During gene expression, the coordinated action of several multiprotein complexes couple transcription, mRNA biogenesis and export, to guarantee the proper maturation of transcripts before their translation in the cytoplasm . mRNA levels are highly regulated by transcription rate adjustments and mRNA decay, to produce the appropriate number of transcripts competent for translation . In yeast, two major cytoplasmic mRNA degradation pathways control transcript turnover: the cytoplasmic exosome and the 5'→3' mRNA decay. Moreover, 5'→3' mRNA decay and translation are interconnected processes providing an exquisite equilibrium between degradation, storage and translation that correlates with the type and localisation of the mRNP in the cell (reviewed in ). Work over the last few years has shown that different classes of mRNPs are found as discrete granules in the cytoplasm. In yeast, different sorts of cytoplasmic mRNP granules have been described. Among them, P-bodies (PBs) and stress granules (SGs) are the best characterised (reviewed in [4, 5]). P-bodies are implicated in translational repression, mRNA storage and 5'→3' mRNA decay . The composition of PBs has been thoroughly studied. They are made up of a set of proteins that form the core of the particules, such as the decapping enzyme Dcp1/Dcp2, activators of decapping Dhh1, Pat1, Lsm1-7, Edc3 and the 5'→3' exonuclease Xrn1 . Other proteins involved in different processes, such as nonsense-mediated decay (Upf1-3)  and translation (eIF4E, eIF4G and Pab1) [9, 10] have also been reported to accumulate in these granules, but only under specific conditions.
A second class of well studied cytoplasmic mRNP structures are the stress granules (reviewed in ). SGs are cytoplasmic mRNP accumulations that appear when translation initiation is impaired. Study of stress granule formation has suggested that they contain mRNAs stalled in the process of translation initiation. In yeast they characteristically contain poly(A) mRNA, the poly(A)-binding protein Pab1, 40S ribosomal subunits and the translation factors eIF4E, eIF4G, eIF3 (reviewed in [4, 5]).
In yeast but also in other organisms, both types of granules are interconnected (reviewed in ). Strikingly, assembly of stress granules depends on P-body formation and several factors are present in both granules, suggesting a crosstalk between them [6, 10, 11].
One key factor involved at different stages of nuclear mRNA metabolism is the conserved Sus1 protein, which is part of two stable nuclear complexes: the transcriptional coactivator SAGA and the nuclear pore associated TREX2 . Biochemical and functional data have suggested a crucial nuclear role for Sus1 in coupling transcription activation and mRNA export. Previously, we have shown that Sus1 participates in histone H2B deubiquitination and histone H3 methylation together with the SAGA-DUB subunits Ubp8 and Sgf11 . Sus1 mediates transcription activation through its associated with chromatin promoters as part of SAGA and is recruited to coding regions where it is necessary for transcription elongation . Interestingly, Sus1 is also required for nuclear post-transcriptional events. After transcriptional shut off, Sus1 affects both the morphology as well as the persistent tethering of the mRNPs to their cognate gene, reinforcing the broad role of Sus1 in nuclear mRNA biogenesis . Furthermore, Sus1 is crucial for TREX2-NPC interaction and its absence provokes a dramatic defect in mRNA export [12–14]. Altogether, Sus1 participates in many nuclear events from early epigenetic modifications to mRNA export through the nuclear pore (reviewed in ).
Strikingly, although Sus1's described functions take place in the nucleus it was also observed in the cytoplasm of yeast and Drosophila[14, 17], thus suggesting additional roles outside of the nucleus.
In this study, we describe genetic and functional links between SUS1 and several components of P-bodies and stress granules. We demonstrate that SUS1 deletion is synthetic lethal with 5'→3' decay machinery components LSM1 and PAT1 and has a strong genetic interaction with LSM6 and DHH1. Interestingly, Sus1 overexpression leads to an accumulation of Sus1 at cytoplasmic granules, which can co-localise with P-bodies and stress granules. In addition, through affinity purification of TAP tagged Sus1, we have identified novel physical interactions between Sus1 and factors associated with P-body/stress granule. Finally, absence of LSM1 and PAT1 slightly promotes association between Sus1-TREX2. Taken together, our results reveal a novel link between the transcription/export factor Sus1 and cytoplasmic mRNA decay factors. Thus, Sus1 plays a broad role in mRNA metabolism.