Involved in mRNA export coupled transcription activation by association with both the TREX-2 and the SAGA complexes. The transcription regulatory histone acetylation (HAT) complex SAGA is involved in RNA polymerase II-dependent regulation of approximately 10% of yeast genes. At the promoters, SAGA is required for recruitment of the basal transcription machinery. It influences RNA polymerase II transcriptional activity through different activities such as TBP interaction (SPT3, SPT8 and SPT20) and promoter selectivity, interaction with transcription activators (GCN5, ADA2, ADA3 and TRA1), and chromatin modification through histone acetylation (GCN5) and deubiquitination (UBP8). Within the SAGA complex, participates in a subcomplex with SGF11, SGF73 and UBP8 required for deubiquitination of H2B and for the maintenance of steady-state H3 methylation levels. The TREX-2 complex functions in docking export-competent ribonucleoprotein particles (mRNPs) to the nuclear entrance of the nuclear pore complex (nuclear basket), by association with components of the nuclear mRNA export machinery (MEX67-MTR2 and SUB2) in the nucleoplasm and the nucleoporin NUP1 at the nuclear basket. TREX-2 participates in mRNA export and accurate chromatin positioning in the nucleus by tethering genes to the nuclear periphery. SUS1 has also a role in mRNP biogenesis and maintenance of genome integrity through preventing RNA-mediated genome instability. Has a role in response to DNA damage induced by methyl methane sulfonate (MMS) and replication arrest induced by hydroxyurea. May also be involved in cytoplasmic mRNA decay by interaction with components of P-bodies. Component of the nuclear pore complex (NPC)-associated TREX-2 complex (transcription and export complex 2), composed of at least SUS1, SAC3, THP1, SEM1, and CDC31. TREX-2 contains 2 SUS1 chains. The TREX-2 complex interacts with the mRNA export factors MEX67, MTR2 and SUB2, and the nucleoporin NUP1. Component of the 1.8 MDa SAGA transcription coactivator-HAT complex, which consists of at least of TRA1, CHD1, SPT7, TAF5, ADA3, SGF73, SPT20/ADA5, SPT8, TAF12, TAF6, HFI1/ADA1, UBP8, GCN5, ADA2, SPT3, SGF29, TAF10, TAF9, SGF11 and SUS1. TAF5, TAF6, TAF9, TAF19, TAF12 and ADA1 seem to be present in 2 copies. SAGA is built of 5 distinct domains with specialized functions. Domain I (containing TRA1) probably represents the activator interaction surface. Domain II (containing TAF5 and TAF6, and probably TAF9 and TAF10), domain III (containing GCN5, TAF10, SPT7, TAF5 and ADA1, and probably ADA2, ADA3 and TAF12), and domain IV (containing HFI1/ADA1 and TAF6, and probably TAF9) are believed to play primarily an architectural role. Domain III also harbors the HAT activity. Domain V (containing SPT3 and SPT20, and probably SPT8) represents the TBP-interacting module, which may be associated transiently with SAGA. Within the SAGA complex, SUS1, SGF11, SGF73 and UBP8 form an additional subcomplex of SAGA called the DUB module (deubiquitination module). Interacts directly with THP1, SAC3, SGF11 and with the RNA polymerase II. Interacts with YRA1 and MEX67. The nucleoplasm localization requires the presence of UBP8 and SGF11 to bind part of SUS1 to the SAGA complex. The gene for SUS1 bears 2 introns, and its expression is highly regulated by splicing, translation and decay. The presence of the introns thereby play a key role for SUS1 function in yeast. Belongs to the ENY2 family.