part of spliceosome conformational change to release U4 (or U4atac) and U1 (or U11) Rearrangement of the pre-catalytic spliceosome containing U4 (or U4atac) and U1 (or U11) snRNPs to unpair U4 (or U4atac) from U6 (or U6atac) and release it from the spliceosomal complex along with U1 (or U11). Note that this step represents formation of the A1 complex (yeast) or the B2 complex (mammalian). spliceosomal A1 complex formation biological_process U12-type spliceosome conformational change to release U4atac and U11 spliceosomal A1 complex biosynthesis 3'-splice site cleavage, exon ligation GO:0000396 U2-type spliceosome conformational change to release U4 and U1 GO:0000397 spliceosomal B2 complex biosynthesis spliceosomal B2 complex formation GO:0000388 spliceosomal conformational changes to generate catalytic conformation protein-RNA complex assembly