Cotranscriptional recruitment of the serine-arginine-rich (SR)-like proteins Gbp2 and Hrb1 to nascent mRNA via the TREX complex.

The TREX (transcription/export) complex couples transcription elongation to the nuclear export of mRNAs. In this article, we show that the poly(A)(+) RNA-binding proteins Gbp2 and Hrb1, which resemble the serine-arginine-rich (SR) family of splicing factors found in higher eukaryotes, are specifically associated with the yeast TREX complex. We also show that Gbp2 and Hrb1 interact with Ctk1, a kinase that phosphorylates the C-terminal domain of RNA polymerase II during transcription elongation. Consistent with these findings, Gbp2 and Hrb1 associate with actively transcribed genes throughout their entire lengths. By using an RNA immunoprecipitation assay, we show that Gbp2 and Hrb1 also are bound to transcripts that are derived from these genes. We conclude that recruitment of the SR-like proteins Gbp2 and Hrb1 to mRNA occurs cotranscriptionally by means of association with the TREX complex and/or Ctk1.