Sgt1 associates with Hsp90: an initial step of assembly of the core kinetochore complex

Insights into Hsp90 mechanism and in vivo functions learned from studies in the yeast, Saccharomyces cerevisiae

The molecular chaperone Hsp90 (Heat shock protein, 90 kDa) is an abundant and essential cytosolic protein required for the stability and/or folding of hundreds of client proteins. Hsp90, along with helper cochaperone proteins, assists client protein folding in an ATP-dependent pathway. The laboratory of Susan Lindquist, in collaboration with other researchers, was the first to establish the yeast Saccharomyces cerevisiae as a model organism to study the functional interaction between Hsp90 and clients. Important insights from studies in her lab were that Hsp90 is essential, and that Hsp90 functions and cochaperone interactions are highly conserved between yeast and mammalian cells. Here, we describe key mechanistic insights into the Hsp90 folding cycle that were obtained using the yeast system. We highlight the early contributions of the laboratory of Susan Lindquist and extend our analysis into the broader use of the yeast system to analyze the understanding of the conformational cycle of Hsp90 and the impact of altered Hsp90 function on the proteome.

Saccharomyces cerevisiae as a tool for deciphering Hsp90 molecular chaperone function

Yeast is a valuable model organism for their ease of genetic manipulation, rapid growth rate, and relative similarity to higher eukaryotes. Historically, Saccharomyces cerevisiae has played a major role in discovering the function of complex proteins and pathways that are important for human health and disease. Heat shock protein 90 (Hsp90) is a molecular chaperone responsible for the stabilization and activation of hundreds of integral members of the cellular signaling network. Much important structural and functional work, including many seminal discoveries in Hsp90 biology are the direct result of work carried out in S. cerevisiae. Here, we have provided a brief overview of the S. cerevisiae model system and described how this eukaryotic model organism has been successfully applied to the study of Hsp90 chaperone function.

Impact of Co-chaperones and Posttranslational Modifications Toward Hsp90 Drug Sensitivity

Posttranslational modifications (PTMs) regulate myriad cellular processes by modulating protein function and protein-protein interaction. Heat shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone whose activity is responsible for the stabilization and maturation of more than 300 client proteins. Hsp90 is a substrate for numerous PTMs, which have diverse effects on Hsp90 function. Interestingly, many Hsp90 clients are enzymes that catalyze PTM, demonstrating one of the several modes of regulation of Hsp90 activity. Approximately 25 co-chaperone regulatory proteins of Hsp90 impact structural rearrangements, ATP hydrolysis, and client interaction, representing a second layer of influence on Hsp90 activity. A growing body of literature has also established that PTM of these co-chaperones fine-tune their activity toward Hsp90; however, many of the identified PTMs remain uncharacterized. Given the critical role of Hsp90 in supporting signaling in cancer, clinical evaluation of Hsp90 inhibitors is an area of great interest. Interestingly, differential PTM and co-chaperone interaction have been shown to impact Hsp90 binding to its inhibitors. Therefore, understanding these layers of Hsp90 regulation will provide a more complete understanding of the chaperone code, facilitating the development of new biomarkers and combination therapies.

Expression Analysis Reveals That Sorghum Disease Resistance Protein SbSGT1 Is Regulated by Auxin

SGT1 (suppressor of the skp1 G2 allele) is an important plant disease resistance-related protein, which plays an important role in plant resistance to pathogens and regulates signal transduction during the process of plant disease resistance. In this study, we analyzed the expression profile of SbSGT1 in sorghum under phytohormones treatment. Quantitative real-time PCR results showed that SbSGT1 was most expressed in sorghum leaves, and could respond to plant hormones such as auxin, abscisic acid, salicylic acid, and brassinolide. Subsequently, we determined the optimal soluble prokaryotic expression conditions for SbSGT1 and purified it using a protein purification system in order to evaluate its potential interactions with plant hormones. Microscale thermophoretic analysis showed that SbSGT1 exhibited significant interactions with indole-3-acetic acid (IAA), with a Kd value of 1.5934. Furthermore, the transient expression of SbSGT1 in Nicotiana benthamiana indicated that treatment with exogenous auxin could inhibit SbSGT1 expression, both at the transcriptional and translational level, demonstrating that there exists an interaction between SbSGT1 and auxin.

Proteomic-based identification of oocyte maturation-related proteins in mouse germinal vesicle oocytes

Oocyte proteins play an important role in oocyte maturation, fertilization and embryonic development. However, the protein composition of mouse germinal vesicle (GV) oocytes is still unclear. Using one-dimensional Sodium Dodecyl Sulphate Polyacrylamide Gel Electrophoresis (1D SDS-PAGE) and Reverse-phase liquid chromatography tandem mass spectrometry (RP-LC-MS/MS), we constructed a protein profile of mouse GV oocytes. First, our proteomics profile identified 1,405 different proteins from 11,000 mouse GV oocytes lacking zona pellucida. Second, with detailed bioinformatics analysis, a group of proteins that play an essential role in oocyte maturation was screened. In addition, the expression and localization of suppressor of G2 allele of skp1(SUGT1, also called SGT1), heterogeneous nuclear ribonucleoprotein K (Hnrpk), Seruin, Cullin1(Clu1) and nuclear distribution protein C (Nudc) in mouse ovaries and early embryos were also captured and investigated in this study. Moreover, the protein profile was submitted to the Proteomics Identifications Database (PRIDE) and is available via ProteomeXchange with the identifier PXD014314. Our research provides valuable resources for the study of oocyte proteins and oocyte maturation and helps to clarify the mechanisms of oocyte maturation.

Network Analysis of UBE3A/E6AP-Associated Proteins Provides Connections to Several Distinct Cellular Processes

Perturbations in activity and dosage of the UBE3A ubiquitin-ligase have been linked to Angelman syndrome and autism spectrum disorders. UBE3A was initially identified as the cellular protein hijacked by the human papillomavirus E6 protein to mediate the ubiquitylation of p53, a function critical to the oncogenic potential of these viruses. Although a number of substrates have been identified, the normal cellular functions and pathways affected by UBE3A are largely unknown. Previously, we showed that UBE3A associates with HERC2, NEURL4, and MAPK6/ERK3 in a high-molecular-weight complex of unknown function that we refer to as the HUN complex (HERC2, UBE3A, and NEURL4). In this study, the combination of two complementary proteomic approaches with a rigorous network analysis revealed cellular functions and pathways in which UBE3A and the HUN complex are involved. In addition to finding new UBE3A-associated proteins, such as MCM6, SUGT1, EIF3C, and ASPP2, network analysis revealed that UBE3A-associated proteins are connected to several fundamental cellular processes including translation, DNA replication, intracellular trafficking, and centrosome regulation. Our analysis suggests that UBE3A could be involved in the control and/or integration of these cellular processes, in some cases as a component of the HUN complex, and also provides evidence for crosstalk between the HUN complex and CAMKII interaction networks. This study contributes to a deeper understanding of the cellular functions of UBE3A and its potential role in pathways that may be affected in Angelman syndrome, UBE3A-associated autism spectrum disorders, and human papillomavirus-associated cancers.

