The Cryptococcus neoformans Rho-GDP dissociation inhibitor mediates intracellular survival and virulence

Impact of Lysine Succinylation on the Biology of Fungi

Post-translational modifications (PTMs) play a crucial role in protein functionality and the control of various cellular processes and secondary metabolites (SMs) in fungi. Lysine succinylation (Ksuc) is an emerging protein PTM characterized by the addition of a succinyl group to a lysine residue, which induces substantial alteration in the chemical and structural properties of the affected protein. This chemical alteration is reversible, dynamic in nature, and evolutionarily conserved. Recent investigations of numerous proteins that undergo significant succinylation have underscored the potential significance of Ksuc in various biological processes, encompassing normal physiological functions and the development of certain pathological processes and metabolites. This review aims to elucidate the molecular mechanisms underlying Ksuc and its diverse functions in fungi. Both conventional investigation techniques and predictive tools for identifying Ksuc sites were also considered. A more profound comprehension of Ksuc and its impact on the biology of fungi have the potential to unveil new insights into post-translational modification and may pave the way for innovative approaches that can be applied across various clinical contexts in the management of mycotoxins.

Microglia are not protective against cryptococcal meningitis

Microglia provide protection against a range of brain infections including bacteria, viruses and parasites, but how these glial cells respond to fungal brain infections is poorly understood. We investigated the role of microglia in the context of cryptococcal meningitis, the most common cause of fungal meningitis in humans. Using a series of transgenic- and chemical-based microglia depletion methods we found that, contrary to their protective role during other infections, loss of microglia did not affect control of Cryptococcus neoformans brain infection which was replicated with several fungal strains. At early time points post-infection, we found that microglia depletion lowered fungal brain burdens, which was related to intracellular residence of C. neoformans within microglia. Further examination of extracellular and intracellular fungal populations revealed that C. neoformans residing in microglia were protected from copper starvation, whereas extracellular yeast upregulated copper transporter CTR4. However, the degree of copper starvation did not equate to fungal survival or abundance of metals within different intracellular niches. Taken together, these data show how tissue-resident myeloid cells may influence fungal phenotype in the brain but do not provide protection against this infection, and instead may act as an early infection reservoir.

Phospholipase B Is Critical for Cryptococcus neoformans Survival in the Central Nervous System

Cryptococcus neoformans (Cn) is an opportunistic, encapsulated, yeast-like fungus that causes severe meningoencephalitis, especially in countries with high HIV prevalence. In addition to its well-known polysaccharide capsule, Cn has other virulence factors such as phospholipases, a heterogeneous group of enzymes that hydrolyze ester linkages in glycerophospholipids. Phospholipase B (PLB1) has been demonstrated to play a key role in Cn pathogenicity. In this study, we used a PLB1 mutant (plb1) and its reconstituted strain (Rec1) to assess the importance of this enzyme on Cn brain infection in vivo and in vitro. Mice infected with the plb1 strain survive significantly longer, have lower peripheral and central nervous system (CNS) fungal loads, and have fewer and smaller cryptococcomas or biofilm-like brain lesions compared to H99- and Rec1-infected animals. PLB1 causes extensive brain tissue damage and changes microglia morphology during cryptococcal disease, observations which can have important implications in patients with altered mental status or dementia as these manifestations are related to poorer survival outcomes. plb1 cryptococci are significantly more phagocytosed and killed by NR-9460 microglia-like cells. plb1 cells have altered capsular polysaccharide biophysical properties which impair their ability to stimulate glial cell responses or morphological changes. Here, we provide significant evidence demonstrating that Cn PLB1 is an important virulence factor for fungal colonization of and survival in the CNS as well as in the progression of cryptococcal meningoencephalitis. These findings may potentially help fill in a gap of knowledge in our understanding of cerebral cryptococcosis and provide novel research avenues in Cn pathogenesis. IMPORTANCE Cryptococcal meningoencephalitis (CME) is a serious disease caused by infection by the neurotropic fungal pathogen Cryptococcus neoformans. Due to the increasing number of cases in HIV-infected individuals, as well as the limited therapies available, investigation into potential targets for new therapeutics has become critical. Phospholipase B is an enzyme synthesized by Cn that confers virulence to the fungus through capsular enlargement, immunomodulation, and intracellular replication. In this study, we examined the properties of PLB1 by comparing infection of a Cn PLB1 mutant strain with both the wild-type and a PLB1-reconstituted strain. We show that PLB1 augments the survival and proliferation of the fungus in the CNS and strengthens virulence by modulating the immune response and enhancing specific biophysical properties of the fungus. PLB1 expression causes brain tissue damage and impacts glial cell functions, which may be responsible for the dementia observed in patients which may persist even after resolving from CME. The implications of PLB1 inhibition reveal its involvement in Cn infection and suggest that it may be a possible molecular target in the development of antifungal therapies. The results of this study support additional investigation into the mechanism of PLB1 to further understand the intricacies of cerebral Cn infection.

The Vacuolar Morphogenesis Protein Vam6-Like Protein Vlp1 Is Required for Pathogenicity of Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated yeast pathogen that infects immunocompromised patients to cause fungal meningitis, resulting in hundreds of thousands of deaths each year. F-box protein Fbp1, the key component of the E3 ubiquitin ligase, plays a critical role in fungal development and virulence in fungal pathogens. In this study, we identified a potential substrate of Fbp1, the vacuolar morphogenesis protein Vam6-like protein Vlp1, and evaluated its role in virulence in C. neoformans. Deletion or overexpression of the VLP1 gene results in abnormal capsule formation and melanin production of C. neoformans. Stress tolerance assay showed that the vlp1Δ mutant was sensitive to SDS and NaCl but not to CFW or Congo red, indicating that Vlp1 might regulate the cell membrane integrity in C. neoformans. Fungal virulence assay showed that Vlp1 was essential for the pathogenicity of C. neoformans, as vlp1Δ mutants are avirulent in the mouse systematic infection model of cryptococcosis. The progression of fungal infection revealed that the vlp1Δ mutants were gradually eliminated from the lungs of the mice after infection. Moreover, the vlp1Δ mutants showed a proliferation defect inside macrophages and a viability defect in the host complement system, which likely contributes to the virulence attenuation of the vlp1Δ mutants. In summary, our results revealed that the vacuolar morphogenesis protein Vam6-like protein Vlp1 is essential for the pathogenicity of C. neoformans.

