A cyclophilin functions in pre-mRNA splicing.
We report that the cyclophilin USA-CyP is part of distinct complexes with two spliceosomal proteins and is involved in both steps of pre-mRNA splicing. The splicing factors hPrp18 and hPrp4 have a short region of homology that defines a high affinity binding site for USA-CyP in each protein. USA-CyP forms separate, stable complexes with hPrp18 and hPrp4 in which the active site of the cyclophilin is exposed. The cyclophilin inhibitor cyclosporin A slows pre-mRNA splicing in vitro, and we show that its inhibition of the second step of splicing is caused by blocking the action of USA-CyP within its complex with hPrp18. Cyclosporin A also slows splicing in vivo, and we show that this slowing results specifically from inhibition of USA-CyP. Our results lead to a model in which USA-CyP is carried into the spliceosome in complexes with hPrp4 and hPrp18, and USA-CyP acts during splicing within these complexes. These results provide an example of the function of a cyclophilin in a complex process and provide insight into the mechanisms of action of cyclophilins.
Horowitz
David S
DS
Department of Biochemistry and Molecular Biology, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA. dhorowitz@usuhs.mil
Lee
Edward J
EJ
Mabon
Stephen A
SA
Misteli
Tom
T
D000595:Amino Acid Sequence; D021983:Cyclophilins / Q000235:genetics*; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics; D012322:RNA Precursors / Q000235:genetics*; D012326:RNA Splicing; D000072260:RNA Splicing Factors; D016601:RNA-Binding Proteins; D017414:Ribonucleoprotein, U4-U6 Small Nuclear / Q000235:genetics
D009687:Nuclear Proteins; C448981:PRPF18 protein, human; C489982:PRPF4 protein, human; D012322:RNA Precursors; D000072260:RNA Splicing Factors; D016601:RNA-Binding Proteins; D017414:Ribonucleoprotein, U4-U6 Small Nuclear; D017346:Protein Serine-Threonine Kinases; D021983:Cyclophilins