An essential nuclear envelope integral membrane protein, Brr6p, required for nuclear transport

Abstract

Despite rapid advances in our understanding of the function of the nuclear pore complex in nuclear transport, little is known about the role the nuclear envelope itself may play in this critical process. A small number of integral membrane proteins speci®c to the envelope have been identi®ed in budding yeast, however, none has been reported to affect transport. We have identi®ed an essential gene, BRR6, whose product, Brr6p, behaves like a nuclear envelope integral membrane protein. Notably, the brr6-1 mutant speci®cally affects transport of mRNA and a protein reporter containing a nuclear export signal. In addition, Brr6p depletion alters nucleoporin distribution and nuclear envelope morphology, suggesting that the protein is required for the spatial organization of nuclear pores. BRR6 interacts genetically with a subset of nucleoporins, and Brr6-green¯uorescent protein (GFP) localizes in a punctate nuclear rim pattern, suggesting location at or near the nuclear pore. However, Brr6-GFP fails to redistribute in a Dnup133 mutant, distinguishing Brr6p from known proteins of the pore membrane domain. We hypothesize that Brr6p is located adjacent to the nuclear pore and interacts functionally with the pore and transport machinery. Keywords: integral membrane protein/mRNA export/ nuclear envelope/nuclear transport IntroductionThe presence of a nuclear envelope (NE) in eukaryotes divides the cell into nuclear and cytoplasmic compartments, greatly enhancing its capacity to control gene expression. Transport of proteins and ribonucleoprotein particles (RNPs) between the nucleus and cytoplasm through the nuclear pore complex (NPC) is central to this ability (reviewed in Go Èrlich and Kutay, 1999;Nakielny and Dreyfuss, 1999). The structure of the NPC is conserved from yeast to humans, consisting of a highly symmetrical core structure organized around a central pore channel as well as peripheral cytoplasmic ®laments and nucleoplasmic basket structures thought to play key roles in initiation and termination of transport pathways (reviewed in Franke and Scheer, 1974;Davis, 1995;Stof¯er et al., 1999;Wente, 2000). Transport of different types of protein and RNP cargoes through the NPC is mediated by speci®c protein carriers, including members of the karyopherin b family and a number of unrelated transport factors such as Ntf2 and Mex67 (reviewed in Pemberton et al., 1998;Wozniak et al., 1998;Go Èrlich and Kutay, 1999).Interactions between carriers and nucleoporins lining the channel of the pore are thought to be important for transport (reviewed in Fabre and Hurt, 1997;Pemberton et al., 1997;Wozniak et al., 1998;Go Èrlich and Kutay, 1999;Wente, 2000), although the precise roles of particular nucleoporins are not well understood. Our knowledge of the functional architecture of the yeast nuclear pore has bene®ted greatly from a detailed study of isolated NPCs carried out recently by Rout et al. (2000). The study produced a working model of the NPC, con®rming the presence of~30 yeast nucleoporins at the pore and placing them spat...

Identification of the NovelNup188-brr7Allele in a Screen for Cold-Sensitive mRNA Export Mutants inSaccharomyces cerevisiae

“…The BRR6 gene encodes a nuclear envelope transmembrane protein (de Bruyn Kops and Guthrie 2001; Scarcelli et al 2007; Hodge et al 2010; Tamm et al 2011; Lone et al 2015; Zhang et al 2018, reviewed, Schneiter and Cole 2010; Jaspersen and Ghosh 2012); we recently found that the export defect in this strain stems from impaired transcriptional regulation of the major poly A binding protein gene, PAB1 (A. de Bruyn Kops, J. E. Burke and C. Guthrie, unpublished data). brr7-1 ( nup188-brr7 ) is a novel cs allele of the nucleoporin, NUP188 (Nehrbass et al 1996; Zabel et al 1996).…”

Fantastic nuclear envelope herniations and where to find them

“…In yeast, such a role could be performed by the essential integral NE proteins Brl1 and Brr6, which localize at INM evaginations and physically interact with nups[73]. Consistent with the interpretation that they are required for NPC assembly, NE herniations build up in the absence Brr6[60,74] or Brl1/Brr6 function[73]. While it remains to be determined how Brl1 or Brr6 contribute to INM-ONM fusion, it is remarkable that overexpression of BRL1 prevents the formation of herniations (or perhaps drives their resolution) in nup116Δ strains[73].…”

Yeast Integral Membrane Proteins Apq12, Brl1, and Brr6 Form a Complex Important for Regulation of Membrane Homeostasis and Nuclear Pore Complex Biogenesis

“…BBR6 was identified initially in a screen for cold-sensitive mutants affecting mRNA export; depletion of BRR6 led to abnormalities in NPC distribution and NE morphology (18). We identified BRR6 as a dosage suppressor of the growth and mRNA export defects seen in apq12⌬ cells and showed that brr6 mutant cells display defects in nuclear envelope structure and NPC assembly very similar to those seen in apq12⌬ cells (15).…”

“…Brr6 and Brl1 are low-abundance proteins, and in previous studies, overexpression was used to permit easy detection of these proteins fused to GFP (18,21,28). Overexpressed tagged proteins were found primarily in the NE/ER.…”

“…Whereas S. cerevisiae and many closely related fungi have both genes, S. pombe, Pneumocystis carinii, and other fungi distantly related to S. cerevisiae have a unique gene that is more closely related to Brl1 than to Brr6 (19,22). Brl1 is not a nucleoporin; its distribution in the NE is unaffected in strains carrying nucleoporin mutations that cause NPCs to cluster together (18,21). Brr6 and Brl1 of budding yeast show synthetic genetic interactions.…”