A Predicted Mannoprotein Cmp1 Regulates Fungal Virulence in Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans consists of glucuronoxylomannan (GXM), glucuronoxylomannogalactan (GXMGal), and mannoproteins (MPs). MPs are a kind of glycoproteins with low content but high immunogenicity, which can stimulate the immune protection of the host. However, there is not much information about the role of mannoproteins in virulence of the human fungal pathogen C. neoformans. In this study, we reported the identification and functional analysis of a predicted mannoprotein Cmp1 that regulates fungal virulence in C. neoformans. Gene expression pattern analysis indicates that the CMP1 gene was ubiquitously expressed at all stages of cryptococcal development. Subcellular localization analysis indicated that Cmp1 was localized in the cytoplasm of cryptococcal cells. Disruption or overexpression of CMP1 results in impairing capsule formation in Cryptococcus, but it does not affect the melanin production and sensitivity under various stress conditions, nor does it affect the sexual reproduction process of Cryptococcus. Survival assay showed that the pathogenicity of the cmp1Δ mutant or the CMP1 overexpression strain was significantly attenuated in a murine inhalation model of cryptococcosis. In conclusion, our findings implied that the mannoprotein Cmp1 is required for the virulence of C. neoformans.

A Fungal Arrestin Protein Contributes to Cell Cycle Progression and Pathogenesis

To survive under unpredictable conditions, all organisms must adapt to stressors by regulating adaptive cellular responses. Arrestin proteins are conserved regulators of adaptive cellular responses in eukaryotes. Studies that have been limited to mammals and model fungi have demonstrated that the disruption of arrestin-regulated pathways is detrimental for viability. The human fungal pathogen Cryptococcus neoformans causes more than 180,000 infection-related deaths annually, especially among immunocompromised patients. In addition to being genetically tractable, C. neoformans has a small arrestin family of four members, lending itself to a comprehensive characterization of its arrestin family. This study serves as a functional analysis of arrestins in a pathogen, particularly in the context of fungal fitness and virulence. We investigate the functions of one arrestin protein, Ali1, and define its novel contributions to cytokinesis. We additionally explore the virulence contributions of the C. neoformans arrestin family and find that they contribute to disease establishment and progression.

Arrestins, a structurally specialized and functionally diverse group of proteins, are central regulators of adaptive cellular responses in eukaryotes. Previous studies on fungal arrestins have demonstrated their capacity to modulate diverse cellular processes through their adaptor functions, facilitating the localization and function of other proteins. However, the mechanisms by which arrestin-regulated processes are involved in fungal virulence remain unexplored. We have identified a small family of four arrestins, Ali1, Ali2, Ali3, and Ali4, in the human fungal pathogen Cryptococcus neoformans. Using complementary microscopy, proteomic, and reverse genetics techniques, we have defined a role for Ali1 as a novel contributor to cytokinesis, a fundamental cell cycle-associated process. We observed that Ali1 strongly interacts with proteins involved in lipid synthesis, and that ali1Δ mutant phenotypes are rescued by supplementation with lipid precursors that are used to build cellular membranes. From these data, we hypothesize that Ali1 contributes to cytokinesis by serving as an adaptor protein, facilitating the localization of enzymes that modify the plasma membrane during cell division, specifically the fatty acid synthases Fas1 and Fas2. Finally, we assessed the contributions of the C. neoformans arrestin family to virulence to better understand the mechanisms by which arrestin-regulated adaptive cellular responses influence fungal infection. We observed that the C. neoformans arrestin family contributes to virulence, and that the individual arrestin proteins likely fulfill distinct functions that are important for disease progression.

Pbp1-Interacting Protein Mkt1 Regulates Virulence and Sexual Reproduction in Cryptococcus neoformans

The Mkt1–Pbp1 complex promotes mating-type switching by regulating the translation of HO mRNA in Saccharomyces cerevisiae. Here, we performed in vivo immunoprecipitation assays and mass spectrometry analyses in the human fungal pathogen Cryptococcus neoformans to show that Pbp1, a poly(A)-binding protein-binding protein, interacts with Mkt1 containing a PIN like-domain. Association of Pbp1 with Mkt1 was confirmed by co-immunoprecipitation assays. Results of spot dilution growth assays showed that unlike pbp1 deletion mutant strains, mkt1 deletion mutant strains were not resistant to heat stress compared with wild-type. However, similar to the pbp1 deletion mutant strains, the mkt1 deletion mutants exhibited both, defective dikaryotic hyphal production and reduced pheromone gene (MFα1) expression during mating. In addition, deletion of mkt1 caused attenuated virulence in a murine intranasal inhalation model. Taken together, our findings reveal that Mkt1 plays a crucial role in sexual reproduction and virulence in C. neoformans.

Identification of a basidiomycete-specific Vilse-like GTPase activating proteins (GAPs) and its roles in the production of virulence factors in Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycetous pathogenic yeast that causes fatal infections in both immunocompetent and immunocompromised patients. Regulation on the production of its virulence factors is not fully understood. Here we reported the characterization of a gene, named CVH1(CNA06260), encoding a Drosophila Vilse-like RhoGAP homolog, which is hallmarked by three conserved functional domains: WW, MyTH4 and RhoGAP. Phylogenetic analysis suggests that CVH1 is highly conserved from protists to mammals and interestingly in basidiomycetes, but absent in plants or Ascomycota and other lower fungi. This phylogenetic distribution indicates an evolutionary link among these groups of organisms. Functional analyses demonstrated that CVH1 was involved in stress tolerance and virulence factor production. By disrupting CVH1, we created a second mutant cvh1Δ with the CRISPR-Cas9 editing tool. The mutant strain exhibited hypersensitivity to osmotic stress by 2 M sorbitol and NaCl, suggesting defects in the HOG signaling pathway and an interaction of Cvh1 with the HOG pathway. Hypersensitivity of cvh1Δ to 1% Congo red and 0.01% SDS suggests that the cell wall integrity was impaired in the mutant. And cvh1Δ hardly produced the pigment melanin and capsule. Our study for the first time demonstrates that the fungal Vilse-like RhoGAP CVH1 is an important regulator of multiple biological processes in C. neoformans, and provides novel insights into the regulatory circuit of stress resistance/cell wall integrity, and laccase and capsule synthesis in C. neoformans.

