The chaperone Chs7 forms a stable complex with Chs3 and promotes its activity at the cell surface.

The polytopic yeast protein Chs3 (chitin synthase III) relies on a dedicated membrane-localized chaperone, Chs7, for its folding and expression at the cell surface. In the absence of Chs7, Chs3 forms high molecular weight aggregates and is retained in the endoplasmic reticulum (ER). Chs7 was reported to be an ER resident protein, but its role in Chs3 folding and transport was not well characterized. Here, we show that Chs7 itself exits the ER and localizes with Chs3 at the bud neck and intracellular compartments. We identified mutations in the Chs7 C-terminal cytosolic domain that do not affect its chaperone function, but cause it to dissociate from Chs3 at a post-ER transport step. Mutations that prevent the continued association of Chs7 with Chs3 do not block delivery of Chs3 to the cell surface, but dramatically reduce its catalytic activity. This suggests that Chs7 engages in functionally distinct interactions with Chs3 to first promote its folding and ER exit, and subsequently to regulate its activity at the plasma membrane.

Differential regulation of TNL-mediated immune signaling by redundant helper CNLs.

Higher plants utilize nucleotide-binding leucine-rich repeat domain proteins (NLRs) as intracellular immune receptors to recognize pathogen-derived effectors and trigger a robust defense. The Activated Disease Resistance 1 (ADR1) family of coiled-coil NLRs (CNLs) have evolved as helper NLRs that function downstream of many TIR-type sensor NLRs (TNLs). Close homologs of ADR1s form the N REQUIREMENT GENE 1 (NRG1) family in Arabidopsis, the function of which is unclear. Through CRISPR/Cas9 gene editing methods, we discovered that the tandemly repeated NRG1A and NRG1B are functionally redundant and operate downstream of TNLs with differential strengths. Interestingly, ADR1s and NRG1s function in two distinct parallel pathways contributing to TNL-specific immunity. Synergistic effects on basal and TNL-mediated defense were detected among ADR1s and NRG1s. An intact P-loop of NRG1s is not required for mediating signals from sensor TNLs, whereas auto-active NRG1A exhibits autoimmunity. Importantly, NRG1s localize to the cytosol and endomembrane network regardless of the presence of effectors, suggesting a cytosolic activation mechanism. Taken together, different sensor TNLs differentially use two groups of helper NLRs, ADR1s and NRG1s, to transduce downstream defense signals.

Septin-associated protein kinase Gin4 affects localization and phosphorylation of Chs4, the regulatory subunit of the Baker's yeast chitin synthase III complex.

Chitin is mainly formed by the chitin synthase III complex (CSIII) in yeast cells. This complex is considered to be composed of the catalytic subunit Chs3 and the regulatory subunit Chs4, both of which are phosphoproteins and transported to the plasma membrane by different trafficking routes. During cytokinesis, Chs3 associates with Chs4 and other proteins at the septin ring, which results in an active CSIII complex. In this study, we focused on the role of Chs4 as a regulatory subunit of the CSIII complex. We analyzed the dynamic localization and interaction of Chs3 and Chs4 during cell division, and found that both proteins transiently co-localize and physically interact only during bud formation and later in a period during septum formation and cytokinesis. To identify unknown binding partners of Chs4, we conducted different screening approaches, which yielded several novel candidates of Chs4-binding proteins including the septin-associated kinase Gin4. Our further studies confirmed this interaction and provided first evidence that Chs4 phosphorylation is partially dependent on Gin4, which is required for proper localization of Chs4 at the bud neck.

CaGdt1 plays a compensatory role for the calcium pump CaPmr1 in the regulation of calcium signaling and cell wall integrity signaling in Candida albicans.

