Mechanism Underlying the Iron-dependent Nuclear Export of the Iron-responsive Transcription Factor Aft1p inSaccharomyces cerevisiae

Abstract

Aft1p is an iron-responsive transcriptional activator that plays a central role in maintaining iron homeostasis in Saccharomyces cerevisiae. Aft1p is regulated primarily by iron-induced shuttling of the protein between the nucleus and cytoplasm, but its nuclear import is not regulated by iron. Here, we have shown that the nuclear export of Aft1p is promoted in the presence of iron and that Msn5p is the nuclear export receptor (exportin) for Aft1p. Msn5p recognizes Aft1p in the iron-replete condition. Phosphorylation of S210 and S224 in Aft1p, which is not iron dependent, and the iron-induced intermolecular interaction of Aft1p are both essential for its recognition by Msn5p. Mutation of Cys291 of Aft1p to Phe, which causes Aft1p to be retained in the nucleus and results in constitutive activation of Aft1-target genes, disrupts the intermolecular interaction of Aft1p. Collectively, these results suggest that iron induces a conformational change in Aft1p, in which Aft1p Cys291 plays a critical role, and that, in turn, Aft1p is recognized by Msn5p and exported into the cytoplasm in an iron-dependent manner.

The Basis for Evolution of DNA-Binding Specificity of the Aft1 Transcription Factor in Yeasts

“…This agrees with a recent study showing that the iron-dependent dissociation of Aft1 from the promoter is the critical step in the regulation of iron metabolism (Ueta et al 2012). Thus, unlike what was previously suggested (Yamaguchi-Iwai et al 2002;Ueta et al 2003Ueta et al , 2007, the way Aft1 regulates iron homeostasis depends more on its capacity to bind its target DNA sequence than on its capacity to shift from the cytosol to the nucleus. Comparative sequence analysis of the DNA-binding domains of various Aft proteins showed that Aft1(152-203), which we named "VS", is a rapidly evolving sequence, bordered by two conserved regions designated CS1 and CS2.…”

Histidine 103 in Fra2 Is an Iron-Sulfur Cluster Ligand in the [2Fe-2S] Fra2-Grx3 Complex and Is Required for in Vivo Iron Signaling in Yeast

“…These results also lead to the question of how [2Fe-2S] Fra2-Grx3/4 influences the transcriptional activity of Aft1/2. As mentioned earlier, the FraGrx signaling pathway is proposed to induce oligomerization of Aft1 (and presumably Aft2) under iron-replete conditions (5,33). This conformational change, in turn, may promote export of Aft1/2 from the nucleus or prevent interaction of Aft1/2 with the target DNA.…”

Derepression of RNA Polymerase III Transcription by Phosphorylation and Nuclear Export of Its Negative Regulator, Maf1

“…It has been suggested previously that Msn5 specifically exports phosphoproteins because several Msn5 cargoes, such as Pho4, Mig1, Crz1, Aft1, Cdh1, and HO, have to be phosphorylated to be relocated out of the nucleus (17)(18)(19)(20)(21)(22)(23). However, binding of Msn5 to other cargoes, such as Far1, does not depend on the cargo phosphorylation state (24).…”