The Cys2His2 zinc finger protein Zfp1 regulates sexual reproduction and virulence in Cryptococcus neoformans

Cryptococcus neoformans is a ubiquitous yeast pathogen that often infects the human central nervous system (CNS) to cause meningitis in immunocompromised individuals. Although numerous signaling pathways and factors important for fungal sexual reproduction and virulence have been investigated, their precise mechanism of action remains to be further elucidated. In this study, we identified and characterized a novel zinc finger protein Zfp1 that regulates fungal sexual reproduction and virulence in C. neoformans. qRT-PCR and ZFP1 promoter regulatory activity assays revealed a ubiquitous expression pattern of ZFP1 in all stages during mating. Subcellular localization analysis indicates that Zfp1 is targeted to the cytoplasm of C. neoformans. In vitro assays of stress responses showed that zfp1Δ mutants and the ZFP1 overexpressed strains ZFP1^OE are hypersensitive to SDS, but not Congo red, indicating that Zfp1 may regulate cell membrane integrity. Zfp1 is also essential for fungal sexual reproduction because basidiospore production was blocked in bilateral mating between zfp1Δ mutants or ZFP1 overexpressed strains. Fungal nuclei development assay showed that nuclei in the bilateral mating of zfp1Δ mutants or ZFP1 overexpressed strains failed to undergo meiosis after fusion, indicating Zfp1 is important for regulating meiosis during mating. Although zfp1Δ mutants showed normal growth and produced normal major virulence factors, virulence was attenuated in a murine model. Interestingly, we found that the ZFP1 overexpressed strains were avirulent in a murine systemic-infection model. Overall, our study showed that the zinc finger protein Zfp1 is essential for fungal sporulation and virulence in C. neoformans.

Characterization of additional components of the environmental pH-sensing complex in the pathogenic fungus Cryptococcus neoformans

Pathogenic microorganisms must adapt to changes in their immediate surroundings, including alterations in pH, to survive the shift from the external environment to that of the infected host. In the basidiomycete fungal pathogen Cryptococcus neoformans, these pH changes are primarily sensed by the fungus-specific, alkaline pH-sensing Rim/Pal pathway. The C. neoformans Rim pathway has diverged significantly from that described in ascomycete fungi. We recently identified the C. neoformans putative pH sensor Rra1, which activates the Rim pathway in response to elevated pH. In this study, we probed the function of Rra1 by analyzing its cellular localization and performing protein co-immunoprecipitation to identify potential Rra1 interactors. We found that Rra1 does not strongly colocalize or interact with immediate downstream Rim pathway components. However, these experiments identified a novel Rra1 interactor, the previously uncharacterized C. neoformans nucleosome assembly protein 1 (Nap1), which was required for Rim pathway activation. We observed that Nap1 specifically binds to the C-terminal tail of the Rra1 sensor, probably promoting Rra1 protein stability. This function of Nap1 is conserved in fungi closely related to C. neoformans that contain Rra1 orthologs, but not in the more distantly related ascomycete fungus Saccharomyces cerevisiae In conclusion, our findings have revealed the sophisticated, yet distinct, molecular mechanisms by which closely and distantly related microbial phyla rapidly adapt to environmental signals and changes, such as alterations in pH.

The first succinylome profile of Trichophyton rubrum reveals lysine succinylation on proteins involved in various key cellular processes

BACKGROUND

Dermatophytes, the most common cause of fungal infections, affect millions of individuals worldwide. They pose a major threat to public health because of the severity and longevity of infections caused by dermatophytes and their refractivity to therapy. Trichophyton rubrum (T. rubrum), the most common dermatophyte species, is a promising model organism for dermatophyte research. Post-translational modifications (PTMs) have been shown to be essential for many biological processes, particularly in the regulation of key cellular processes that contribute to pathogenicity. Although PTMs have important roles, little is known about their roles in T. rubrum and other dermatophytes. Succinylation is a new PTM that has recently been identified. In this study, we assessed the proteome-wide succinylation profile of T. rubrum. This study sought to systematically identify the succinylated sites and proteins in T. rubrum and to reveal the roles of succinylated proteins in various cellular processes as well as the differences in the succinylation profiles in different growth stages of the T. rubrum life cycle.

RESULTS

A total of 569 succinylated lysine sites were identified in 284 proteins. These succinylated proteins are involved in various cellular processes, such as metabolism, translation and epigenetic regulation. Additionally, 24 proteins related to pathogenicity were found to be succinylated. Comparison of the succinylome at the conidia and mycelia stages revealed that most of the succinylated proteins and sites were growth-stage specific. In addition, the succinylation modifications on histone and ribosomal proteins were significantly different between these two growth stages. Moreover, the sequence features surrounding the succinylated sites were different in the two stages, thus indicating the specific recognition of succinyltransferases in each growth phase.

CONCLUSIONS

In this study, we explored the first T. rubrum succinylome, which is also the first PTM analysis of dermatophytes reported to date. These results revealed the major roles of the succinylated proteins involved in T. rubrum and the differences in the succinylomes between the two major growth stages. These findings should improve understanding of the physiological and pathogenic properties of dermatophytes and facilitate future development of novel drugs and therapeutics for treating superficial fungal infections.

CSN1201, a subunit of the COP9 signalosome, regulates the virulence in Cryptococcus neoformans infection

The COP9 signalosome (CSN) is a multisubunit protein complex, and it now has been found to participate in diverse cellular and developmental processes in various eukaryotic organisms. Cryptococcus neoformans (C. neoformans) is an important basidiomycete pathogen that causes life-threatening meningoencephalitis primarily in the immune compromised population. Here, we generated CSN deletion mutants to investigate the role in Cryptococcus infection. Compared to other CSN mutants, we identified a CSN1201 mutant exhibited severely attenuated virulence. Deletion of CSN1201 made cryptococcal cells more susceptible to nearly all in vitro stresses. Furthermore, deletion of CSN1201 obviously impaired survival of C. neoformans. At the same time, in vivo virulence assay of mouse infection models demonstrated that CSN1201 significantly enhanced the virulence of C. neoformans compared with the other CSN subunit strains, while ELISA analysis of C. neoformans infection in innate or adaptive immune response showed that deletion of CSN1201 significantly impaired cytokines and interferon expression. In vitro model of the blood-brain barrier (BBB) analysis indicated that deletion of CSN1201 reduced the invasion efficacy of Cryptococcusto cross BBB. Taken together, our findings reveal a novel mechanism of CSN1201, which plays a critical role for the virulence composite of C. neoformans, and also provides an additional yeast survival and propagation advantage in the host.

