CBC links pre-mRNA capping to transcription elongation and alternative splicing through P-TEFb

Promoter-proximal pausing of RNAPII coincides with the formation of the cap structure at the 5′ end of pre-mRNA, which is bound by the cap-binding protein complex (CBC). Although the P-TEFb elongation factor stimulates the release of RNAPII from pausing and promotes transcription elongation and alternative splicing by phosphorylating the RNAPII CTD at Ser2 (S2-P RNAPII), it is unknown whether CBC facilitates these events. In this study, we report that CBC interacts with P-TEFb and transcriptionally engaged RNAPII and is globally required for optimal levels of S2-P RNAPII. Quantitative nascent RNA immunoprecipitation and ChIP experiments reveal that depletion of CBC attenuates HIV-1 Tat transactivation and transcription elongation of investigated CBC-dependent genes by decreasing the levels of P-TEFb and S2-P RNAPII, leading to accumulation of RNAPII in the body of these genes. Finally, CBC is essential for the promotion of alternative splicing through facilitating P‑TEFb, S2-P RNAPII and SF2/ASF occupancy at a minigene. These findings disclose a vital role of CBC in connecting pre-mRNA capping to transcription elongation and alternative splicing via P-TEFb.