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).
U2-type spliceosome conformational change to release U4 and U1