BACKGROUND: Saccharomyces cerevisiae ScGdt1 and mammalian TMEM165 are two members of the UPF0016 membrane protein family that is likely to form a new group of Ca2+/H+ antiporter and/or a Mn2+ transporter in the Golgi apparatus. We have previously shown that Candida albicans CaGDT1 is a functional ortholog of ScGDT1 in the response of S. cerevisiae to calcium stress. However, how CaGdt1 together with the Golgi calcium pump CaPmr1 regulate calcium homeostasis and cell wall integrity in this fungal pathogen remains unknown. METHODS: Chemical sensitivity was tested by dilution assay. Cell survival was examined by measuring colony-forming units and staining with Annexin V-FITC and propidium iodide. Calcium signaling was examined by expression of downstream target gene CaUTR2, while cell wall integrity signaling was revealed by detection of phosphorylated Mkc1 and Cek1. Subcellular localization of CaGdt1 was examined through direct and indirect immunofluorescent approaches. Transcriptomic analysis was carried out with RNA sequencing. RESULTS: This study shows that Candida albicans CaGDT1 is also a functional ortholog of ScGDT1 in the response of S. cerevisiae to cell wall stress. CaGdt1 is localized in the Golgi apparatus but at distinct sites from CaPmr1 in C. albicans. Loss of CaGDT1 increases the sensitivity of cell lacking CaPMR1 to cell wall and ER stresses. Deletion of CaGDT1 and/or CaPMR1 increases calcium uptake and activates the calcium/calcineurin signaling. Transcriptomic profiling reveals that core functions shared by CaGdt1 and CaPmr1 are involved in the regulation of cellular transport of metal ions and amino acids. However, CaGdt1 has distinct functions from CaPmr1. Chitin synthase gene CHS2 is up regulated in all three mutants, while CHS3 is only up regulated in the pmr1/pmr1 and the gdt1/gdt1 pmr1/pmr1 mutants. Five genes (DIE2, STT3, OST3, PMT1 and PMT4) of glycosylation pathway and one gene (SWI4) of the cell wall integrity (CWI) pathway are upregulated due to deletion of CaGDT1 and/or CaPMR1. Consistently, deletion of either CaPMR1 or CaGDT1 activates the CaCek1-mediated CWI signaling in a cell wall stress-independent fashion. Calcineurin function is required for the integrity of the cell wall and vacuolar compartments of cells lacking both GDT1 and CaPMR1. CONCLUSIONS: CaPmr1 is the major player in the regulation of calcium homeostasis and cell wall stress, while CaGdt1 plays a compensatory role for CaPmr1 in the Golgi compartment in C. albicans.

In Vitro and In Vivo Studies on the Structural Organization of Chs3 from Saccharomyces cerevisiae.

Chitin biosynthesis in yeast is accomplished by three chitin synthases (Chs) termed Chs1, Chs2 and Chs3, of which the latter accounts for most of the chitin deposited within the cell wall. While the overall structures of Chs1 and Chs2 are similar to those of other chitin synthases from fungi and arthropods, Chs3 lacks some of the C-terminal transmembrane helices raising questions regarding its structure and topology. To fill this gap of knowledge, we performed bioinformatic analyses and protease protection assays that revealed significant information about the catalytic domain, the chitin-translocating channel and the interfacial helices in between. In particular, we identified an amphipathic, crescent-shaped α-helix attached to the inner side of the membrane that presumably controls the channel entrance and a finger helix pushing the polymer into the channel. Evidence has accumulated in the past years that chitin synthases form oligomeric complexes, which may be necessary for the formation of chitin nanofibrils. However, the functional significance for living yeast cells has remained elusive. To test Chs3 oligomerization in vivo, we used bimolecular fluorescence complementation. We detected oligomeric complexes at the bud neck, the lateral plasma membrane, and in membranes of Golgi vesicles, and analyzed their transport route using various trafficking mutants.

Transcriptome analysis revealing the effect of Bupleurum scorzonerifolium Willd association with endophytic fungi CHS3 on the production of saikosaponin D.