Structural basis for assembly of the CBF3 kinetochore complex

Eukaryotic chromosomes contain a specialised region known as the centromere, which forms the platform for kinetochore assembly and microtubule attachment. The centromere is distinguished by the presence of nucleosomes containing the histone H3 variant, CENP-A. In budding yeast, centromere establishment begins with the recognition of a specific DNA sequence by the CBF3 complex. This in turn facilitates CENP-ACse4 nucleosome deposition and kinetochore assembly. Here, we describe a 3.6 Å single-particle cryo-EM reconstruction of the core CBF3 complex, incorporating the sequence-specific DNA-binding protein Cep3 together with regulatory subunits Ctf13 and Skp1. This provides the first structural data on Ctf13, defining it as an F-box protein of the leucine-rich-repeat family, and demonstrates how a novel F-box-mediated interaction between Ctf13 and Skp1 is responsible for initial assembly of the CBF3 complex.

Modeling the assembly order of multimeric heteroprotein complexes

Protein-protein interactions are the cornerstone of numerous biological processes. Although an increasing number of protein complex structures have been determined using experimental methods, relatively fewer studies have been performed to determine the assembly order of complexes. In addition to the insights into the molecular mechanisms of biological function provided by the structure of a complex, knowing the assembly order is important for understanding the process of complex formation. Assembly order is also practically useful for constructing subcomplexes as a step toward solving the entire complex experimentally, designing artificial protein complexes, and developing drugs that interrupt a critical step in the complex assembly. There are several experimental methods for determining the assembly order of complexes; however, these techniques are resource-intensive. Here, we present a computational method that predicts the assembly order of protein complexes by building the complex structure. The method, named Path-LzerD, uses a multimeric protein docking algorithm that assembles a protein complex structure from individual subunit structures and predicts assembly order by observing the simulated assembly process of the complex. Benchmarked on a dataset of complexes with experimental evidence of assembly order, Path-LZerD was successful in predicting the assembly pathway for the majority of the cases. Moreover, when compared with a simple approach that infers the assembly path from the buried surface area of subunits in the native complex, Path-LZerD has the strong advantage that it can be used for cases where the complex structure is not known. The path prediction accuracy decreased when starting from unbound monomers, particularly for larger complexes of five or more subunits, for which only a part of the assembly path was correctly identified. As the first method of its kind, Path-LZerD opens a new area of computational protein structure modeling and will be an indispensable approach for studying protein complexes.

Evidence for Hsp90 Co-chaperones in Regulating Hsp90 Function and Promoting Client Protein Folding

Molecular chaperones are a diverse group of highly conserved proteins that transiently interact with partially folded polypeptide chains during normal cellular processes such as protein translation, translocation, and disassembly of protein complexes. Prior to folding or after denaturation, hydrophobic residues that are normally sequestered within a folded protein are exposed to the aqueous environment and are prone to aggregation or misfolding. Multiple classes of molecular chaperones, such as Hsp70s and Hsp40s, recognize and transiently bind polypeptides with exposed hydrophobic stretches in order to prevent misfolding. Other types of chaperones, such as Hsp90, have more specialized functions in that they appear to interact with only a subset of cellular proteins. This chapter focuses on the role of Hsp90 and partner co-chaperones in promoting the folding and activation of a diverse group of proteins with critical roles in cellular signaling and function.

The Plasticity of the Hsp90 Co-chaperone System

The Hsp90 system in the eukaryotic cytosol is characterized by a cohort of co-chaperones that bind to Hsp90 and affect its function. Although progress has been made regarding the underlying biochemical mechanisms, how co-chaperones influence Hsp90 client proteins in vivo has remained elusive. By investigating the effect of 12 Hsp90 co-chaperones on the activity of different client proteins in yeast, we find that deletion of co-chaperones can have a neutral or negative effect on client activity but can also lead to more active clients. Only a few co-chaperones are active on all clients studied. Closely related clients and even point mutants can depend on different co-chaperones. These effects are direct because differences in client-co-chaperone interactions can be reconstituted in vitro. Interestingly, some co-chaperones affect client conformation in vivo. Thus, co-chaperones adapt the Hsp90 cycle to the requirements of the client proteins, ensuring optimal activation.

SGT1-HSP90 complex is required for CENP-A deposition at centromeres

The centromere plays an essential role in accurate chromosome segregation, and defects in its function lead to aneuploidy and thus cancer. The centromere-specific histone H3 variant CENP-A is proposed to be the epigenetic mark of the centromere, as active centromeres require CENP-A–containing nucleosomes to direct the recruitment of multiple kinetochore proteins. CENP-A K124 ubiquitylation, mediated by CUL4A-RBX1-COPS8 E3 ligase activity, is required for CENP-A deposition at the centromere. However, the mechanism that controls the E3 ligase activity of the CUL4A-RBX1-COPS8 complex remains obscure. We have discovered that the SGT1-HSP90 complex is required for recognition of CENP-A by COPS8. Thus, the SGT1-HSP90 complex contributes to the E3 ligase activity of the CUL4A complex that is necessary for CENP-A ubiquitylation and CENP-A deposition at the centromere.

Mechanisms of Hsp90 regulation

Heat shock protein 90 (Hsp90) is a molecular chaperone that is involved in the activation of disparate client proteins. This implicates Hsp90 in diverse biological processes that require a variety of co-ordinated regulatory mechanisms to control its activity. Perhaps the most important regulator is heat shock factor 1 (HSF1), which is primarily responsible for upregulating Hsp90 by binding heat shock elements (HSEs) within Hsp90 promoters. HSF1 is itself subject to a variety of regulatory processes and can directly respond to stress. HSF1 also interacts with a variety of transcriptional factors that help integrate biological signals, which in turn regulate Hsp90 appropriately. Because of the diverse clientele of Hsp90 a whole variety of co-chaperones also regulate its activity and some are directly responsible for delivery of client protein. Consequently, co-chaperones themselves, like Hsp90, are also subject to regulatory mechanisms such as post translational modification. This review, looks at the many different levels by which Hsp90 activity is ultimately regulated.