Relative Contributions of Prenylation and Postprenylation Processing in Cryptococcus neoformans Pathogenesis

Cryptococcus neoformans is an important human fungal pathogen that causes disease and death in immunocompromised individuals. The growth and morphogenesis of this fungus are controlled by conserved Ras-like GTPases, which are also important for its pathogenicity. Many of these proteins require proper subcellular localization for full function, and they are directed to cellular membranes through a posttranslational modification process known as prenylation. These studies investigate the roles of one of the prenylation enzymes, farnesyltransferase, as well as the postprenylation processing enzymes in C. neoformans. We demonstrate that the postprenylation processing steps are dispensable for the localization of certain substrate proteins. However, both protein farnesylation and the subsequent postprenylation processing steps are required for full pathogenesis of this fungus.

Prenyltransferase enzymes promote the membrane localization of their target proteins by directing the attachment of a hydrophobic lipid group at a conserved C-terminal CAAX motif. Subsequently, the prenylated protein is further modified by postprenylation processing enzymes that cleave the terminal 3 amino acids and carboxymethylate the prenylated cysteine residue. Many prenylated proteins, including Ras1 and Ras-like proteins, require this multistep membrane localization process in order to function properly. In the human fungal pathogen Cryptococcus neoformans, previous studies have demonstrated that two distinct forms of protein prenylation, farnesylation and geranylgeranylation, are both required for cellular adaptation to stress, as well as full virulence in animal infection models. Here, we establish that the C. neoformans RAM1 gene encoding the farnesyltransferase β-subunit, though not strictly essential for growth under permissive in vitro conditions, is absolutely required for cryptococcal pathogenesis. We also identify and characterize postprenylation protease and carboxyl methyltransferase enzymes in C. neoformans. In contrast to the prenyltransferases, deletion of the genes encoding the Rce1 protease and Ste14 carboxyl methyltransferase results in subtle defects in stress response and only partial reductions in virulence. These postprenylation modifications, as well as the prenylation events themselves, do play important roles in mating and hyphal transitions, likely due to their regulation of peptide pheromones and other proteins involved in development.

IMPORTANCECryptococcus neoformans is an important human fungal pathogen that causes disease and death in immunocompromised individuals. The growth and morphogenesis of this fungus are controlled by conserved Ras-like GTPases, which are also important for its pathogenicity. Many of these proteins require proper subcellular localization for full function, and they are directed to cellular membranes through a posttranslational modification process known as prenylation. These studies investigate the roles of one of the prenylation enzymes, farnesyltransferase, as well as the postprenylation processing enzymes in C. neoformans. We demonstrate that the postprenylation processing steps are dispensable for the localization of certain substrate proteins. However, both protein farnesylation and the subsequent postprenylation processing steps are required for full pathogenesis of this fungus.

The Cryptococcus neoformans alkaline response pathway: identification of a novel rim pathway activator

The Rim101/PacC transcription factor acts in a fungal-specific signaling pathway responsible for sensing extracellular pH signals. First characterized in ascomycete fungi such as Aspergillus nidulans and Saccharomyces cerevisiae, the Rim/Pal pathway maintains conserved features among very distantly related fungi, where it coordinates cellular adaptation to alkaline pH signals and micronutrient deprivation. However, it also directs species-specific functions in fungal pathogens such as Cryptococcus neoformans, where it controls surface capsule expression. Moreover, disruption of the Rim pathway central transcription factor, Rim101, results in a strain that causes a hyper-inflammatory response in animal infection models. Using targeted gene deletions, we demonstrate that several genes encoding components of the classical Rim/Pal pathway are present in the C. neoformans genome. Many of these genes are in fact required for Rim101 activation, including members of the ESCRT complex (Vps23 and Snf7), ESCRT-interacting proteins (Rim20 and Rim23), and the predicted Rim13 protease. We demonstrate that in neutral/alkaline pH, Rim23 is recruited to punctate regions on the plasma membrane. This change in Rim23 localization requires upstream ESCRT complex components but does not require other Rim101 proteolysis components, such as Rim20 or Rim13. Using a forward genetics screen, we identified the RRA1 gene encoding a novel membrane protein that is also required for Rim101 protein activation and, like the ESCRT complex, is functionally upstream of Rim23-membrane localization. Homologs of RRA1 are present in other Cryptococcus species as well as other basidiomycetes, but closely related genes are not present in ascomycetes. These findings suggest that major branches of the fungal Kingdom developed different mechanisms to sense and respond to very elemental extracellular signals such as changing pH levels.

A multi-host approach for the systematic analysis of virulence factors in Cryptococcus neoformans

A multi-host approach was followed to screen a library of 1201 signature-tagged deletion strains of Cryptococcus neoformans mutants to identify previously unknown virulence factors. The primary screen was performed using a Caenorhabditis elegans-C. neoformans infection assay. The hits among these strains were reconfirmed as less virulent than the wild type in the insect Galleria mellonella-C. neoformans infection assay. After this 2-stage screen, and to prioritize hits, we performed serial evaluations of the selected strains, using the C. elegans model. All hit strains identified through these studies were validated in a murine model of systemic cryptococcosis. Twelve strains were identified through a stepwise screening assay. Among them, 4 (CSN1201, SRE1, RDI1, and YLR243W) were previously discovered, providing proof of principle for this approach, while the role of the remaining 8 genes (CKS101, CNC5600, YOL003C, CND1850, MLH3, HAP502, MSL5, and CNA2580) were not previously described in cryptococcal virulence. The multi-host approach is an efficient method of studying the pathogenesis of C. neoformans. We used diverse model hosts, C. elegans, G. mellonella, and mice, with physiological differences and identified 12 genes associated with mammalian infection. Our approach may be suitable for large pathogenesis screens.

Molecular mechanisms of hypoxic responses via unique roles of Ras1, Cdc24 and Ptp3 in a human fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans encounters a low oxygen environment when it enters the human host. Here, we show that the conserved Ras1 (a small GTPase) and Cdc24 (the guanine nucleotide exchange factor for Cdc42) play an essential role in cryptococcal growth in hypoxia. Suppressor studies indicate that PTP3 functions epistatically downstream of both RAS1 and CDC24 in regulating hypoxic growth. Ptp3 shares sequence similarity to the family of phosphotyrosine-specific protein phosphatases and the ptp3Δ strain failed to grow in 1% O₂. We demonstrate that RAS1, CDC24 and PTP3 function in parallel to regulate thermal tolerance but RAS1 and CDC24 function linearly in regulating hypoxic growth while CDC24 and PTP3 reside in compensatory pathways. The ras1Δ and cdc24Δ strains ceased to grow at 1% O₂ and became enlarged but viable single cells. Actin polarization in these cells, however, was normal for up to eight hours after transferring to hypoxic conditions. Double deletions of the genes encoding Rho GTPase Cdc42 and Cdc420, but not of the genes encoding Rac1 and Rac2, caused a slight growth retardation in hypoxia. Furthermore, growth in hypoxia was not affected by the deletion of several central genes functioning in the pathways of cAMP, Hog1, or the two-component like phosphorylation system that are critical in the cryptococcal response to osmotic and genotoxic stresses. Interestingly, although deletion of HOG1 rescued the hypoxic growth defect of ras1Δ, cdc24Δ, and ptp3Δ, Hog1 was not hyperphosphorylated in these three mutants in hypoxic conditions. RNA sequencing analysis indicated that RAS1, CDC24 and PTP3 acted upon the expression of genes involved in ergosterol biosynthesis, chromosome organization, RNA processing and protein translation. Moreover, growth of the wild-type strain under low oxygen conditions was affected by sub-inhibitory concentrations of the compounds that inhibit these biological processes, demonstrating the importance of these biological processes in the cryptococcal hypoxia response.