Saikosaponin D (SSd) is a naturally active product with strong pharmacological activity found in Bupleurum scorzonerifolium Willd. Studies have shown that endophytic fungi have great potential as sources of natural medicines. Fusarium acuminatum (CHS3), an SSd-producing endophytic fungus, was isolated from B. scorzonerifolium. To elucidate the effect of host plants on the production of SSd in CHS3, CHS3 was co-cultured with suspension cells of B. scorzonerifolium and SSd was detected using high-performance liquid chromatography (HPLC). Transcriptome sequencing (RNA-Seq) of CHS3 before and after co-culture was performed using an Illumina HiSeq 2500 platform. The results indicated that the content of SSd synthesised by CHS3 increased after co-culture with suspension cells of B. scorzonerifolium. Transcriptome analysis of CHS3 with differentially expressed genes (DEGs) showed that 1202 and 1049 genes were upregulated and downregulated, respectively, after co-culture. Thirty genes associated with SSd synthesis and 11 genes related to terpene backbone biosynthesis were annotated to the Kyoto Encyclopaedia of Genes and Genomes (KEGG). Combined with transcriptome data, it was speculated that the mevalonate (MVA) pathway is a possible pathway for SSd synthesis in CHS3, and the expression of key enzyme genes (HMGR, HMGCS, GGPS1, MVK, FDFT1, FNTB) was validated by qRT-PCR. In conclusion, the endophytic fungus CHS3 can form an interactive relationship with its host, thereby promoting SSd biosynthesis and accumulation by upregulating the expression of key enzyme genes in the biosynthesis pathway.

Allelic variation in the Arabidopsis TNL CHS3/CSA1 immune receptor pair reveals two functional cell-death regulatory modes.

Some plant NLR immune receptors are encoded in head-to-head "sensor-executor" pairs that function together. Alleles of the NLR pair CHS3/CSA1 form three clades. The clade 1 sensor CHS3 contains an integrated domain (ID) with homology to regulatory domains, which is lacking in clades 2 and 3. In this study, we defined two cell-death regulatory modes for CHS3/CSA1 pairs. One is mediated by ID domain on clade 1 CHS3, and the other relies on CHS3/CSA1 pairs from all clades detecting perturbation of an associated pattern-recognition receptor (PRR) co-receptor. Our data support the hypothesis that an ancestral Arabidopsis CHS3/CSA1 pair gained a second recognition specificity and regulatory mechanism through ID acquisition while retaining its original specificity as a "guard" against PRR co-receptor perturbation. This likely comes with a cost, since both ID and non-ID alleles of the pair persist in diverse Arabidopsis populations through balancing selection.

The Arabidopsis TIR-NBS-LRR protein CSA1 guards BAK1-BIR3 homeostasis and mediates convergence of pattern- and effector-induced immune responses.

Arabidopsis BAK1/SERK3, a co-receptor of leucine-rich repeat pattern recognition receptors (PRRs), mediates pattern-triggered immunity (PTI). Genetic inactivation of BAK1 or BAK1-interacting receptor-like kinases (BIRs) causes cell death, but the direct mechanisms leading to such deregulation remains unclear. Here, we found that the TIR-NBS-LRR protein CONSTITUTIVE SHADE AVOIDANCE 1 (CSA1) physically interacts with BIR3, but not with BAK1. CSA1 mediates cell death in bak1-4 and bak1-4 bir3-2 mutants via components of effector-triggered immunity-(ETI) pathways. Effector HopB1-mediated perturbation of BAK1 also results in CSA1-dependent cell death. Likewise, microbial pattern pg23-induced cell death, but not PTI responses, requires CSA1. Thus, we show that CSA1 guards BIR3 BAK1 homeostasis and integrates pattern- and effector-mediated cell death pathways downstream of BAK1. De-repression of CSA1 in the absence of intact BAK1 and BIR3 triggers ETI cell death. This suggests that PTI and ETI pathways are activated downstream of BAK1 for efficient plant immunity.

