The evolutionarily conserved protein Sem1/Dss1 is a subunit of the regulatory particle (RP) of the proteasome, and, in mammalian cells, binds the tumor suppressor protein BRCA2. Here, we describe a new function for yeast Sem1. We show that sem1 mutants are impaired in messenger RNA (mRNA) export and transcription elongation, and induce strong transcription-associated hyper-recombination phenotypes. Importantly, Sem1, independent of the RP, is functionally linked to the mRNA export pathway. Biochemical analyses revealed that, in addition to the RP, Sem1 coenriches with components of two other multisubunit complexes: the nuclear pore complex (NPC)-associated TREX-2 complex that is required for transcription-coupled mRNA export, and the COP9 signalosome, which is involved in deneddylation. Notably, targeting of Thp1, a TREX-2 component, to the NPC is perturbed in a sem1 mutant. These findings reveal an unexpected nonproteasomal function of Sem1 in mRNA export and in prevention of transcription-associated genome instability. Thus, Sem1 is a versatile protein that might stabilize multiple protein complexes involved in diverse pathways.
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