When Cryptococcus neoformans, an environmental fungal pathogen, enters the human host, it encounters a low oxygen condition. The well conserved Ras1 and Cdc24 proteins are known for their key roles in maintenance of the actin cytoskeletal integrity in eukaryotic cells. In this work, we show a unique role of RAS1 and CDC24 in the growth of C. neoformans in a low oxygen environment. Actin polarization, however, appeared normal in the ras1Δ and cdc24Δ strains under hypoxic conditions for up to eight hours. We show that PTP3 is required for hypoxic growth and it can rescue the hypoxic growth defect in ras1Δ and cdc24Δ. Genetic analysis suggested that RAS1 and CDC24 function linearly while CDC24 and PTP3 function parallelly in regulating hypoxic growth. RNA sequencing combined with analysis by small molecular inhibitors revealed that RAS1, CDC24 and PTP3 regulate several biological processes such as ergosterol biosynthesis, chromosome organization, RNA processing and protein translation which are required in the cryptococcal response to hypoxic conditions.

Restricted substrate specificity for the geranylgeranyltransferase-I enzyme in Cryptococcus neoformans: implications for virulence

Proper cellular localization is required for the function of many proteins. The CaaX prenyltransferases (where CaaX indicates a cysteine followed by two aliphatic amino acids and a variable amino acid) direct the subcellular localization of a large group of proteins by catalyzing the attachment of hydrophobic isoprenoid moieties onto C-terminal CaaX motifs, thus facilitating membrane association. This group of enzymes includes farnesyltransferase (Ftase) and geranylgeranyltransferase-I (Ggtase-1). Classically, the variable (X) amino acid determines whether a protein will be an Ftase or Ggtase-I substrate, with Ggtase-I substrates often containing CaaL motifs. In this study, we identify the gene encoding the β subunit of Ggtase-I (CDC43) and demonstrate that Ggtase-mediated activity is not essential. However, Cryptococcus neoformans CDC43 is important for thermotolerance, morphogenesis, and virulence. We find that Ggtase-I function is required for full membrane localization of Rho10 and the two Cdc42 paralogs (Cdc42 and Cdc420). Interestingly, the related Rac and Ras proteins are not mislocalized in the cdc43Δ mutant even though they contain similar CaaL motifs. Additionally, the membrane localization of each of these GTPases is dependent on the prenylation of the CaaX cysteine. These results indicate that C. neoformans CaaX prenyltransferases may recognize their substrates in a unique manner from existing models of prenyltransferase specificity. It also suggests that the C. neoformans Ftase, which has been shown to be more important for C. neoformans proliferation and viability, may be the primary prenyltransferase for proteins that are typically geranylgeranylated in other species.

Ras1 acts through duplicated Cdc42 and Rac proteins to regulate morphogenesis and pathogenesis in the human fungal pathogen Cryptococcus neoformans

Proliferation and morphogenesis in eukaryotic cells depend on the concerted activity of Rho-type GTPases, including Ras, Cdc42, and Rac. The sexually dimorphic fungus Cryptococcus neoformans, which encodes paralogous, non-essential copies of all three, provides a unique model in which to examine the interactions of these conserved proteins. Previously, we demonstrated that RAS1 mediates C. neoformans virulence by acting as a central regulator of both thermotolerance and mating. We report here that ras1Δ mutants accumulate defects in polarized growth, cytokinesis, and cell cycle progression. We demonstrate that the ras1Δ defects in thermotolerance and mating can be largely explained by the compromised activity of four downstream Rho-GTPases: the Cdc42 paralogs, Cdc42 and Cdc420; and the Rac paralogs, Rac1 and Rac2. Further, we demonstrate that the separate GTPase classes play distinct Ras-dependent roles in C. neoformans morphogenesis and pathogenesis. Cdc42 paralogs primarily control septin localization and cytokinesis, while Rac paralogs play a primary role in polarized cell growth. Together, these duplicate, related signaling proteins provide a robust system to allow microbial proliferation in the presence of host-derived cell stresses.

Two Rac paralogs regulate polarized growth in the human fungal pathogen Cryptococcus neoformans

A genome wide analysis of the human fungal pathogen Cryptococcus neoformans var. grubii has revealed a number of duplications of highly conserved genes involved in morphogenesis. Previously, we reported that duplicate Cdc42 paralogs provide C. neoformans with niche-specific responses to environmental stresses: Cdc42 is required for thermotolerance, while Cdc420 supports the formation of titan cells. The related Rho-GTPase Rac1 has been shown in C. neoformans var. neoformans to play a major role in filamentation and to share Cdc42/Cdc420 binding partners. Here we report the characterization of a second Rac paralog in C. neoformans, Rac2, and describe its overlapping function with the previously described CnRac, Rac1. Further, we demonstrate that the Rac paralogs play a primary role in polarized growth via the organization of reactive oxygen species and play only a minor role in the organization of actin. Finally, we provide preliminary evidence that pharmacological inhibitors of Rac activity and actin stability have synergistic activity.

Centrosomal localization of RhoGDIβ and its relevance to mitotic processes in cancer cells

Rho GDP-dissociation inhibitors (RhoGDIs) are regulators of Rho family GTPases. RhoGDIβ has been implicated in cancer progression, but its precise role remains unclear. We determined the subcellular localization of RhoGDIβ and examined the effects of its overexpression and RNAi knockdown in cancer cells. Immunofluorescence staining showed that RhoGDIβ localized to centrosomes in human cancer cells. In HeLa cells, exogenous GFP-tagged RhoGDIβ localized to centrosomes and its overexpression caused prolonged mitosis and aberrant cytokinesis in which the cell shape was distorted. RNAi knockdown of RhoGDIβ led to increased incidence of monopolar spindle mitosis resulting in polyploid cells. These results suggest that RhoGDIβ has mitotic functions, including regulation of cytokinesis and bipolar spindle formation. The dysregulated expression of RhoGDIβ may contribute to cancer progression by disrupting these processes.