HSP90 Contributes to chs3-2D-Mediated Autoimmunity.

Plants employ multi-layered immune system to fight against pathogen infections. Different receptors are able to detect the invasion activities of pathogens, transduce signals to downstream components, and activate defense responses. Among those receptors, nucleotide-binding domain leucine-rich repeat containing proteins (NLRs) are the major intracellular ones. CHILLING SENSITIVE 3 (CHS3) is an Arabidopsis NLR with an additional Lin-11, Isl-1 and Mec-3 (LIM) domain at its C terminus. The gain-of-function mutant, chs3-2D, exhibiting severe dwarfism and constitutively activated defense responses, was selected as a genetic background in this study for a forward genetic screen. A mutant allele of hsp90.2 was isolated as a partial suppressor of chs3-2D, suggesting that HSP90 is required for CHS3-mediated defense signaling. In addition, HSP90 is also required for the autoimmunity of the Dominant Negative (DN)-SNIPER1 and gain-of-function ADR1-L2 D484V transgenic lines, suggesting a broad role for HSP90 in NLR-mediated defense. Overall, our work indicates a larger contribution of HSP90 not only at the sensor, but also the helper NLR levels.

Echinocandin resistance in Candida parapsilosis sensu stricto: Role of alterations in CHS3, FKS1 and Rho gene expression.

OBJECTIVES: The rate of resistance of Candida parapsilosis to echinocandins remains unexplored in Iran. The main aims of this study were to investigate the susceptibility patterns and possible mechanisms of echinocandin resistance in echinocandin-resistant clinical C. parapsilosis isolates in Iran. METHODS: A total of 105 isolates of C. parapsilosis sensu stricto underwent antifungal susceptibility testing to echinocandins by the broth microdilution reference method. Sequences of the CpERG3 and CpFKS1 genes were analysed using MEGA6 software, and alterations in CHS3, FKS1 and Rho gene expression were evaluated by quantitative reverse transcription (RT-qPCR). REST® software was used to analyse the results. RESULTS: The rate of echinocandin cross-resistance was 2.9% (3/105). No substitutions were detected in Fks1p except for the naturally occurring P660A amino acid substitution observed in isolates both with high and low minimum inhibitory concentrations (MICs). Moreover, the G111R amino acid substitution was not found in Erg3p. Following echinocandin exposure, expression of Rho and FKS1 genes was significantly increased in resistant isolates, whilst the CHS3 gene showed no change. CONCLUSION: Alterations in the expression of some key genes may be responsible for echinocandin resistance among C. parapsilosis isolates. Understanding the mechanisms responsible for drug resistance in C. parapsilosis is not only crucial for the development of new antifungals but is also important in choosing appropriate antifungals for patient treatment at the earliest stage.

Long-Chain Polyprenols Promote Spore Wall Formation in Saccharomyces cerevisiae.

Dolichols are isoprenoid lipids of varying length that act as sugar carriers in glycosylation reactions in the endoplasmic reticulum. In Saccharomyces cerevisiae, there are two cis-prenyltransferases that synthesize polyprenol-an essential precursor to dolichol. These enzymes are heterodimers composed of Nus1 and either Rer2 or Srt1. Rer2-Nus1 and Srt1-Nus1 can both generate dolichol in vegetative cells, but srt1∆ cells grow normally while rer2∆ grows very slowly, indicating that Rer2-Nus1 is the primary enzyme used in mitotically dividing cells. In contrast, SRT1 performs an important function in sporulating cells, where the haploid genomes created by meiosis are packaged into spores. The spore wall is a multilaminar structure and SRT1 is required for the generation of the outer chitosan and dityrosine layers of the spore wall. Srt1 specifically localizes to lipid droplets associated with spore walls, and, during sporulation there is an SRT1-dependent increase in long-chain polyprenols and dolichols in these lipid droplets. Synthesis of chitin by Chs3, the chitin synthase responsible for chitosan layer formation, is dependent on the cis-prenyltransferase activity of Srt1, indicating that polyprenols are necessary to coordinate assembly of the spore wall layers. This work shows that a developmentally regulated cis-prenyltransferase can produce polyprenols that function in cellular processes besides protein glycosylation.