The crystal structure of the Sgt1-Skp1 complex: the link between Hsp90 and both SCF E3 ubiquitin ligases and kinetochores

The essential cochaperone Sgt1 recruits Hsp90 chaperone activity to a range of cellular factors including SCF E3 ubiquitin ligases and the kinetochore in eukaryotes. In these pathways Sgt1 interacts with Skp1, a small protein that heterodimerizes with proteins containing the F-box motif. We have determined the crystal structure of the interacting domains of Saccharomyces cerevisiae Sgt1 and Skp1 at 2.8 Å resolution and validated the interface in the context of the full-length proteins in solution. The BTB/POZ domain of Skp1 associates with Sgt1 via the concave surface of its TPR domain using residues that are conserved in humans. Dimerization of yeast Sgt1 occurs via an insertion that is absent from monomeric human Sgt1. We identify point mutations that disrupt dimerization and Skp1 binding in vitro and find that the interaction with Skp1 is an essential function of Sgt1 in yeast. Our data provide a structural rationale for understanding the phenotypes of temperature-sensitive Sgt1 mutants and for linking Skp1-associated proteins to Hsp90-dependent pathways.

Application of SGT1-Hsp90 chaperone complex for soluble expression of NOD1 LRR domain in E. coli

NOD1 is an intracellular sensor of innate immunity which is related to a number of inflammatory diseases. NOD1 is known to be difficult to express and purify for structural and biochemical studies. Based on the fact that Hsp90 and its cochaperone SGT1 are necessary for the stabilization and activation of NOD1 in mammals, SGT1 was chosen as a fusion partner of the leucine-rich repeat (LRR) domain of NOD1 for its soluble expression in Escherichia coli. Fusion of human SGT1 (hSGT1) to NOD1 LRR significantly enhanced the solubility, and the fusion protein was stabilized by coexpression of mouse Hsp90α. The expression level of hSGT1-NOD1 LRR was further enhanced by supplementation of rare codon tRNAs and exchange of antibiotic marker genes.

CacyBP/SIP--Structure and variety of functions

BACKGROUND

CacyBP/SIP (Calcyclin-Binding Protein and Siah-1 Interacting Protein) is a small modular protein implicated in a wide range of cellular processes. It is expressed in different tissues of mammals but homologs are also found in some lower organisms. In mammals, a high level of CacyBP/SIP is present in tumor cells and in neurons. CacyBP/SIP binds several target proteins such as members of the S100 family, components of a ubiquitin ligase complex, and cytoskeletal proteins.

SCOPE OF REVIEW

CacyBP/SIP has been shown to be involved in protein de-phosphorylation, ubiquitination, cytoskeletal dynamics, regulation of gene expression, cell proliferation, differentiation, and tumorigenesis. This review focuses on very recent reports on CacyBP/SIP structure and function in these important cellular processes.

MAJOR CONCLUSIONS

CacyBP/SIP is a multi-domain and multi-functional protein. Altered levels of CacyBP/SIP in several cancers implicate its involvement in the maintenance of cell homeostasis. Changes in CacyBP/SIP subcellular localization in neurons of AD brains suggest that this protein is strongly linked to neurodegenerative diseases. Elucidation of CacyBP/SIP structure and cellular function is leading to greater understanding of its role in normal physiology and disease pathologies.

GENERAL SIGNIFICANCE

The available results suggest that CacyBP/SIP is a key player in multiple biological processes. Detailed characterization of the physical, biochemical and biological properties of CacyBP/SIP will provide better insight into the regulation of its diverse functions in vivo, and given the association with specific diseases, will help clarify the potential of therapeutic targeting of this protein.

The oncogenic role of the cochaperone Sgt1

Sgt1/Sugt1, a cochaperone of Hsp90, is involved in several cellular activities including Cullin E3 ubiqutin ligase activity. The high level of Sgt1 expression in colorectal and gastric tumors suggests that Sgt1 is involved in tumorigenesis. Here, we report that Sgt1 is overexpressed in colon, breast and lung tumor tissues and in Ewing sarcoma and rhabdomyosarcoma xenografts. We also found that Sgt1 heterozygous knockout resulted in suppressed Hras-mediated transformation in vitro and tumor formation in p53^−/− mouse embryonic fibroblast cells and significantly increased survival of p53^−/− mice. Moreover, depletion of Sgt1 inhibited the growth of Ewing sarcoma and rhabdomyosarcoma cells and destabilized EWS-FLI1 and PAX3-FOXO1 oncogenic fusion proteins, respectively, which are required for cellular growth. Our results suggest that Sgt1 contributes to cancer development by stabilizing oncoproteins and that Sgt1 is a potential therapeutic target.

TaRAR1 and TaSGT1 associate with TaHsp90 to function in bread wheat (Triticum aestivum L.) seedling growth and stripe rust resistance

RAR1 and SGT1 are important co-chaperones of Hsp90. We previously showed that TaHsp90.1 is required for wheat seedling growth, and that TaHsp90.2 and TaHsp90.3 are essential for resistance (R) gene mediated resistance to stripe rust fungus. Here, we report the characterization of TaRAR1 and TaSGT1 genes in bread wheat. TaRAR1 and TaSGT1 each had three homoeologs, which were located on wheat groups 2 and 3 chromosomes, respectively. Strong inhibition of seedling growth was observed after silencing TaSGT1 but not TaRAR1. In contrast, decreasing the expression of TaRAR1 or TaSGT1 could all compromise R gene mediated resistance to stripe rust fungus infection. Protein-protein interactions were found among TaRAR1, TaSGT1 and TaHsp90. The N-terminus of TaHsp90, the CHORD-I and CHORD-II domains of TaRAR1 and the CS domain of TaSGT1 may be instrumental for the interactions among the three proteins. Based on this work and our previous study on TaHsp90, we speculate that the TaSGT1-TaHsp90.1 interaction is important for maintaining bread wheat seedling growth. The TaRAR1-TaSGT1-TaHsp90.2 and TaRAR1-TaSGT1-TaHsp90.3 interactions are involved in controlling the resistance to stripe rust disease. The new information obtained here should aid further functional investigations of TaRAR1-TaSGT1-TaHsp90 complexes in regulating bread wheat growth and disease resistance.

