Antisense oligonucleotide binding to U5 snRNP induces a conformational change that exposes the conserved loop of U5 snRNA.

Conformational rearrangements of the spliceosomal small nuclear RNAs (U snRNAs) are essential for proper assembly of the active site prior to the first catalytic step of splicing. We have previously shown that conformational changes caused by binding of an antisense 2'-O-methyl RNA oligonucleotide (BU5Ae) to U5 snRNA nt 68-88 disrupted the U4/U5/U6 complex and induced formation of the U1/U4/U5 and U2/U6 complexes. Here we show that the conformational change induced by BU5Ae exposes the invariant loop of U5 that binds the 5'exon and also reorganizes internal loop 1 (IL1) and the top of stem 2. Interestingly, we have also previously found that the U1/U4/U5 complex induced by BU5Ae brings the invariant loop of U5 into close proximity with the 5'-end of U1. Taken together, these data suggest that U1 and U5 may both contribute to the ability of the U1/U4/U5 complex to bind the 5' splice site.