As part of the spliceosome, plays a role in pre-mRNA splicing. Probable inactive PPIase with no peptidyl-prolyl cis-trans isomerase activity. Part of the activated spliceosome B/catalytic step 1 spliceosome, one of the forms of the spliceosome which has a well-formed active site but still cannot catalyze the branching reaction and is composed at least of 52 proteins, the U2, U5 and U6 snRNAs and the pre-mRNA. Recruited during early steps of activated spliceosome B maturation, it is probably one of the first proteins released from this complex as he matures to the spliceosome C complex. Belongs to the cyclophilin-type PPIase family. Despite the fact that it belongs to the cyclophilin-type PPIase family, it has probably no peptidyl-prolyl cis-trans isomerase activity.