. Tel: +39 0651501 590; Fax: +39 0651501 512; e-mail: t.achsel{at}hsantalucia.it
Cyclophilins are a family of proteins that share a common, highly conserved sequence motif. Cyclophilins bind transiently to other proteins and facilitate their folding. One member of the family, hCypH, is part of the human spliceosomal [U4/U6·U5] tri-snRNP complex; it associates specifically and stably with the U4/U6-specific protein 60K. Here, we demonstrate that recombinant hCypH exhibits peptidyl–prolyl isomerase (PPIase) activity, and describe mutagenesis studies demonstrating that it shares the catalytic pocket with other members of the cyclophilin family. However, neither the PPIase activity nor the catalytic pocket is required for binding of protein 60K. Rather, hCypH contains a small insertion in a loop of the otherwise conserved cyclophilin backbone, and this minor change creates a highly specific binding site that is responsible for the association of this cyclophilin, but not others, with protein 60K. hCypH is thus the first small cyclophilin shown to have a second protein–protein interaction site and the ability to bind stably to another protein. Since the catalytic pocket and the second binding site are located on opposite sides of the cyclophilin structure, this opens up the interesting possibility that hCypH may serve as a bridge mediating interactions between protein 60K of the U4/U6 snRNP and other as yet unknown factors.
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