Cotranscriptional Recruitment to the mRNA Export Receptor Mex67p Contributes to Nuclear Pore Anchoring of Activated Genes

Abstract

Transcription activation of some Saccharomyces cerevisiae genes is paralleled by their repositioning to the nuclear periphery, but the mechanism underlying gene anchoring is poorly defined. We show that the nuclear pore complex-associated Mlp1p and the shuttling mRNA export receptor Mex67p contribute to the stable association of the activated GAL10 and HSP104 genes with the nuclear periphery. However, we find no obligatory link between gene positioning and gene expression. Furthermore, gene anchoring correlates with the cotranscriptional recruitment of Mex67p to transcribing genes. Notably, the association of Mex67p with chromatin is not mediated by RNA. Interestingly, a mutant GAL2 gene lacking the coding region is still able to recruit Mex67p upon transcriptional activation and to relocate to the nuclear periphery. Together these data suggest that, at least for GAL2, nascent messenger ribonucleoprotein does not play a major role in gene anchoring and that the early recruitment of Mex67p contributes to gene repositioning by virtue of an RNA-independent process.A growing number of recent studies point to a functional relationship between gene expression and nuclear organization of chromatin. Indeed, the nucleus is subdivided into spatially defined domains, and the nonrandom distribution of chromosomes within these domains is thought to regulate their transcriptional state (8,25). Notably, the nuclear periphery has first been viewed as a transcriptionally repressive zone. In Saccharomyces cerevisiae, telomeres and mating-type loci, consisting of silent chromatin, concentrate in nuclear peripheral regions (16), and artificial tethering of nonsilenced DNA to the envelope induces its repression (2). In particular, the nuclear pore complex (NPC) has been implicated in perinuclear gene silencing and maintenance of gene expression states. Indeed, loss of a subset of NPC components, including the nuclear basketanchored Mlp1p and Mlp2p, results in the activation of subtelomeric reporter genes (10,12,20). However, another study showed that artificial tethering of nuclear transport factors to a partially silenced mating-type locus allowed its expression. Thus, recruitment of genes to the nuclear periphery can also have important effects on their activation (24). Consistent with these observations, genome-wide analyses of NPC-bound loci identified preferential association of highly transcribed genes with the nuclear periphery. These studies also showed that transcriptional activation of genes induced by galactose or ␣-factor is accompanied by their relocation from the nuclear interior to the nuclear periphery (5, 6). INO1 gene activation is also paralleled by its repositioning to the periphery, and this relocation contributes to optimal INO1 gene expression (3).Importantly, recent studies pointed to a direct physical link between Sus1p, a component of the SAGA histone deacetylase coactivator complex, and the Sac3-Thp1 complex, which is part of the mRNA export machinery associated with pores (31). These data together suggest...

Actively Transcribed GAL Genes Can Be Physically Linked to the Nuclear Pore by the SAGA Chromatin Modifying Complex

“…Taken together, these findings indicate that it is the physical presence of the SAGA complex at the GAL promoter that is required for association of the GAL genes with the NPC. Our findings are complemented by recent results suggesting that Mlp1 is an important connection between the NPC and actively transcribed genes (14), which suggests that the Mlp/SAGA link analyzed here could be a key element of locus recruitment to the NPC.…”

“…First, Mlp1 and Mlp2 interact with highly transcribed loci in genome-wide chromatin immunoprecipitation (ChIP) experiments (5). In fact, a recent study demonstrated a requirement for Mlp1 in GAL locus recruitment to the nuclear periphery (14). Second, Mlp1 has been indirectly linked to the Spt-Ada-Gcn5-acetyltransferase (SAGA) histone acetyltransferase complex through interactions between the Sac3-Thp1 mRNA export complex and the Sus1 component of SAGA (19,20).…”

“…Transcription factors, chromatin modifying complexes, and the transcription machinery itself have each been independently implicated in directing active loci to the NPC (8 -12), perhaps suggesting a recruitment mechanism dependent upon transcription initiation. In addition, some interactions between components of the NPC and active loci are RNA-dependent (7), and mRNA processing and export factors have also been implicated in tethering loci to the NPC (9,13,14), suggesting that the interaction between active genes and the NPC may instead be dependent upon ongoing transcription and mRNA maturation. Recent, independent studies of GAL and HXK1 recruitment have yielded different results regarding the role of the transcription machinery and transcriptional co-activators in this process (8 -10), raising the possibility that distinct mechanisms of recruitment may operate for individual loci.…”

Nuclear organization in genome stability: SUMO connections

“…INO1 or HXK1, like X-linked genes in male flies, show a modest but reproducible increase in expression when allowed to associate with nuclear pores [12,15,19]. On the other hand, there are other genes, notably a set of yeast ribosomal biogenesis genes, whose expression levels are lower when they are associated with nuclear pores [11,18]. This subset of promoters has a particular constellation of regulators and their localization at the nuclear pore depends on the actin-related protein Arp6 [18].…”

“…Although there is no evidence yet showing that specific pores bind specific sets of genes, recent studies implicate nuclear pores in a range of cellular processes, well beyond their function in macromolecular transport. Notably, in budding yeast and Drosophila certain highly transcribed, and in some cases stress-induced, genes are associated with nuclear pores [11][12][13][14][15][16][17][18][19]. This association not only promotes efficient export of mRNAs through coupling of transcription with export [10,11,13,14], but also helps modulate transcript levels of some loci.…”