Studies on the roles of clathrin-mediated membrane trafficking and zinc transporter Cis4 in the transport of GPI-anchored proteins in fission yeast

HcZnT2 is a highly mycorrhiza-induced zinc transporter from Hebeloma cylindrosporum in association with pine

Zinc (Zn) shortage is a common micronutrient deficiency affecting plants worldwide, while Zn toxicity may occur when this metal is in excess. Ectomycorrhizal (ECM) fungi are known to be able to modulate the transfer of macro- and microelements, among them Zn, to the plant. However, the underlying mechanisms are not well understood. We identified the HcZnT2 gene from the ECM fungus Hebeloma cylindrosporum, encoding a member of the Cation Diffusion Facilitator (CDF) family including Zn transporters, and analyzed its transcriptional regulation, the transport function by yeast complementation experiments, and its subcellular localization using a GFP fusion protein in yeast. HcZnT2 is highly induced during mycorrhization of Pinus pinaster, and upregulated in presence of the host plant root even without any direct contact. However, HcZnT2 is repressed by Zn excess conditions. By functional expression in yeast, our results strongly support the ability of HcZnT2 to transport Zn and, to a lesser extent, manganese. HcZnT2 localization was associated with the endoplasmic reticulum of yeast. Mycorrhizal gene activation at low external Zn suggests that the Zn transporter HcZnT2 might be important for the early establishment of the ECM symbiosis during Zn deficiency, rather than under Zn excess. HcZnT2 arises as an extremely remarkable candidate playing a key role in Zn homeostasis and regulation in ectomycorrhiza.

Zinc homeostasis in Schizosaccharomyces pombe

Most metal ions such as iron, calcium, zinc, or copper are essential for all eukaryotes. Organisms must maintain homeostasis of these metal ions because excess or deficiency of metal ions could cause damage to organisms. The steady state of many metal ions such as iron and copper has been well studied in detail. However, how to regulate zinc homeostasis in Schizosaccharomyces pombe is still confusing. In this review, we provide an overview of the molecular mechanisms that how S. pombe is able to maintain the balance of zinc levels in the changes of environment. In response to high levels of zinc, the transcription factor Loz1 represses the expression of several genes involved in the acquisition of zinc. Meanwhile, the CDF family proteins transport excess zinc to the secretory pathway. When zinc levels are limited, Loz1 was inactivated and could not inhibit the expression of zinc acquisition genes, and zinc stored in the secretory pathway is released for use by the cells. Besides, other factors that regulate zinc homeostasis are also discussed.

Involvement of BbTpc1, an important Zn(II)2Cys6 transcriptional regulator, in chitin biosynthesis, fungal development and virulence of an insect mycopathogen

Chitin is one of the major components of the fungal cell wall and contributes to the mechanical strength and shape of the fungal cell. Zn(II)₂Cys₆ transcription factors are unique to the fungal kingdom and have a variety of functions in some fungi. However, the mechanisms by which Zn(II)₂Cys₆ proteins affect entomopathogenic fungi are largely unknown. Here, we characterized the Zn(II)₂Cys₆ transcription factor BbTpc1 in the insect pathogenic fungus Beauveria bassiana. Disruption of BbTpc1 resulted in a distinct changes in vegetative growth and septation patterns, and a significant decrease in conidia and blastospore yield. The ΔBbTpc1 mutant displayed impaired resistance to chemical stresses and heat shock and attenuated virulence in topical and intrahemocoel injection assays. Importantly, the ΔBbTpc1 mutant had an abnormal cell wall with altered wall thickness and chitin synthesis, which were accompanied by transcriptional repression of the chitin synthetase family genes. In addition, comparative transcriptomics revealed that deletion of BbTpc1 altered fungal asexual reproduction via different genetic pathways. These data revealed that BbTpc1 regulates fungal development, chitin synthesis and biological control potential in B. bassiana.

Transcription factors and transporters in zinc homeostasis: lessons learned from fungi

Zinc is an essential nutrient for all organisms because this metal serves as a critical structural or catalytic cofactor for many proteins. These zinc-dependent proteins are abundant in the cytosol as well as within organelles of eukaryotic cells such as the nucleus, mitochondria, endoplasmic reticulum, Golgi, and storage compartments such as the fungal vacuole. Therefore, cells need zinc transporters so that they can efficiently take up the metal and move it around within cells. In addition, because zinc levels in the environment can vary drastically, the activity of many of these transporters and other components of zinc homeostasis is regulated at the level of transcription by zinc-responsive transcription factors. Mechanisms of post-transcriptional control are also important for zinc homeostasis. In this review, the focus will be on our current knowledge of zinc transporters and their regulation by zinc-responsive transcription factors and other mechanisms in fungi because these organisms have served as useful paradigms of zinc homeostasis in all organisms. With this foundation, extension to other organisms will be made where warranted.

Genetic evidence for involvement of membrane trafficking in the action of 5-fluorouracil

To identify novel genes that mediate cellular sensitivity and resistance to 5-fluorouracil (5-FU), we performed a genome-wide genetic screening to identify altered susceptibility to 5-FU by Schizosaccharomyces pombe haploid nonessential gene deletion library containing 3004 deletion mutants. We identified 50 hypersensitive and 12 resistant mutants to this drug. Mutants sensitive or resistant to 5-FU were classified into various categories based on their putative functions. The largest group of the genes whose disruption renders cells altered susceptibility to 5-FU is involved in nucleic acid metabolism, but to our surprise, the second largest group is involved in membrane trafficking. In addition, several other membrane traffic mutants examined including gdi1-i11, ypt3-i5, Δryh1, Δric1, and Δaps1 exhibited hypersensitivity to 5-FU. Furthermore, we found that 5-FU in low concentration that generally do not affect cell growth altered the localization of Syb1, a secretory vesicle SNARE synaptobrevin which is cycled between the plasma membrane and the endocytic pathway. Notably, 5-FU at such low concentration also significantly inhibited the secretion of acid phosphatase. Altogether, our findings revealed the first evidence that 5-FU influences membrane trafficking as the potential underlying mechanism of the drug action.

E3 ubiquitin ligase Pub1 is implicated in endocytosis of a GPI-anchored protein Ecm33 in fission yeast

We previously identified three glycosylphosphatidylinositol (GPI)-anchored proteins including Ecm33, as multicopy suppressors of the phenotypes of a mutant allele of cis4⁺ that encodes a zinc transporter in fission yeast. Here, we further identified two multicopy suppressor genes, ubi1⁺ and ubc4⁺, encoding ubiquitin-ribosomal fusion protein and ubiquitin conjugating enzyme E2, respectively. In addition, Ubi1 or Ubc4 overexpression failed to suppress the phenotypes of the double deletion of cis4⁺ and pub1⁺ gene, which encodes a HECT-type ubiquitin ligase E3. During exponential phase GFP-Ecm33 localized at the growing cell tips of the cell surface and the medial region in wild-type cells. Notably, during the post-exponential and stationary phase, GFP-Ecm33 in wild-type cells was internalized and mostly localized to the Golgi/endosomes, but it was still stably localized at the cell surface in Δpub1 cells. The Δpub1 cells showed osomoremedial phenotypes to various drugs indicating their defects in cell wall integrity. Altogether, our findings reveal a novel role for Pub1 in endocytosis of Ecm33 and regulation of cell wall integrity in fission yeast.