THO complex

Abstract: In yeast, the TREX complex contains the THO transcription elongation complex, which functions in direct cotranscriptional recruitment of the mRNA export proteins Sub2 and Yra1 to nascent transcripts. Here we report the identification of the human THO complex and show that it associates with spliced mRNA, but not with unspliced pre-mRNA in vitro. Transcription is not required for this recruitment. We also show that the human THO complex colocalizes with splicing factors in nuclear speckle domains in vivo. Considering that splicing occurs cotranscriptionally in humans, our data indicate that recruitment of the human TREX complex to spliced mRNA is not directly coupled to transcription, but is instead coupled to transcription indirectly through splicing.

Abstract: Yra1p/REF participates in mRNA export by recruiting the export receptor Mex67p to messenger ribonucleoprotein (mRNP) complexes. Yra1p also binds Sub2p, a DEAD box ATPase/RNA helicase implicated in splicing and required for mRNA export. We identified genetic and physical interactions between Yra1p, Sub2p, and Hpr1p, a protein involved in transcription elongation whose deletion leads to poly(A) RNA accumulation in the nucleus. By chromatin immunoprecipitation (ChIP) experiments, we show that Hpr1p, Sub2p, and Yra1p become associated with active genes during transcription elongation and that Hpr1p is required for the efficient recruitment of Sub2p and Yra1p. The data indicate that transcription and export are functionally linked and that mRNA export defects may be due in part to inefficient loading of essential mRNA export factors on the growing mRNP. We also identified functional interactions between Yra1p and the exosome components Rrp45p and Rrp6p. We show that yra1, sub2, and hpr1 mutants all present defects in mRNA accumulation and that deletion of RRP6 in yra1 mutants restores normal mRNA levels. The data support the hypothesis that an exosome-dependent surveillance mechanism targets improperly assembled mRNPs for degradation. mRNAs are exported from the nucleus as messenger ribonucleoprotein (mRNP) complexes which begin to be assembled during transcription. mRNA biogenesis requires multiple processing steps, including the addition of a 5 cap, splicing, and 3-end formation, which have to be completed before the mRNA can be exported. Fully mature mRNPs are subsequently recognized by the essential mRNA export receptor Mex67p (TAP in metazoans), which mediates the interaction between the mRNP and components of the nuclear pore complex (42, 51). The recruitment of Mex67p/TAP to the mRNP is facilitated by Yra1p (Aly in metazoans), an essential mRNA export factor which binds RNA and directly interacts with