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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 Δhpr1 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.
Our special thanks go to R. Sahli, R. Imoberdorf, and S. Creton for help and advice in ChIPs and real-time PCR. Our thanks also go to D. Libri and T. H. Jensen for communicating and discussing results before publication. We are grateful to V. Müller for excellent technical assistance. We also thank D. Libri for yeast strains and plasmids, E. Izaurralde and A. Aguilera for plasmids, and M. Collart, D. Görlich, and M. Strubin for anti-TBP, anti-CBP80, and anti-CTD antibodies, respectively. We thank T. H. Jensen, D. Libri, and M. Collart for comments on the manuscript.
This work was supported by a grant from the Swiss National Science Foundation (no. 31-61378-00) to F.S.