Discrete elements of the U2 snRNP regulate its intranuclear trafficking and recruitment to nascent transcripts

In this study, I characterize the elements of the U2 snRNP that control its intranuclear trafficking and association with nascent transcripts. Through a mutational analysis, I demonstrate that stem loops 3 and 4 (SL3 and SL4) are both required for U2 snRNP association with nascent transcripts and splicing speckles, while the Sm binding site is sufficient for U2 snRNP association with Cajal bodies after nuclear entry. For SL4, I further show that the associated protein U2B” rather than SL4 itself provides the targeting activity, which establishes a critical new role for U2B” in the pre-mRNA splicing pathway. Additionally, I demonstrate that a lack of SL3, SL4, or U2B” results in an accumulation of the U2 snRNP within Cajal bodies. Collectively, my results support a paradigm where the recruitment of U2 snRNP to nascent transcripts is independent of its splicing activity and a model where Cajal bodies are domains that control snRNP assembly before association with nascent transcripts