Nematode Sgt1-homologue D1054.3 binds open and closed conformations of Hsp90 via distinct binding sites

Heat shock protein 90 (Hsp90) is a highly conserved ATP-driven machine involved in client protein maturation, folding, and activation. The chaperone is supported by a set of cochaperones that confer client specificities. One of those proteins is the suppressor of G2 allele of skp1 (Sgt1), which participates together with Hsp90 in the immune responses of plants. Sgt1 consists of three domains: a TPR-, CS-, and SGS-domain, conserved in plants, yeast, and humans. The TPR-domain though is lacking in nematodes and insects. We observe that the Caenorhabditis elegans Sgt1 homologue D1054.3 binds to Hsp90 in the absence of nucleotides but much stronger in the presence of ATP and ATPγS. The latter binding mode is similar to p23, another CS-domain containing Hsp90 cofactor, even though binding is not observable for p23 in the absence of nucleotides. We use point mutations in Hsp90, which accumulate different conformations in the ATPase cycle, to differentiate between binding to open and closed Hsp90 conformations. These data support a strong contribution of the Hsp90 conformation to Sgt1 binding and highlight the ability of this cofactor to interact with all known Hsp90 conformations albeit with different affinities.

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

The Hv-SGT1 gene from Haynaldia villosa contributes to resistances towards both biotrophic and hemi-biotrophic pathogens in common wheat (Triticum aestivum L.)

The SGT1 protein is essential for R protein-mediated and PAMPs-triggered resistance in many plant species. Here we reported the isolation and characterization of the Hv-SGT1 gene from Haynaldiavillosa (2n = 14, VV). Analysis of the subcellular location of Hv-SGT1 by transient expression of a fusion to GFP indicated its presence in the cytoplasm and nucleus. Levels of Hv-SGT1 transcripts were increased by inoculation with either the biotrophic pathogen Blumeriagraminis DC. f. Sp. tritici (Bgt) or the hemi-biotrophic pathogen Fusariumgraminearum (Fg). Levels of Hv-SGT1 showed substantial increase following treatment with H₂O₂ and methyl jasmonate (MeJA), only slightly induced following exposure to ethephon or abscisic acid, but not changed following exposure to salicylic acid. The demonstration that silencing of Hv-SGT1 substantially reduced resistance to Bgt indicated that Hv-SGT1 was an essential component of disease resistance in H. villosa. The over-expression of Hv-SGT1 in Yangmai 158 enhanced resistance to powdery mildew, and this correlated with increased levels of whole-cell reactive oxygen intermediates at the sites of penetration by the pathogens. Compared with wild-type plants, the expression levels of genes related to the H₂O₂ and JA signaling pathways were lower in the Hv-SGT1 silenced plants and higher in the Hv-SGT1 over-expressing plants. Therefore, the involvement of Hv-SGT1 in H₂O₂ production correlates with the hypersensitive response and jasmonic acid signaling. Our novel demonstration that wheat with over-expressed Hv-SGT1 showed enhanced resistance to both powdery mildew and FHB suggests that it could served as a transgenic genetic resource in wheat breeding for multiple disease resistance.

OsCYP2, a chaperone involved in degradation of auxin-responsive proteins, plays crucial roles in rice lateral root initiation

Auxin plays a pivotal role in many facets of plant development. It acts by inducing the interaction between auxin-responsive [auxin (AUX)/indole-3-acetic acid (IAA)] proteins and the ubiquitin protein ligase SCF(TIR) to promote the degradation of the AUX/IAA proteins. Other cofactors and chaperones that participate in auxin signaling remain to be identified. Here, we characterized rice (Oryza sativa) plants with mutations in a cyclophilin gene (OsCYP2). cyp2 mutants showed defects in auxin responses and exhibited a variety of auxin-related growth defects in the root. In cyp2 mutants, lateral root initiation was blocked after nuclear migration but before the first anticlinal division of the pericycle cell. Yeast two-hybrid and in vitro pull-down results revealed an association between OsCYP2 and the co-chaperone Suppressor of G2 allele of skp1 (OsSGT1). Luciferase complementation imaging assays further supported this interaction. Similar to previous findings in an Arabidopsis thaliana SGT1 mutant (atsgt1b), degradation of AUX/IAA proteins was retarded in cyp2 mutants treated with exogenous 1-naphthylacetic acid. Our results suggest that OsCYP2 participates in auxin signal transduction by interacting with OsSGT1.

Fungal Hsp90: a biological transistor that tunes cellular outputs to thermal inputs

Heat shock protein 90 (HSP90) is an essential, abundant and ubiquitous eukaryotic chaperone that has crucial roles in protein folding and modulates the activities of key regulators. The fungal Hsp90 interactome, which includes numerous client proteins such as receptors, protein kinases and transcription factors, displays a surprisingly high degree of plasticity that depends on environmental conditions. Furthermore, although fungal Hsp90 levels increase following environmental challenges, Hsp90 activity is tightly controlled via post-translational regulation and an autoregulatory loop involving heat shock transcription factor 1 (Hsf1). In this Review, we discuss the roles and regulation of fungal Hsp90. We propose that Hsp90 acts as a biological transistor that modulates the activity of fungal signalling networks in response to environmental cues via this Hsf1-Hsp90 autoregulatory loop.

Plk1 phosphorylates Sgt1 at the kinetochores to promote timely kinetochore-microtubule attachment

Accurate chromosome segregation during cell division maintains genomic integrity and requires the proper establishment of kinetochore-microtubule attachment in mitosis. As a key regulator of mitosis, Polo-like kinase 1 (Plk1) is essential for this attachment process, but the molecular mechanism remains elusive. Here we identify Sgt1, a cochaperone for Hsp90, as a novel Plk1 substrate during mitosis. We show that Sgt1 dynamically localizes at the kinetochores, which lack microtubule attachments during prometaphase. Plk1 is required for the kinetochore localization of Sgt1 and phosphorylates serine 331 of Sgt1 at the kinetochores. This phosphorylation event enhances the association of the Hsp90-Sgt1 chaperone with the MIS12 complex to stabilize this complex at the kinetochores and thus coordinates the recruitment of the NDC80 complex to form efficient microtubule-binding sites. Disruption of Sgt1 phosphorylation reduces the MIS12 and NDC80 complexes at the kinetochores, impairs stable microtubule attachment, and eventually results in chromosome misalignment to delay the anaphase onset. Our results demonstrate a mechanism for Plk1 in promoting kinetochore-microtubule attachment to ensure chromosome stability.

