The DECD box putative ATPase Sub2p is an early mRNA export factor

Nuclear mRNA metabolism relies on the interplay between transcription, proc essing, and nuclear export. RNA polymerase II transcripts experience major rearrangements within the nucleus, which include alterations in the structu re of the mRNA precursors as well as the addition and perhaps even removal of proteins prior to transport across the nuclear membrane. Such mRNP-remod eling steps are thought to require the activity of RNA helicases/ATPases. O ne such protein, the DECD box RNA-dependent ATPase Sub2p/UAP56, is involved in both early and late steps of spliceosome assembly [1-4]. Here, we repor t a more general function of Saccharomyces cerevisiae Sub2p in mRNA nuclear export. We observe a rapid and dramatic nuclear accumulation of poly(A)(+) RNA in strains carrying mutant alleles of sub2. Strikingly, an intronless transcript, HSP104, also accumulates in nuclei, suggesting that Sub2p funct ion is not restricted to splicing events. The HSP104 transcripts are locali zed in a single nuclear focus that is suggested to be at or near their site of transcription. Intriguingly, Sub2p shows strong genetic and functional interactions with the RNA polymerase II-associated DNA/DNA:RNA helicase Rad 3p as well as the nuclear RNA exosome component Rrp6p, which was independen tly implicated in the retention of mRNAs at transcription sites [5]. Taken together, our data suggest that Sub2p functions at an early step in the mRN A export process.