Identification of genes from the fungal pathogen Cryptococcus neoformans related to transmigration into the central nervous system

Background

A mouse brain transmigration assessment (MBTA) was created to investigate the central nervous system (CNS) pathogenesis of cryptococcal meningoencephalitis.

Methodology/Principal Findings

Two cryptococcal mutants were identified from a pool of 109 pre-selected mutants that were signature-tagged with the nourseothricin acetyltransferase (NAT) resistance cassette. These two mutants displayed abnormal transmigration into the central nervous system. One mutant displaying decreased transmigration contains a null mutation in the putative FNX1 gene, whereas the other mutant possessing a null mutation in the putative RUB1 gene exhibited increased transmigration into the brain. Two macrophage adhesion-defective mutants in the pool, 12F1 and 3C9, showed reduced phagocytosis by macrophages, but displayed no defects in CNS entry suggesting that transit within macrophages (the “Trojan horse” model of CNS entry) is not the primary mechanism for C. neoformans migration into the CNS in this MBTA.

Conclusions/Significance

This research design provides a new strategy for genetic impact studies on how Cryptococcus passes through the blood-brain barrier (BBB), and the specific isolated mutants in this assay support a transcellular mechanism of CNS entry.

Ras and Rho small G proteins: insights from the Schizophyllum commune genome sequence and comparisons to other fungi

Unlike in animal cells and yeasts, the Ras and Rho small G proteins and their regulators have not received extensive research attention in the case of the filamentous fungi. In an effort to begin to rectify this deficiency, the genome sequence of the basidiomycete mushroom Schizophyllum commune was searched for all known components of the Ras and Rho signalling pathways. The results of this study should provide an impetus for further detailed investigations into their role in polarized hyphal growth, sexual reproduction and fruiting body development. These processes have long been the targets for genetic and cell biological research in this fungus.

Pleiotropic effects of deubiquitinating enzyme Ubp5 on growth and pathogenesis of Cryptococcus neoformans

Ubiquitination is a reversible protein modification that influences various cellular processes in eukaryotic cells. Deubiquitinating enzymes remove ubiquitin, maintain ubiquitin homeostasis and regulate protein degradation via the ubiquitination pathway. Cryptococcus neoformans is an important basidiomycete pathogen that causes life-threatening meningoencephalitis primarily in the immunocompromised population. In order to understand the possible influence deubiquitinases have on growth and virulence of the model pathogenic yeast Cryptococcus neoformans, we generated deletion mutants of seven putative deubiquitinase genes. Compared to other deubiquitinating enzyme mutants, a ubp5Δ mutant exhibited severely attenuated virulence and many distinct phenotypes, including decreased capsule formation, hypomelanization, defective sporulation, and elevated sensitivity to several external stressors (such as high temperature, oxidative and nitrosative stresses, high salts, and antifungal agents). Ubp5 is likely the major deubiquitinating enzyme for stress responses in C. neoformans, which further delineates the evolutionary divergence of Cryptococcus from the model yeast S. cerevisiae, and provides an important paradigm for understanding the potential role of deubiquitination in virulence by other pathogenic fungi. Other putative deubiquitinase mutants (doa4Δ and ubp13Δ) share some phenotypes with the ubp5Δ mutant, illustrating functional overlap among deubiquitinating enzymes in C. neoformans. Therefore, deubiquitinating enzymes (especially Ubp5) are essential for the virulence composite of C. neoformans and provide an additional yeast survival and propagation advantage in the host.

Function of Cryptococcus neoformans KAR7 (SEC66) in karyogamy during unisexual and opposite-sex mating

The human basidiomycetous fungal pathogen Cryptococcus neoformans serves as a model fungus to study sexual development and produces infectious propagules, basidiospores, via the sexual cycle. Karyogamy is the process of nuclear fusion and an essential step to complete mating. Therefore, regulation of nuclear fusion is central to understanding sexual development of C. neoformans. However, our knowledge of karyogamy genes was limited. In this study, using a BLAST search with the Saccharomyces cerevisiae KAR genes, we identified five C. neoformans karyogamy gene orthologs: CnKAR2, CnKAR3, CnKAR4, CnKAR7 (or CnSEC66), and CnKAR8. There are no apparent orthologs of the S. cerevisiae genes ScKAR1, ScKAR5, and ScKar9 in C. neoformans. Karyogamy involves the congression of two nuclei followed by nuclear membrane fusion, which results in diploidization. ScKar7 (or ScSec66) is known to be involved in nuclear membrane fusion. In C. neoformans, kar7 mutants display significant defects in hyphal growth and basidiospore chain formation during both a-α opposite and α-α unisexual reproduction. Fluorescent nuclear imaging revealed that during kar7 × kar7 bilateral mutant matings, the nuclei congress but fail to fuse in the basidia. These results demonstrate that the KAR7 gene plays an integral role in both opposite-sex and unisexual mating, indicating that proper control of nuclear dynamics is important. CnKAR2 was found to be essential for viability, and its function in mating is not known. No apparent phenotypes were observed during mating of kar3, kar4, or kar8 mutants, suggesting that the role of these genes may be dispensable for C. neoformans mating, which demonstrates a different evolutionary trajectory for the KAR genes in C. neoformans compared to those in S. cerevisiae.

Aimless mutants of Cryptococcus neoformans: failure to disseminate

The pathogenic fungus Cryptococcus neoformans exhibits a striking propensity to cause central nervous system (CNS) disease in people with HIV/AIDS. Given that cryptococcal infections are generally initiated by pulmonary colonization, dissemination requires that the fungus withstand phagocytic killing, cross the alveolar-capillary interface in the lung, survive in the circulatory system and breach the blood-brain barrier. We know little about the molecular mechanisms underlying dissemination, but there is a rapidly growing list of mutants that fail to cause CNS disease. These mutants reveal a remarkable diversity of functions and therefore illustrate the complexity of the cryptococcal-host interaction. The challenge now is to extend the analysis of these mutants to acquire a detailed understanding of each step in dissemination.

