Phosphorylation and Proteasome Recognition of the mRNA-Binding Protein Cth2 Facilitates Yeast Adaptation to Iron Deficiency

Abstract

Iron is a vital element for many metabolic pathways, including the synthesis of DNA and proteins, and the generation of energy via oxidative phosphorylation. Therefore, living organisms have developed tightly controlled mechanisms to properly distribute iron, since imbalances lead to nutritional deficiencies, multiple diseases, and vulnerability against pathogens. Saccharomyces cerevisiae Cth2 is a conserved mRNA-binding protein that coordinates a global reprogramming of iron metabolism in response to iron deficiency in order to optimize its utilization. Here we report that the phosphorylation of Cth2 at specific serine residues is essential to regulate the stability of the protein and adaptation to iron depletion. We identify the kinase and ubiquitination machinery implicated in this process to establish a posttranscriptional regulatory model. These results and recent findings for both mammals and plants reinforce the privileged position of E3 ubiquitin ligases and phosphorylation events in the regulation of eukaryotic iron homeostasis.

Iron Regulatory Mechanisms in Saccharomyces cerevisiae

“…Cth2 also limits the accumulation of iron into the vacuole when iron levels are low by promoting the degradation of the CCC1 transcript encoding for the vacuolar iron importer (Puig et al, 2005). The overexpression of a functional CTH1 or CTH2 gene is highly cytotoxic (Thompson et al, 1996;Pedro-Segura et al, 2008;Romero et al, 2018b), so yeast cells have to tightly control their expression. Interestingly, both mRNAs contain AREs that allow a negative cross-and auto-regulation that limits their expression and that is important for the activation of respiration and for optimal adaptation to the transit from iron-deficient to iron-sufficient conditions (Martinez-Pastor et al, 2013b).…”

“…The higher relevance of Cth2 in the adaptation of yeast cells to iron depletion is highlighted by the growth defect that cth2 Δ mutant cells display under these conditions, which is exacerbated in cth1 Δ cth2 Δ double mutants ( Puig et al, 2005 ). Cth2 is a highly unstable nucleocytoplasmic shuttling protein ( Vergara et al, 2011 ; Romero et al, 2018b ). A nuclear localization signal (NLS) embedded in its TZF motif enables its import to the nucleus, where it co-transcriptionally associates to ARE-containing mRNAs ( Prouteau et al, 2008 ; Vergara et al, 2011 ).…”

Mitochondrial Biogenesis Is Positively Regulated by Casein Kinase I Hrr25 Through Phosphorylation of Puf3 in Saccharomyces cerevisiae

“…We reported that Hrr25 is a negative regulator of the transcriptional activator Haa1, which is important in cellular adaptation to acetic acid stress (COLLINS et al 2017). Hrr25 has also been reported to regulate Cth2, an RNA-binding protein that plays an important role in iron hemostasis (ROMERO et al 2018). Phosphorylation of Cth2 by Hrr25 leads to Cth2's recognition by the SCF Grr1 E3 ubiquitin ligase and degradation via the 26S proteasome.…”

A genome-wide transcriptional study reveals that iron deficiency inhibits the yeast TORC1 pathway

“…However, other factors could participate in this regulation. For instance, the E3 ubiquitin ligase complex SCF Grr1 , which positively regulates RTG signaling by promoting the ubiquitination and degradation of the Rtg1/Rtg3-inhibitor protein Mks1, has been recently implicated in the degradation of the iron-regulated protein Cth2, and grr1Δ cells are unable to grow in iron-deficient conditions [36].…”