Mechanistic study of regulation of mRNA export by proteasome in a non-proteolytic manner

Abstract

Over the years, gene expression is an intricate process tightly linked from the gene activation to the nuclear export of mRNA. Previous reports have indicated that the proteasome is essential for gene expression regulation. The 19S regulatory particle (19S RP) of the proteasome binds to the Spt-Ada-Gcn5-acetyltransferase (SAGA) complex and affects transcription in an ATP-dependent manner. However, the mechanism as to how the chaperone activity of the 19S RP of proteasome modulates SAGA by direct interactions is largely unknown. Here, we found that the proteasomal ATPase Rpt2p binds directly to Sgf73p, a component of SAGA complex in Saccharomyces cerevisiae. We also demonstrate that the rpt2-1 strain, a proteasomal ATPase Rpt2p mutant, is defective in specific binding to Sgf73p. Interestingly, a specific interaction between Rpt2p and Sgf73p leads to the dissociation of the H2Bub1-deubiquitylating module (herein designated Sgf73-DUBm) from SAGA both in vitro and in vivo. Furthermore, the localization of the Sgf73-DUBm on chromatin is perturbed in rpt2-1. The rpt2-1 mutant also exhibits impaired localization of the Thp1-Sac3-Sus1-Cdc31 (THSC, also termed TREX-2) complex and Poly-(A) RNA binding protein involved in nuclear mRNA exporter (MEX67-MTR2), interrupts the association of TREX-2 complex with the nuclear pore complex (NPC) and is defective in mRNA export. Taken together, these data indicate that the proteasome-mediated remodeling of the SAGA complex is a prerequisite for proper mRNA export.