Axl2 integrates polarity establishment, maintenance, and environmental stress response in the filamentous fungus Ashbya gossypii

In budding yeast, new sites of polarity are chosen with each cell cycle and polarization is transient. In filamentous fungi, sites of polarity persist for extended periods of growth and new polarity sites can be established while existing sites are maintained. How the polarity establishment machinery functions in these distinct growth forms found in fungi is still not well understood. We have examined the function of Axl2, a transmembrane bud site selection protein discovered in Saccharomyces cerevisiae, in the filamentous fungus Ashbya gossypii. A. gossypii does not divide by budding and instead exhibits persistent highly polarized growth, and multiple axes of polarity coexist in one cell. A. gossypii axl2Δ (Agaxl2Δ) cells have wavy hyphae, bulbous tips, and a high frequency of branch initiations that fail to elongate, indicative of a polarity maintenance defect. Mutant colonies also have significantly lower radial growth and hyphal tip elongation speeds than wild-type colonies, and Agaxl2Δ hyphae have depolarized actin patches. Consistent with a function in polarity, AgAxl2 localizes to hyphal tips, branches, and septin rings. Unlike S. cerevisiae Axl2, AgAxl2 contains a Mid2 homology domain and may function to sense or respond to environmental stress. In support of this idea, hyphae lacking AgAxl2 also display hypersensitivity to heat, osmotic, and cell wall stresses. Axl2 serves to integrate polarity establishment, polarity maintenance, and environmental stress response for optimal polarized growth in A. gossypii.

Cryptococcus neoformans requires a functional glycolytic pathway for disease but not persistence in the host

Cryptococcus neoformans is an important fungal pathogen of immunocompromised individuals, with a close relative, Cryptococcus gattii, emerging as a serious threat for the immunocompetent. During initial infection, C. neoformans colonizes the airspaces of the lungs, resulting in pneumonia, and subsequently migrates to the central nervous system (CNS). We sought to understand fungal carbon utilization during colonization of these fundamentally different niches within the host, in particular the roles of gluconeogenesis and glycolysis. We created mutants at key points in the gluconeogenesis/glycolysis metabolic pathways that are restricted for growth on lactate and glucose, respectively. A phosphoenolpyruvate carboxykinase mutant (the pck1∆ mutant), blocked for entry of 2- and 3-carbon substrates into gluconeogenesis and attenuated for virulence in a murine inhalation model, showed wild-type (WT) persistence in a rabbit cerebrospinal fluid (CSF) model of cryptococcosis. Conversely, both the pyruvate kinase (pyk1∆) and the hexose kinase I and II (hxk1∆/hxk2∆) mutants, which show impaired glucose utilization, exhibited severely attenuated virulence in the murine inhalation model of cryptococcosis and decreased persistence in the CNS in both the rabbit CSF and the murine inhalation models while displaying adequate persistence in the lungs of mice. These data suggest that glucose utilization is critical for virulence of C. neoformans and persistence of the yeast in the CNS.

Cryptococcus neoformans is an emerging fungal pathogen of humans and is responsible for approximately 625,000 deaths annually among those suffering from HIV infection/AIDS. The ability of this fungus to persist in the host, coupled with its propensity to colonize the CNS, makes the understanding of nutrient acquisition in the host a primary concern. In this study, we report a requirement of glucose utilization for virulence of C. neoformans that is separate from its role in ATP production in the pathogen. Furthermore, we show that inhibition of glycolysis is a viable antifungal drug target, and impaired ATP production via the PYK1 deletion may serve as a model for dormant/chronic fungal infection in the host. Taken together, these results demonstrate the critical importance of understanding basic metabolic processes of the fungus in the context of host-pathogen interactions.

Survival defects of Cryptococcus neoformans mutants exposed to human cerebrospinal fluid result in attenuated virulence in an experimental model of meningitis

Cryptococcus neoformans is a fungal pathogen that encounters various microenvironments during growth in the mammalian host, including intracellular vacuoles, blood, and cerebrospinal fluid (CSF). Because the CSF is isolated by the blood-brain barrier, we hypothesize that CSF presents unique stresses that C. neoformans must overcome to establish an infection. We assayed 1,201 mutants for survival defects in growth media, saline, and human CSF. We assessed CSF-specific mutants for (i) mutant survival in both human bronchoalveolar lavage (BAL) fluid and fetal bovine serum (FBS), (ii) survival in macrophages, and (iii) virulence using both Caenorhabditis elegans and rabbit models of cryptococcosis. Thirteen mutants exhibited significant survival defects unique to CSF. The mutations of three of these mutants were recreated in the clinical serotype A strain H99: deletions of the genes for a cation ATPase transporter (ena1Δ), a putative NEDD8 ubiquitin-like protein (rub1Δ), and a phosphatidylinositol 4-kinase (pik1Δ). Mutant survival rates in yeast media, saline, and BAL fluid were similar to those of the wild type; however, survival in FBS was reduced but not to the levels in CSF. These mutant strains also exhibited decreased intracellular survival in macrophages, various degrees of virulence in nematodes, and severe attenuation of survival in a rabbit meningitis model. We analyzed the CSF by mass spectrometry for candidate compounds responsible for the survival defect. Our findings indicate that the genes required for C. neoformans survival in CSF ex vivo are necessary for survival and infection in this unique host environment.

Interaction of Cryptococcus neoformans Rim101 and protein kinase A regulates capsule

Cryptococcus neoformans is a prevalent human fungal pathogen that must survive within various tissues in order to establish a human infection. We have identified the C. neoformans Rim101 transcription factor, a highly conserved pH-response regulator in many fungal species. The rim101Δ mutant strain displays growth defects similar to other fungal species in the presence of alkaline pH, increased salt concentrations, and iron limitation. However, the rim101Δ strain is also characterized by a striking defect in capsule, an important virulence-associated phenotype. This capsular defect is likely due to alterations in polysaccharide attachment to the cell surface, not in polysaccharide biosynthesis. In contrast to many other C. neoformans capsule-defective strains, the rim101Δ mutant is hypervirulent in animal models of cryptococcosis. Whereas Rim101 activation in other fungal species occurs through the conserved Rim pathway, we demonstrate that C. neoformans Rim101 is also activated by the cAMP/PKA pathway. We report here that C. neoformans uses PKA and the Rim pathway to regulate the localization, activation, and processing of the Rim101 transcription factor. We also demonstrate specific host-relevant activating conditions for Rim101 cleavage, showing that C. neoformans has co-opted conserved signaling pathways to respond to the specific niche within the infected host. These results establish a novel mechanism for Rim101 activation and the integration of two conserved signaling cascades in response to host environmental conditions.

