Core glycan in the yeast multicopper ferroxidase, Fet3p: a case study of N-linked glycosylation, protein maturation, and stability

Proteome plasticity in response to persistent environmental change

Temperature is a variable component of the environment, and all organisms must deal with or adapt to temperature change. Acute temperature change activates cellular stress responses, resulting in refolding or removal of damaged proteins. However, how organisms adapt to long-term temperature change remains largely unexplored. Here we report that budding yeast responds to long-term high temperature challenge by switching from chaperone induction to reduction of temperature-sensitive proteins and re-localizing a portion of its proteome. Surprisingly, we also find that many proteins adopt an alternative conformation. Using Fet3p as an example, we find that the temperature-dependent conformational difference is accompanied by distinct thermostability, subcellular localization, and, importantly, cellular functions. We postulate that, in addition to the known mechanisms of adaptation, conformational plasticity allows some polypeptides to acquire new biophysical properties and functions when environmental change endures.

Alpha-synuclein inhibits Snx3-retromer-mediated retrograde recycling of iron transporters in S. cerevisiae and C. elegans models of Parkinson's disease

We probed the role of alpha-synuclein (α-syn) in modulating sorting nexin 3 (Snx3)-retromer-mediated recycling of iron transporters in Saccharomyces cerevisiae and Caenorhabditis elegans. In yeast, the membrane-bound heterodimer Fet3/Ftr1 is the high affinity iron importer. Fet3 is a membrane-bound multicopper ferroxidase, whose ferroxidase domain is orthologous to human ceruloplasmin (Cp), that oxidizes external Fe+2 to Fe+3; the Fe+3 ions then channel through the Ftr1 permease into the cell. When the concentration of external iron is low (<1 µM), Fet3/Ftr1 is maintained on the plasma membrane by retrograde endocytic-recycling; whereas, when the concentration of external iron is high (>10 µM), Fet3/Ftr1 is endocytosed and shunted to the vacuole for degradation. We discovered that α-syn expression phenocopies the high iron condition: under the low iron condition (<1 µM), α-syn inhibits Snx3-retromer-mediated recycling of Fet3/Ftr1 and instead shunts Fet3/Ftr1 into the multivesicular body pathway to the vacuole. α-Syn inhibits recycling by blocking the association of Snx3-mCherry molecules with endocytic vesicles, possibly by interfering with the binding of Snx3 to phosphatidylinositol-3-monophosphate. In C. elegans, transgenic worms expressing α-syn exhibit an age-dependent degeneration of dopaminergic neurons that is partially rescued by the iron chelator desferoxamine. This implies that α-syn-expressing dopaminergic neurons are susceptible to changes in iron neurotoxicity with age, whereby excess iron enhances α-syn-induced neurodegeneration. In vivo genetic analysis indicates that α-syn dysregulates iron homeostasis in worm dopaminergic neurons, possibly by inhibiting SNX-3-mediated recycling of a membrane-bound ortholog of Cp (F21D5.3), the iron exporter ferroportin (FPN1.1), or both.

Novel insight into the expression and function of the multicopper oxidases in Candida albicans

Iron is an essential element required for most organisms. The high-affinity iron-uptake systems in the opportunistic pathogen Candida albicans are activated under iron-limited conditions and are also required for virulence. Here one component of high-affinity iron-uptake systems, the multicopper oxidase (MCO) genes, was characterized. We examined the expression of five MCO genes and demonstrated that CaFET3 and CaFET34 were the major MCO genes in response to iron deficiency. Complementation of the Saccharomyces cerevisiae fet3Δ mutant showed that CaFET34 could effectively rescue the growth phenotype in iron-limited medium. Deletion of CaFET33 and CaFET34 in C. albicans decreased cellular iron content and iron acquisition during iron starvation. However, the fet33Δ/Δ and fet34Δ/Δ mutants exhibited no obvious growth defect in solid iron-limited medium while the fet34Δ/Δ mutant showed a slight growth defect in liquid medium. Further analysis shows that other members of the five MCO genes, especially CaFET3, would compensate for the absence of CaFET33 and CaFET34. Furthermore, for the first time, we provide evidence that CaFET34 is implicated in hyphal development in an iron-independent manner and is required for C. albicans virulence in a mouse model of systemic infection. Together, our results not only expand our understanding about the expression of the MCO genes in C. albicans, but also provide a novel insight into the role of CaFET34 in iron metabolism, hyphal development and virulence.

Elimination of manganese(II,III) oxidation in Pseudomonas putida GB-1 by a double knockout of two putative multicopper oxidase genes

Bacterial manganese(II) oxidation impacts the redox cycling of Mn, other elements, and compounds in the environment; therefore, it is important to understand the mechanisms of and enzymes responsible for Mn(II) oxidation. In several Mn(II)-oxidizing organisms, the identified Mn(II) oxidase belongs to either the multicopper oxidase (MCO) or the heme peroxidase family of proteins. However, the identity of the oxidase in Pseudomonas putida GB-1 has long remained unknown. To identify the P. putida GB-1 oxidase, we searched its genome and found several homologues of known or suspected Mn(II) oxidase-encoding genes (mnxG, mofA, moxA, and mopA). To narrow this list, we assumed that the Mn(II) oxidase gene would be conserved among Mn(II)-oxidizing pseudomonads but not in nonoxidizers and performed a genome comparison to 11 Pseudomonas species. We further assumed that the oxidase gene would be regulated by MnxR, a transcription factor required for Mn(II) oxidation. Two loci met all these criteria: PputGB1_2447, which encodes an MCO homologous to MnxG, and PputGB1_2665, which encodes an MCO with very low homology to MofA. In-frame deletions of each locus resulted in strains that retained some ability to oxidize Mn(II) or Mn(III); loss of oxidation was attained only upon deletion of both genes. These results suggest that PputGB1_2447 and PputGB1_2665 encode two MCOs that are independently capable of oxidizing both Mn(II) and Mn(III). The purpose of this redundancy is unclear; however, differences in oxidation phenotype for the single mutants suggest specialization in function for the two enzymes.

Functional characterization of the ferroxidase, permease high-affinity iron transport complex from Candida albicans

Saccharomyces cerevisiae expresses two proteins that together support high-affinity Fe-uptake. These are a multicopper oxidase, Fet3p, with specificity towards Fe²⁺ and a ferric iron permease, Ftr1p, which supports Fe-accumulation. Homologues of the genes encoding these two proteins are found in all fungal genomes including those for the pathogens, Candida albicans and Cryptococcus neoformans. At least one of these loci represents a virulence factor for each pathogen suggesting that this complex would be an appropriate pharmacologic target. However, the mechanism by which this protein pair supports Fe-uptake in any fungal pathogen has not been elucidated. Taking advantage of the robust molecular genetics available in S. cerevisiae, we identify the two of five candidate ferroxidases likely involved in high-affinity Fe-uptake in C. albicans, Fet31 and Fet34. Both localize to the yeast plasma membrane and both support Fe-uptake along with an Ftr1 protein, either from C. albicans or from S. cerevisiae. We express and characterize Fet34, demonstrating that it is functionally homologous to ScFet3p. Using S. cerevisiae as host for the functional expression of the C. albicans Fe-uptake proteins, we demonstrate that they support a mechanism of Fe-trafficking that involves channelling of the CaFet34-generated Fe³⁺ directly to CaFtr1 for transport into the cytoplasm.