Cyclophilins (Cyps) catalyze the cis/trans
isomerization of peptidyl-prolyl bonds, a rate-limiting
step in protein folding. In some cases, cyclophilins have
also been shown to form stable complexes with specific
proteins in vivo and may thus also act as chaperone-like
molecules. We have characterized the 20kD protein of the
spliceosomal 25S [U4/U6.U5] tri-snRNP complex
from HeLa cells and show that it is a novel human cyclophilin
(denoted SnuCyp-20). Purified [U4/U6.U5] tri-snRNPs,
but not U1, U2, or U5 snRNPs, exhibit peptidyl-prolyl cis/trans
isomerase activity in vitro, which is cyclosporin A-sensitive,
suggesting that SnuCyp-20 is an active isomerase. Consistent
with its specific association with tri-snRNPs in vitro,
immunofluorescence microscopy studies showed that SnuCyp-20
is predominantly located in the nucleus, where it colocalizes
in situ with typical snRNP-containing structures referred
to as nuclear speckles. As a first step toward the identification
of possible targets of SnuCyp-20, we have investigated
the interaction of SnuCyp-20 with other proteins of the
tri-snRNP. Fractionation of RNA-free protein complexes
dissociated from isolated tri-snRNPs by treatment with
high salt revealed that SnuCyp-20 is part of a biochemically
stable heteromer containing additionally the U4/U6-specific
60kD and 90kD proteins. By coimmunoprecipitation experiments
performed with in vitro-translated proteins, we could further
demonstrate a direct interaction between SnuCyp-20 and
the 60kD protein, but failed to detect a protein complex
containing the 90kD protein. The formation of a stable
SnuCyp-20/60kD/90kD heteromer may thus require additional
factors not present in our in vitro reconstitution system.
We discuss possible roles of SnuCyp-20 in the assembly
of [U4/U6.U5] tri-snRNPs and/or in conformational
changes occurring during the splicing process.