The Hsp90 co-chaperone Sgt1 governs Candida albicans morphogenesis and drug resistance

The molecular chaperone Hsp90 orchestrates regulatory circuitry governing fungal morphogenesis, biofilm development, drug resistance, and virulence. Hsp90 functions in concert with co-chaperones to regulate stability and activation of client proteins, many of which are signal transducers. Here, we characterize the first Hsp90 co-chaperone in the leading human fungal pathogen, Candida albicans. We demonstrate that Sgt1 physically interacts with Hsp90, and that it governs C. albicans morphogenesis and drug resistance. Genetic depletion of Sgt1 phenocopies depletion of Hsp90, inducing yeast to filament morphogenesis and invasive growth. Sgt1 governs these traits by bridging two morphogenetic regulators: Hsp90 and the adenylyl cyclase of the cAMP-PKA signaling cascade, Cyr1. Sgt1 physically interacts with Cyr1, and depletion of either Sgt1 or Hsp90 activates cAMP-PKA signaling, revealing the elusive link between Hsp90 and the PKA signaling cascade. Sgt1 also mediates tolerance and resistance to the two most widely deployed classes of antifungal drugs, azoles and echinocandins. Depletion of Sgt1 abrogates basal tolerance and acquired resistance to azoles, which target the cell membrane. Depletion of Sgt1 also abrogates tolerance and resistance to echinocandins, which target the cell wall, and renders echinocandins fungicidal. Though Sgt1 and Hsp90 have a conserved impact on drug resistance, the underlying mechanisms are distinct. Depletion of Hsp90 destabilizes the client protein calcineurin, thereby blocking crucial responses to drug-induced stress; in contrast, depletion of Sgt1 does not destabilize calcineurin, but blocks calcineurin activation in response to drug-induced stress. Sgt1 influences not only morphogenesis and drug resistance, but also virulence, as genetic depletion of C. albicans Sgt1 leads to reduced kidney fungal burden in a murine model of systemic infection. Thus, our characterization of the first Hsp90 co-chaperone in a fungal pathogen establishes C. albicans Sgt1 as a global regulator of morphogenesis and drug resistance, providing a new target for treatment of life-threatening fungal infections.

Several components of SKP1/Cullin/F-box E3 ubiquitin ligase complex and associated factors play a role in Agrobacterium-mediated plant transformation

• Successful genetic transformation of plants by Agrobacterium tumefaciens requires the import of bacterial T-DNA and virulence proteins into the plant cell that eventually form a complex (T-complex). The essential components of the T-complex include the single stranded T-DNA, bacterial virulence proteins (VirD2, VirE2, VirE3 and VirF) and associated host proteins that facilitate the transfer and integration of T-DNA. The removal of the proteins from the T-complex is likely achieved by targeted proteolysis mediated by VirF and the plant ubiquitin proteasome complex. • We evaluated the involvement of the host SKP1/culin/F-box (SCF)-E3 ligase complex and its role in plant transformation. Gene silencing, mutant screening and gene expression studies suggested that the Arabidopsis homologs of yeast SKP1 (suppressor of kinetochore protein 1) protein, ASK1 and ASK2, are required for Agrobacterium-mediated plant transformation. • We identified the role for SGT1b (suppressor of the G2 allele of SKP1), an accessory protein that associates with SCF-complex, in plant transformation. We also report the differential expression of many genes that encode F-box motif containing SKP1-interacting proteins (SKIP) upon Agrobacterium infection. • We speculate that these SKIP genes could encode the plant specific F-box proteins that target the T-complex associated proteins for polyubiquitination and subsequent degradation by the 26S proteasome.

Identification of an Hsp90 mutation that selectively disrupts cAMP/PKA signaling in Saccharomyces cerevisiae

The molecular chaperone Hsp90 cooperates with multiple cochaperone proteins as it promotes the folding and activation of diverse client proteins. Some cochaperones regulate the ATPase activity of Hsp90, while others appear to promote Hsp90 interaction with specific types of client proteins. Through its interaction with the adenylate cyclase Cyr1, the Sgt1 cochaperone modulates the activity of the cAMP pathway in Saccharomyces cerevisiae. A specific mutation in yeast Hsp90, hsc82-W296A, or a mutation in Sgt1, sgt1-K360E, resulted in altered transcription patterns genetically linked to the cAMP pathway. Hsp90 interacted with Cyr1 in vivo and the hsc82-W296A mutation resulted in reduced accumulation of Cyr1. Hsp90-Sgt1 interaction was altered by either the hsc82-W296A or sgt1-K360E mutation, suggesting defective Hsp90-Sgt1 cooperation leads to reduced Cyr1 activity. Microarray analysis of hsc82-W296A cells indicated that over 80 % of all transcriptional changes in this strain may be attributed to altered cAMP signaling. This suggests that a majority of the cellular defects observed in hsc82-W296A cells are due to altered interaction with one specific essential cochaperone, Sgt1 and one essential client, Cyr1. Together our results indicate that specific interaction of Hsp90 and Sgt1 with Cyr1 plays a key role in regulating gene expression, including genes involved in polarized morphogenesis.

The 'active life' of Hsp90 complexes

Hsp90 forms a variety of complexes differing both in clientele and co-chaperones. Central to the role of co-chaperones in the formation of Hsp90 complexes is the delivery of client proteins and the regulation of the ATPase activity of Hsp90. Determining the mechanisms by which co-chaperones regulate Hsp90 is essential in understanding the assembly of these complexes and the activation and maturation of Hsp90's clientele. Mechanistically, co-chaperones alter the kinetics of the ATP-coupled conformational changes of Hsp90. The structural changes leading to the formation of a catalytically active unit involve all regions of the Hsp90 dimer. Their complexity has allowed different orthologues of Hsp90 to evolve kinetically in slightly different ways. The interaction of the cytosolic Hsp90 with a variety of co-chaperones lends itself to a complex set of different regulatory mechanisms that modulate Hsp90's conformation and ATPase activity. It also appears that the conformational switches of Hsp90 are not necessarily coupled under all circumstances. Here, I described different co-chaperone complexes and then discuss in detail the mechanisms and role that specific co-chaperones play in this. I will also discuss emerging evidence that post-translational modifications also affect the ATPase activity of Hsp90, and thus complex formation. Finally, I will present evidence showing how Hsp90's active site, although being highly conserved, can be altered to show resistance to drug binding, but still maintain ATP binding and ATPase activity. Such changes are therefore unlikely to significantly alter Hsp90's interactions with client proteins and co-chaperones. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Control of the function of the transcription and repair factor TFIIH by the action of the cochaperone Ydj1

Yeast rad3-102, a mutant of the TFIIH complex involved in nucleotide excision repair (NER) and transcription, can perform NER initial steps but not late steps of postincision gap filing. Because removal of early-acting NER proteins prevents rad3-102 deleterious action, we used this feature to explore if chaperones act in early NER. We found that the cochaperone Ydj1 is required for NER and that Ydj1 guarantees TFIIH stoichiometry. Importantly, in the absence of Ydj1, the roles of TFIIH in transcription and transactivation, the ability to activate transcription by nuclear receptors in response to hormones, are strongly impaired. We propose that TFIIH constitutes a multitarget complex for Ydj1, as six of the seven TFIIH core components contain biologically relevant Ydj1- binding motives. Our results provide evidence for a role of chaperones in NER and transcription, with implications in cancer and TFIIH-associated syndromes.

