Recognition of mutant and cryptic 5' splice sites by the U1 small nuclear ribonucleoprotein in vitro.

We examined the ability of U1 small nuclear ribonucleoproteins (U1 snRNPs) to recognize mutant and cryptic 5' splice sites on beta-globin pre-mRNA substrates using an RNase T1 protection assay. When U1 snRNPs were prebound to anti-(U1)RNP antibodies, we detected binding to mutant but not to cryptic 5' splice sites on several substrates. By contrast, in a splicing extract at 0 degree C, neither the mutated nor cryptic 5' splice sites of a human beta-globin transcript were selected as protected fragments with the same antibodies. However, after incubation of the transcript in the extract to yield splicing intermediates, fragments that included a cryptic 5' splice site were detected. The results of our analyses suggest that U1 snRNPs are involved in recognizing cryptic 5' splice sites but that interactions with other splicing components are required to stabilize the association.