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. Knockout mice manifest significant embryonic lethality. Growth retardation, lack of neural tube closure, and absence of limb buds are observed at embryonic day 12.5. Surviving mice show growth retardation and retinal dystrophic changes. 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.