Cryptococcus neoformans is an environmental fungus and an opportunistic human pathogen. Survival of this fungus within a human host depends on its ability to sense the host environment and respond with protective cellular changes. It is known that the cAMP/PKA signal transduction cascade is important for sensing host-specific environments and regulating the cellular adaptations, such as capsule and increased iron uptake, that are necessary for growth inside the infected host. Here we document that, unlike what has been described in other fungal species, a C. neoformans Rim101 homologue is directly regulated by PKA. The Rim101 signaling pathway is also involved in capsule regulation and virulence. Our study demonstrates that Rim101 integrates two conserved signal transduction cascades, and it is important in regulating microbial pathogenesis.

Comparative transcriptome analysis reveals novel roles of the Ras and cyclic AMP signaling pathways in environmental stress response and antifungal drug sensitivity in Cryptococcus neoformans

The cyclic AMP (cAMP) pathway plays a central role in the growth, differentiation, and virulence of pathogenic fungi, including Cryptococcus neoformans. Three upstream signaling regulators of adenylyl cyclase (Cac1), Ras, Aca1, and Gpa1, have been demonstrated to control the cAMP pathway in C. neoformans, but their functional relationship remains elusive. We performed a genome-wide transcriptome analysis with a DNA microarray using the ras1Delta, gpa1Delta, cac1Delta, aca1Delta, and pka1Delta pka2Delta mutants. The aca1Delta, gpa1Delta, cac1Delta, and pka1Delta pka2Delta mutants displayed similar transcriptome patterns, whereas the ras1Delta mutant exhibited transcriptome patterns distinct from those of the wild type and the cAMP mutants. Interestingly, a number of environmental stress response genes are modulated differentially in the ras1Delta and cAMP mutants. In fact, the Ras signaling pathway was found to be involved in osmotic and genotoxic stress responses and the maintenance of cell wall integrity via the Cdc24-dependent signaling pathway. Notably, the Ras and cAMP mutants exhibited hypersensitivity to a polyene drug, amphotericin B, without showing effects on ergosterol biosynthesis, which suggested a novel method of antifungal combination therapy. Among the cAMP-dependent gene products that we characterized, two small heat shock proteins, Hsp12 and Hsp122, were found to be involved in the polyene antifungal drug susceptibility of C. neoformans.

Two CDC42 paralogues modulate Cryptococcus neoformans thermotolerance and morphogenesis under host physiological conditions

The precise regulation of morphogenesis is a key mechanism by which cells respond to a variety of stresses, including those encountered by microbial pathogens in the host. The polarity protein Cdc42 regulates cellular morphogenesis throughout eukaryotes, and we explore the role of Cdc42 proteins in the host survival of the human fungal pathogen Cryptococcus neoformans. Uniquely, C. neoformans has two functional Cdc42 paralogues, Cdc42 and Cdc420. Here we investigate the contribution of each paralogue to resistance to host stress. In contrast to non-pathogenic model organisms, C. neoformans Cdc42 proteins are not required for viability under non-stress conditions but are required for resistance to high temperature. The paralogues play differential roles in actin and septin organization and act downstream of C. neoformans Ras1 to regulate its morphogenesis sub-pathway, but not its effects on mating. Cdc42, and not Cdc420, is upregulated in response to temperature stress and is required for virulence in a murine model of cryptococcosis. The C. neoformans Cdc42 proteins likely perform complementary functions with other Rho-like GTPases to control cell polarity, septin organization and hyphal transitions that allow survival in the environment and in the host.

A putative P-type ATPase, Apt1, is involved in stress tolerance and virulence in Cryptococcus neoformans

The export of virulence factors, such as the capsule polysaccharide, to the cell surface is a critical aspect of the pathogenicity of Cryptococcus neoformans. A view of capsule export via exocytosis and extracellular vesicles is emerging, but the molecular mechanisms underlying virulence factor transport pathways remain to be established. In this study, we characterized the APT1 gene, which encodes a predicted integral membrane P-type ATPase belonging to the type IV, Drs2 family of aminophospholipid translocases (flippases) (APTs). APTs maintain the phospholipid asymmetry that is critical in membrane fusion events for trafficking and in establishing cell polarity. Deletion of the APT1 gene resulted in phenotypes consistent with similar roles in C. neoformans. These included altered actin distribution, increased sensitivity to stress conditions (oxidative and nitrosative stress) and to trafficking inhibitors, such as brefeldin A and monensin, a reduction in exported acid phosphatase activity, and hypersensitivity to the antifungal drugs amphotericin B, fluconazole, and cinnamycin. However, there was no difference in growth, capsule size, or melanin production between the wild type and the apt1 mutant strains at either 30 degrees C or 37 degrees C. Despite the absence of an influence on these major virulence factors, Apt1 was required for survival during interactions with macrophages, and apt1 mutants exhibited attenuated virulence in a mouse inhalation model of cryptococcosis. Therefore, Apt1 contributes to virulence and the stress response in C. neoformans through apparent functions in membrane fusion and trafficking that do not influence the deposition of major virulence factors, such as capsule and melanin, outside the cell.

Membrane organization and dynamics in cell polarity

The establishment and maintenance of cell polarity is important to a wide range of biological processes ranging from chemotaxis to embryogenesis. An essential feature of cell polarity is the asymmetric organization of proteins and lipids in the plasma membrane. In this article, we discuss how polarity regulators such as small GTP-binding proteins and phospholipids spatially and kinetically control vesicular trafficking and membrane organization. Conversely, we discuss how membrane trafficking contributes to cell polarization through delivery of polarity determinants and regulators to the plasma membrane.

Subcellular localization directs signaling specificity of the Cryptococcus neoformans Ras1 protein

In the human fungal pathogen Cryptococcus neoformans, Ras signaling mediates sexual differentiation, morphogenesis, and pathogenesis. By studying Ras prenylation and palmitoylation in this organism, we have found that the subcellular localization of this protein dictates its downstream signaling specificity. Inhibiting C. neoformans Ras1 prenylation results in the defective general membrane targeting of this protein and the loss of all Ras function. In contrast, palmitoylation mediates localization of Ras1 to the plasma membrane and is required for normal morphogenesis and survival at high temperatures. However, palmitoylation and plasma membrane localization are not required for Ras-dependent sexual differentiation. Likely as a result of its effect on thermotolerance, Ras1 palmitoylation is also required for the pathogenesis of C. neoformans. These data support an emerging paradigm of compartmentalized Ras signaling. However, our studies also demonstrate fundamental differences between the Ras pathways in different organisms that emphasize the functional flexibility of conserved signaling cascades.