TAF2 condensation in nuclear speckles links basal transcription factor TFIID to RNA splicing

TFIID is an essential basal transcription factor, crucial for RNA polymerase II (pol II) promoter recognition and transcription initiation. The TFIID complex consists of the TATA-binding protein (TBP) and 13 TBP-associated factors (TAFs) that contain intrinsically disordered regions (IDRs) with currently unknown functions. Here, we show that a conserved IDR drives TAF2 condensation in nuclear speckles, independently of other TFIID subunits. Quantitative mass spectrometry analyses reveal that the TAF2 IDR specifically interacts with the nuclear speckle and spliceosome-associated protein SRRM2. Consequently, TAF2 recruits SRRM2 to TFIID to form non-canonical TFIID-SRRM2 complexes. Reduced SRRM2 recruitment elicits alternative splicing events in RNAs coding for proteins involved in transcription and transmembrane transport. Further, genome-wide binding analyses suggest TAF2 shuttling between nuclear speckles and pol II promoters. This study identifies an IDR of the basal transcription machinery as a molecular guide for protein partitioning into nuclear compartments, controlling protein complex composition and pre-mRNA splicing.