Bub1-mediated adaptation of the spindle checkpoint

During cell division, the spindle checkpoint ensures accurate chromosome segregation by monitoring the kinetochore–microtubule interaction and delaying the onset of anaphase until each pair of sister chromosomes is properly attached to microtubules. The spindle checkpoint is deactivated as chromosomes start moving toward the spindles in anaphase, but the mechanisms by which this deactivation and adaptation to prolonged mitotic arrest occur remain obscure. Our results strongly suggest that Cdc28-mediated phosphorylation of Bub1 at T566 plays an important role for the degradation of Bub1 in anaphase, and the phosphorylation is required for adaptation of the spindle checkpoint to prolonged mitotic arrest.

The spindle checkpoint protects cells from aneuploidy by monitoring the status of the kinetochore-microtubule attachment. Defects in this checkpoint pathway and in kinetochore-microtubule attachment can cause substantial aneuploidy in cells. The duration of the mitotic arrest induced by the spindle checkpoint is not indefinite: cells eventually exit from mitosis and re-enter interphase. Because continued activation of the spindle checkpoint is lethal, adaptation to the spindle checkpoint arrest is essential so that cells have a chance for survival as opposed to certain death. However, adaptation of the spindle checkpoint has a flip side—adapted cells could have an increased chance of aneuploidy due to premature mitotic exit. Thus, it is essential that this mechanism be regulated appropriately. Despite the importance of understanding the adaptation of the spindle checkpoint, little is known to date about this mechanism. We found that Cdc28-mediated phosphorylation of Bub1 at T566 plays an important role for adaptation of the spindle checkpoint, a finding providing the molecular insight on how adaptation to prolonged mitotic arrest induced by the spindle checkpoint occurs.

Sgt1 dimerization is negatively regulated by protein kinase CK2-mediated phosphorylation at Ser361

The kinetochore, which consists of centromere DNA and structural proteins, is essential for proper chromosome segregation in eukaryotes. In budding yeast, Sgt1 and Hsp90 are required for the binding of Skp1 to Ctf13 (a component of the core kinetochore complex CBF3) and therefore for the assembly of CBF3. We have previously shown that Sgt1 dimerization is important for this kinetochore assembly mechanism. In this study, we report that protein kinase CK2 phosphorylates Ser(361) on Sgt1, and this phosphorylation inhibits Sgt1 dimerization.

Sgt1, a co-chaperone of Hsp90 stabilizes Polo and is required for centrosome organization

Sgt1 was described previously in yeast and humans to be a Hsp90 co-chaperone and required for kinetochore assembly. We have identified a mutant allele of Sgt1 in Drosophila and characterized its function. Mutations in sgt1 do not affect overall kinetochore assembly or spindle assembly checkpoint. sgt1 mutant cells enter less frequently into mitosis and arrest in a prometaphase-like state. Mutations in sgt1 severely compromise the organization and function of the mitotic apparatus. In these cells, centrioles replicate but centrosomes fail to mature, and pericentriolar material components do not localize normally resulting in highly abnormal spindles. Interestingly, a similar phenotype was described previously in Hsp90 mutant cells and correlated with a decrease in Polo protein levels. In sgt1 mutant neuroblasts, we also observe a decrease in overall levels of Polo. Overexpression of the kinase results in a substantial rescue of the centrosome defects; most cells form normal bipolar spindles and progress through mitosis normally. Taken together, these findings suggest that Sgt1 is involved in the stabilization of Polo allowing normal centrosome maturation, entry and progression though mitosis.

Altered expression of cytosolic/nuclear HSC70-1 molecular chaperone affects development and abiotic stress tolerance in Arabidopsis thaliana

Molecular chaperones of the heat shock cognate 70 kDa (HSC70) family are highly conserved in all living organisms and assist nascent protein folding in normal physiological conditions as well as in biotic and abiotic stress conditions. In the absence of specific inhibitors or viable knockout mutants, cytosolic/nuclear HSC70-1 overexpression (OE) and mutants in the HSC70 co-chaperone SGT1 (suppressor of G(2)/M allele of skp1) were used as genetic tools to identify HSC70/SGT1 functions in Arabidopsis development and abiotic stress responses. HSC70-1 OE caused a reduction in root and shoot meristem activities, thus explaining the dwarfism of those plants. In addition, HSC70-1 OE did not impair auxin-dependent phenotypes, suggesting that SGT1 functions previously identified in auxin signalling are HSC70 independent. While responses to abiotic stimuli such as UV-C exposure, phosphate starvation, or seedling de-etiolation were not perturbed by HSC70-1 OE, it specifically conferred gamma-ray hypersensitivity and tolerance to salt, cadmium (Cd), and arsenic (As). Cd and As perception was not perturbed, but plants overexpressing HSC70-1 accumulated less Cd, thus providing a possible molecular explanation for their tolerance phenotype. In summary, genetic evidence is provided for HSC70-1 involvement in a limited set of physiological processes, illustrating the essential and yet specific functions of this chaperone in development and abiotic stress responses in Arabidopsis.

The HSP90-SGT1 chaperone complex for NLR immune sensors

The nucleotide-binding domain and leucine-rich repeat-containing (NLR) proteins function as immune sensors in both plants and animals. NLR proteins recognize, directly or indirectly, pathogen-derived molecules and trigger immune responses. To function as a sensor, NLR proteins must be correctly folded and maintained in a recognition-competent state in the appropriate cellular location. Upon pathogen recognition, conformational changes and/or translocation of the sensors would activate the downstream immunity signaling pathways. Misfolded or used sensors are a threat to the cell and must be immediately inactivated and discarded to avoid inappropriate activation of downstream pathways. Such maintenance of NLR-type sensors requires the SGT1-HSP90 pair, a chaperone complex that is structurally and functionally conserved in eukaryotes. Deciphering how the chaperone machinery works would facilitate an understanding of the mechanisms of pathogen recognition and signal transduction by NLR proteins in both plants and animals.

