Biochemical and genetic characterization of Yra1p in budding yeast

Yra1p and its vertebrate homologues bind to the mRNA export factor Mex67p/TAP and are thought to play a role in mRNA export in vivo. To further characterize Yra1p, we used immunoaffinity chromatography to purify endogenous Yra1p complexes. These experiments demonstrated that two importin beta homologues (Kap123p and Pse1p) and the poly A tail-binding proteins Pab1p and Nab2p associate with Yra1p. The other major proteins that associate with Yra1p include proteins involved in mRNA and rRNA processing and the Yra1p-related protein Yra2p. Additional biochemical and genetic experiments suggest a close functional relationship between Yra1p and Yra2p. We generated a temperature-sensitive allele of YRA1 and used it to demonstrate that cells which lack the function of both Yra1p and Yra2p are able to exit a G0 arrest and go through several rounds of cell division before arresting. We also identified high-copy suppressors of the yra1-2 temperature-sensitive growth defect. These include SUB2, a splicing factor important in mRNA export, ULP1, a nuclear cysteine protease localized to the nuclear pore and involved in Smt3p/SUMO processing, and YRA2. Taken together, these results suggest that Yra1p has roles in diverse RNA processing events in addition to a role in mRNA export.