Hof1 and Chs4 Interact via F-BAR Domain and Sel1-like Repeats to Control Extracellular Matrix Deposition during Cytokinesis.

Localized extracellular matrix (ECM) remodeling is thought to stabilize the cleavage furrow and maintain cell shape during cytokinesis [1-14]. This remodeling is spatiotemporally coordinated with a cytoskeletal structure pertaining to a kingdom of life, for example the FtsZ ring in bacteria [15], the phragmoplast in plants [16], and the actomyosin ring in fungi and animals [17, 18]. Although the cytoskeletal structures have been analyzed extensively, the mechanisms of ECM remodeling remain poorly understood. In the budding yeast Saccharomyces cerevisiae, ECM remodeling refers to sequential formations of the primary and secondary septa that are catalyzed by chitin synthase-II (Chs2) and chitin synthase-III (the catalytic subunit Chs3 and its activator Chs4), respectively [18, 19]. Surprisingly, both Chs2 and Chs3 are delivered to the division site at the onset of cytokinesis [6, 20]. What keeps Chs3 inactive until secondary septum formation remains unknown. Here, we show that Hof1 binds to the Sel1-like repeats (SLRs) of Chs4 via its F-BAR domain and inhibits Chs3-mediated chitin synthesis during cytokinesis. In addition, Hof1 is required for rapid accumulation as well as efficient removal of Chs4 at the division site. This study uncovers a mechanism by which Hof1 controls timely activation of Chs3 during cytokinesis and defines a novel interaction and function for the conserved F-BAR domain and SLR that are otherwise known for their abilities to bind membrane lipids [21, 22] and scaffold protein complex formation [23].

Mutations in an Atypical TIR-NB-LRR-LIM Resistance Protein Confer Autoimmunity.

In order to defend against microbial infection, plants employ a complex immune system that relies partly on resistance (R) proteins that initiate intricate signaling cascades upon pathogen detection. The resistance signaling network utilized by plants is only partially characterized. A genetic screen conducted to identify novel defense regulators involved in this network resulted in the isolation of the snc6-1D mutant. Positional cloning revealed that this mutant contained a molecular lesion in the chilling sensitive 3 (CHS3) gene, thus the allele was renamed chs3-2D. CHS3 encodes a TIR-NB-LRR R protein that contains a C-terminal zinc-binding LIM (Lin-11, Isl-1, Mec-3) domain. Although this protein has been previously implicated in cold stress and defense response, the role of the LIM domain in modulating protein activity is unclear. The chs3-2D allele contains a G to A point mutation causing a C1340 to Y1340 substitution close to the LIM domain. It encodes a dominant gain-of-function mutation. The chs3-2D mutant is severely stunted and displays curled leaf morphology. Additionally, it constitutively expresses PATHOGENESIS-RELATED (PR) genes, accumulates salicylic acid, and shows enhanced resistance to the virulent oomycete isolate Hyaloperonospora arabidopsidis (H.a.) Noco2. Subcellular localization assays using GFP fusion constructs indicate that both CHS3 and chs3-2D localize to the nucleus. A third chs3 mutant allele, chs3-3D, was identified in an unrelated genetic screen in our lab. This allele contains a C to T point mutation resulting in an M1017 to V1017 substitution in the LRR-LIM linker region. Additionally, a chs3-2D suppressor screen identified two revertant alleles containing secondary mutations that abolish the mutant morphology. Analysis of the locations of these molecular lesions provides support for the hypothesis that the LIM domain represses CHS3 R-like protein activity. This repression may occur through either autoinhibition or binding of a negative defense regulator.