Sgt1 dimerization is required for yeast kinetochore assembly

The kinetochore, which consists of DNA sequence elements and structural proteins, is essential for high-fidelity chromosome transmission during cell division. In budding yeast, Sgt1 and Hsp90 help assemble the core kinetochore complex CBF3 by activating the CBF3 components Skp1 and Ctf13. In this study, we show that Sgt1 forms homodimers by performing in vitro and in vivo immunoprecipitation and analytical ultracentrifugation analyses. Analyses of the dimerization of Sgt1 deletion proteins showed that the Skp1-binding domain (amino acids 1-211) contains the Sgt1 homodimerization domain. Also, the Sgt1 mutant proteins that were unable to dimerize also did not bind Skp1, suggesting that Sgt1 dimerization is important for Sgt1-Skp1 binding. Restoring dimerization activity of a dimerization-deficient sgt1 mutant (sgt1-L31P) by using the CENP-B (centromere protein-B) dimerization domain suppressed the temperature sensitivity, the benomyl sensitivity, and the chromosome missegregation phenotype of sgt1-L31P. These results strongly suggest that Sgt1 dimerization is required for kinetochore assembly.

Structural and functional coupling of Hsp90- and Sgt1-centred multi-protein complexes

Sgt1 is an adaptor protein implicated in a variety of processes, including formation of the kinetochore complex in yeast, and regulation of innate immunity systems in plants and animals. Sgt1 has been found to associate with SCF E3 ubiquitin ligases, the CBF3 kinetochore complex, plant R proteins and related animal Nod-like receptors, and with the Hsp90 molecular chaperone. We have determined the crystal structure of the core Hsp90-Sgt1 complex, revealing a distinct site of interaction on the Hsp90 N-terminal domain. Using the structure, we developed mutations in Sgt1 interfacial residues, which specifically abrogate interaction with Hsp90, and disrupt Sgt1-dependent functions in vivo, in plants and yeast. We show that Sgt1 bridges the Hsp90 molecular chaperone system to the substrate-specific arm of SCF ubiquitin ligase complexes, suggesting a role in SCF assembly and regulation, and providing multiple complementary routes for ubiquitination of Hsp90 client proteins.

Structural and functional analysis of SGT1-HSP90 core complex required for innate immunity in plants

SGT1 (Suppressor of G2 allele of skp1), a co-chaperone of HSP90 (Heat-shock protein 90), is required for innate immunity in plants and animals. Unveiling the cross talks between SGT1 and other co-chaperones such as p23, AHA1 (Activator of HSP90 ATPase 1) or RAR1 (Required for Mla12 resistance) is an important step towards understanding the HSP90 machinery. Nuclear magnetic resonance spectroscopy and mutational analyses of HSP90 revealed the nature of its binding with the CS domain of SGT1. Although CS is structurally similar to p23, these domains were found to non-competitively bind to various regions of HSP90; yet, unexpectedly, full-length SGT1 could displace p23 from HSP90. RAR1 partly shares the same binding site with HSP90 as the CS domain, whereas AHA1 does not. This analysis allowed us to build a structural model of the HSP90-SGT1 complex and to obtain a compensatory mutant pair between both partners that is able to restore virus resistance in vivo through Rx (Resistance to potato virus X) immune sensor stabilization.

Schizosaccharomyces pombe Hsp90/Git10 is required for glucose/cAMP signaling

The fission yeast Schizosaccharomyces pombe senses environmental glucose through a cAMP-signaling pathway. Elevated cAMP levels activate protein kinase A (PKA) to inhibit transcription of genes involved in sexual development and gluconeogenesis, including the fbp1(+) gene, which encodes fructose-1,6-bisphosphatase. Glucose-mediated activation of PKA requires the function of nine glucose-insensitive transcription (git) genes, encoding adenylate cyclase, the PKA catalytic subunit, and seven "upstream" proteins required for glucose-triggered adenylate cyclase activation. We describe the cloning and characterization of the git10(+) gene, which is identical to swo1(+) and encodes the S. pombe Hsp90 chaperone protein. Glucose repression of fbp1(+) transcription is impaired by both git10(-) and swo1(-) mutant alleles of the hsp90(+) gene, as well as by chemical inhibition of Hsp90 activity and temperature stress to wild-type cells. Unlike the swo1(-) mutant alleles, the git10-201 allele supports cell growth at 37 degrees , while severely reducing glucose repression of an fbp1-lacZ reporter, suggesting a separation-of-function defect. Sequence analyses of three swo1(-) alleles and the one git10(-) allele indicate that swo1(-) mutations alter core functional domains of Hsp90, while the git10(-) mutation affects the Hsp90 central domain involved in client protein binding. These results suggest that Hsp90 plays a specific role in the S. pombe glucose/cAMP pathway.

Staying in the fold: The SGT1/chaperone machinery in maintenance and evolution of leucine-rich repeat proteins

The conserved eukaryotic protein SGT1 (suppressor of G(2) allele of skp1) participates in diverse physiological processes such as cell cycle progression in yeast, plant immunity against pathogens and plant hormone signalling. Recent genetic and biochemical studies suggest that SGT1 functions as a novel co-chaperone for cytosolic/nuclear HSP90 and HSP70 molecular chaperones in the folding and maturation of substrate proteins. Since proteins containing the leucine-rich repeat (LRR) protein-protein interaction motif are overrepresented in SGT1-dependent phenomena, we consider whether LRR-containing proteins are preferential substrates of an SGT1/HSP70/HSP90 complex. Such a chaperone organisation is reminiscent of the HOP/HSP70/HSP90 machinery which controls maturation and activation of glucocorticoid receptors in animals. Drawing on this parallel, we discuss the possible contribution of an SGT1-chaperone complex in the folding and maturation of LRR-containing proteins and its evolutionary consequences for the emergence of novel LRR interaction surfaces.

The Hsp90 molecular chaperone: an open and shut case for treatment

The molecular chaperone Hsp90 (90 kDa heat-shock protein) is a remarkably versatile protein involved in the stress response and in normal homoeostatic control mechanisms. It interacts with 'client proteins', including protein kinases, transcription factors and others, and either facilitates their stabilization and activation or directs them for proteasomal degradation. By this means, Hsp90 displays a multifaceted ability to influence signal transduction, chromatin remodelling and epigenetic regulation, development and morphological evolution. Hsp90 operates as a dimer in a conformational cycle driven by ATP binding and hydrolysis at the N-terminus. The cycle is also regulated by a group of co-chaperones and accessory proteins. Here we review the biology of the Hsp90 molecular chaperone, emphasizing recent progress in our understanding of structure-function relationships and the identification of new client proteins. In addition we describe the exciting progress that has been made in the development of Hsp90 inhibitors, which are now showing promise in the clinic for cancer treatment. We also identify the gaps in our current understanding and highlight important topics for future research.

