This entry represents a mRNA export factor Sac3 from budding yeasts. Sac3 is a component of the TREX-2 complex (Sac3p-Thp1p-Sus1p-Cdc31p), which functions in transcription elongation and transcription-coupled mRNA export from the nucleus to the cytoplasm [, , ]. Its N terminus associates with Thp1 and Sem1 to form an mRNA-binding module, as well as with the export receptor Mex67, its C terminus interacts with Sus1, Cdc31 and the Nup1 nucleoporin, providing anchoring to the nuclear pore complex [].
This family includes yeast Sem1 and its mammalian homologue, DSS1.Sem1/DSS1 (also known as rpn15) is a component of lid subcomplex of 26S proteasome regulatory subunit [, , ]. Besides being a subunit of the 26S proteasome, Sem1/DSS1 associates with other protein complexes []. It is a component of 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 [, , ]. Loss of DSS1 in humans has been associated with split hand/split foot malformations [].
The 26S proteasome is the major ATP-dependent protease in eukaryotes which plays a key role in intracellular protein degradation [, ].The lid of the 26S proteasome contains six PCI-domain-containing proteins (Rpn3/5/6/7/9/12), two MPN-domain-containing proteins (Rpn8/11), and one peptide, Sem1 []. The only catalytically active member of the lid is Rpn11, which serves as the essential deubiquitinase of the proteasome []. The C terminus of each subunit in the lid is predicted to form one or more helices. These C-terminal helices are highly conserved and have been predicted to form a helical bundle structure [].In yeast, Rpn9 was found to be necessary for the integrity and efficiency of the 26S proteasome [, ].This entry represents a helix domain C-terminal to the PCI domain found in Rpn9, a subunit of the 26S proteasome. C-terminal truncations of Rpn5 or Rpn9 have been shown not to cause any major lid assembly defects but to prevent the association of Rpn12 [, ].
