The mRNA Export Factor Sus1 Is Involved in Spt/Ada/Gcn5 Acetyltransferase-mediated H2B Deubiquitinylation through Its Interaction with Ubp8 and Sgf11

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Abstract
Sus1 acts in nuclear mRNA export via its association with the nuclear pore-associated Sac3–Thp1–Cdc31 complex. In addition, Sus1 plays a role in transcription through its interaction with the Spt/Ada/Gcn5 acetyltransferase (SAGA) complex. Here, we have analyzed function and interaction of Sus1 within the SAGA complex. We demonstrate that Sus1 is involved in the SAGA-dependent histone H2B deubiquitinylation and maintenance of normal H3 methylation levels. By deletion analyses, we show that binding of Sus1 to SAGA depends on the deubiquitinylating enzyme Ubp8 and Sgf11. Moreover, a stable subcomplex between Sus1, Sgf11, and Ubp8 could be dissociated from SAGA under high salt conditions. In vivo recruitment of Sus1 to the activated GAL1 promoter depends on Ubp8 and vice versa. In addition, histones coenrich during SAGA purification in a Sus1–Sgf11–Ubp8-dependent way. Interestingly, sgf11 deletion enhances the mRNA export defect observed in sus1Δ cells. Thus, the Sus1–Sgf11–Ubp8 module could work at the junction between SAGA-dependent transcription and nuclear mRNA export.