Diverse cellular functions of the Hsp90 molecular chaperone uncovered using systems approaches

A comprehensive understanding of the cellular functions of the Hsp90 molecular chaperone has remained elusive. Although Hsp90 is essential, highly abundant under normal conditions, and further induced by environmental stress, only a limited number of Hsp90 "clients" have been identified. To define Hsp90 function, a panel of genome-wide chemical-genetic screens in Saccharomyces cerevisiae were combined with bioinformatic analyses. This approach identified several unanticipated functions of Hsp90 under normal conditions and in response to stress. Under normal growth conditions, Hsp90 plays a major role in various aspects of the secretory pathway and cellular transport; during environmental stress, Hsp90 is required for the cell cycle, meiosis, and cytokinesis. Importantly, biochemical and cell biological analyses validated several of these Hsp90-dependent functions, highlighting the potential of our integrated global approach to uncover chaperone functions in the cell.

Ybp2 associates with the central kinetochore of Saccharomyces cerevisiae and mediates proper mitotic progression

The spindle checkpoint ensures the accurate segregation of chromosomes by monitoring the status of kinetochore attachment to microtubules. Simultaneous mutations in one of several kinetochore and cohesion genes and a spindle checkpoint gene cause a synthetic-lethal or synthetic-sick phenotype. A synthetic genetic array (SGA) analysis using a mad2Delta query mutant strain of yeast identified YBP2, a gene whose product shares sequence similarity with the product of YBP1, which is required for H(2)O(2)-induced oxidation of the transcription factor Yap1. ybp2Delta was sensitive to benomyl and accumulated at the mitotic stage of the cell cycle. Ybp2 physically associates with proteins of the COMA complex (Ctf19, Okp1, Mcm21, and Ame1) and 3 components of the Ndc80 complex (Ndc80, Nuf2, and Spc25 but not Spc24) in the central kinetochore and with Cse4 (the centromeric histone and CENP-A homolog). Chromatin-immunoprecipitation analyses revealed that Ybp2 associates specifically with CEN DNA. Furthermore, ybp2Delta showed synthetic-sick interactions with mutants of the genes that encode the COMA complex components. Ybp2 seems to be part of a macromolecular kinetochore complex and appears to contribute to the proper associations among the central kinetochore subcomplexes and the kinetochore-specific nucleosome.

Interaction between SGT1 and cytosolic/nuclear HSC70 chaperones regulates Arabidopsis immune responses

The conserved eukaryotic protein SGT1 (for Suppressor of G2 allele of skp1) has characteristics of an HSP90 (for heat shock protein 90 kD) cochaperone and in plants regulates hormone responses and Resistance gene-triggered immunity. We affinity-purified SGT1-interacting proteins from Arabidopsis thaliana leaf extracts and identified by mass spectrometry cytosolic heat shock cognate 70 (HSC70) chaperones as the major stable SGT1 interactors. Arabidopsis SGT1a and SGT1b proteins associate with HSC70 in vivo and distribute with HSC70 in the cytosol and nucleus. An intact C-terminal SGT1-specific (SGS) domain that is required for all known SGT1b functions in immunity and development is needed for HSC70 interaction and for the nuclear accumulation of SGT1b. Interaction assays of transiently expressed proteins or their domains in Nicotiana benthamiana point to a role of SGT1 as a HSC70 cofactor. Expression of two HSC70 isoforms is upregulated by pathogen challenge, and while loss of function of individual cytosolic HSC70 genes has no defense phenotype, HSC70-1 overexpression disables resistance to virulent and avirulent pathogens. Moreover, mutations in SGT1b lead to a similar degree of heat shock tolerance as deregulation of HSC70-1. We conclude that an HSC70-SGT1 chaperone complex is important for multiple plant environmental responses and that the evolutionarily conserved SGS domain of SGT1 is a key determinant of the HSC70-SGT1 association.

Structural and functional analysis of SGT1 reveals that its interaction with HSP90 is required for the accumulation of Rx, an R protein involved in plant immunity

SGT1 (for suppressor of G2 allele of skp1) and RAR1 (for required for Mla12 resistance) are highly conserved eukaryotic proteins that interact with the molecular chaperone HSP90 (for heat shock protein90). In plants, SGT1, RAR1, and HSP90 are essential for disease resistance triggered by a number of resistance (R) proteins. Here, we present structural and functional characterization of plant SGT1 proteins. Random mutagenesis of Arabidopsis thaliana SGT1b revealed that its CS (for CHORD-SGT1) and SGS (for SGT1 specific) domains are essential for disease resistance. NMR-based interaction surface mapping and mutational analyses of the CS domain showed that the CHORD II domain of RAR1 and the N-terminal domain of HSP90 interact with opposite sides of the CS domain. Functional analysis of the CS mutations indicated that the interaction between SGT1 and HSP90 is required for the accumulation of Rx, a potato (Solanum tuberosum) R protein. Biochemical reconstitution experiments suggest that RAR1 may function to enhance the SGT1-HSP90 interaction by promoting ternary complex formation.

Genome-wide fitness test and mechanism-of-action studies of inhibitory compounds in Candida albicans

Candida albicans is a prevalent fungal pathogen amongst the immunocompromised population, causing both superficial and life-threatening infections. Since C. albicans is diploid, classical transmission genetics can not be performed to study specific aspects of its biology and pathogenesis. Here, we exploit the diploid status of C. albicans by constructing a library of 2,868 heterozygous deletion mutants and screening this collection using 35 known or novel compounds to survey chemically induced haploinsufficiency in the pathogen. In this reverse genetic assay termed the fitness test, genes related to the mechanism of action of the probe compounds are clearly identified, supporting their functional roles and genetic interactions. In this report, chemical-genetic relationships are provided for multiple FDA-approved antifungal drugs (fluconazole, voriconazole, caspofungin, 5-fluorocytosine, and amphotericin B) as well as additional compounds targeting ergosterol, fatty acid and sphingolipid biosynthesis, microtubules, actin, secretion, rRNA processing, translation, glycosylation, and protein folding mechanisms. We also demonstrate how chemically induced haploinsufficiency profiles can be used to identify the mechanism of action of novel antifungal agents, thereby illustrating the potential utility of this approach to antifungal drug discovery.