An Erg11 lanosterol 14-α-demethylase-Arv1 complex is required for Candida albicans virulence.

Azole resistant fungal infections remain a health problem for the immune compromised. Current therapies are limited due to rises in new resistance mechanisms. Therefore, it is important to identify new drug targets for drug discovery and novel therapeutics. Arv1 (are1 are2 required for viability 1) function is highly conserved between multiple pathogenic fungal species. Candida albicans (C. albicans) cells lacking CaArv1 are azole hypersusceptible and lack virulence. Saccharomyces cerevisiae (S. cerevisiae) Scarv1 cells are also azole hypersusceptible, a phenotype reversed by expression of CaArv1, indicating conservation in the molecular mechanism for azole susceptibility. To define the relationship between Arv1 function and azole susceptibility, we undertook a structure/function analysis of ScArv1. We identified several conserved amino acids within the ScArv1 homology domain (ScAhd) required for maintaining normal azole susceptibility. Erg11 lanosterol 14-α-demethylase is the rate-limiting enzyme in sterol biosynthesis and is the direct target of azole antifungals, so we used our ScArv1 mutants in order to explore the relationship between ScArv1 and ScErg11. Specific ScArv1 mutants ectopically expressed from a low copy plasmid were unable to restore normal azole susceptibility to Scarv1 cells and had reduced Erg11 protein levels. Erg11 protein stability depended on its ability to form a heterodimeric complex with Arv1. Complex formation was required for maintaining normal azole susceptibility. Scarv1 cells expressing orthologous CaArv1 mutants also had reduced CaErg11 levels, were unable to form a CaArv1-CaErg11 complex, and were azole hypersusceptible. Scarv1 cells expressing CaArv1 mutants unable to interact with CaErg11 could not sustain proper levels of the azole resistant CaErg11Y132F F145L protein. Caarv1/Caarv1 cells expressing CaArv1 mutants unable to interact with CaErg11 were found to lack virulence using a disseminated candidiasis mouse model. Expressing CaErg11Y132F F145L did not reverse the lack of virulence. We hypothesize that the role of Arv1 in Erg11-dependent azole resistance is to stabilize Erg11 protein level. Arv1 inhibition may represent an avenue for treating azole resistance.

A link between very long chain fatty acid elongation and mating-specific yeast cell cycle arrest.

Ceramides and sphingolipid intermediates are well-established regulators of the cell cycle. In the budding yeast Saccharomyces cerevisae, the complex sphingolipid backbone, ceramide, comprises a long chain sphingoid base, a polar head group, and a very long chain fatty acid (VLCFA). While ceramides and long chain bases have been extensively studied as to their roles in regulating cell cycle arrest under multiple conditions, the roles of VLCFAs are not well understood. Here, we used the yeast elo2 and elo3 mutants, which are unable to elongate fatty acids, as tools to explore if maintaining VLCFA elongation is necessary for cell cycle arrest in response to yeast mating. We found that both elo2 and elo3 cells had severely reduced mating efficiencies and were unable to form polarized shmoo projections that are necessary for cell-cell contact during mating. They also lacked functional MAP kinase signaling activity and were defective in initiating a cell cycle arrest in response to pheromone. Additional data suggests that mislocalization of the Ste5 scaffold in elo2 and elo3 mutants upon mating initiation may be responsible for the inability to initiate a cell cycle arrest. Moreover, the lack of proper Ste5 localization may be caused by the inability of mutant cells to mobilize PIP2. We suggest that VLCFAs are required for Ste5 localization, which is a necessary event for initiating MAP kinase signaling and cell cycle arrest during yeast mating initiation.

Ceramide signals for initiation of yeast mating-specific cell cycle arrest.

Sphingolipids are major constituents of membranes. A number of S. cerevisiae sphingolipid intermediates such as long chains sphingoid bases (LCBs) and ceramides act as signaling molecules regulating cell cycle progression, adaptability to heat stress, and survival in response to starvation. Here we show that S. cerevisiae haploid cells must synthesize ceramide in order to induce mating specific cell cycle arrest. Cells devoid of sphingolipid biosynthesis or defective in ceramide synthesis are sterile and harbor defects in pheromone-induced MAP kinase-dependent transcription. Analyses of G1/S cyclin levels indicate that mutant cells cannot reduce Cln1/2 levels in response to pheromone. FACS analysis indicates a lack of ability to arrest. The addition of LCBs to sphingolipid deficient cells restores MAP kinase-dependent transcription, reduces cyclin levels, and allows for mating, as does the addition of a cell permeable ceramide to cells blocked at ceramide synthesis. Pharmacological studies using the inositolphosphorylceramide synthase inhibitor aureobasidin A indicate that the ability to synthesize and accumulate ceramide alone is sufficient for cell cycle arrest and mating. Studies indicate that ceramide also has a role in PI(4,5)P2 polarization during mating, an event necessary for initiating cell cycle arrest and mating itself. Moreover, our studies suggest a third role for ceramide in localizing the mating-specific Ste5 scaffold to the plasma membrane. Thus, ceramide plays a role 1) in pheromone-induced cell cycle arrest, 2) in activation of MAP kinase-dependent transcription, and 3) in PtdIns(4,5)P2 polarization. All three events are required for differentiation during yeast mating.

The oxysterol-binding protein superfamily: new concepts and old proteins.

The Kes1 OSBP (oxysterol-binding protein) is a key regulator of membrane trafficking through the TGN (trans-Golgi network) and endosomal membranes. We demonstrated recently that Kes1 acts as a sterol-regulated rheostat for TGN/endosomal phosphatidylinositol 4-phosphate signalling. Kes1 utilizes its dual lipid-binding activities to integrate endosomal lipid metabolism with TORC1 (target of rapamycin complex 1)-dependent proliferative pathways and transcriptional control of nutrient signalling.

Arv1 lipid transporter function is conserved between pathogenic and nonpathogenic fungi.

The lipid transporter Arv1 regulates sterol trafficking, and glycosylphosphatidylinositol and sphingolipid biosyntheses in Saccharomyces cerevisiae. ScArv1 contains an Arv1 homology domain (AHD) that is conserved at the amino acid level in the pathogenic fungal species, Candida albicans and Candida glabrata. Here we show S. cerevisiae cells lacking Arv1 are highly susceptible to antifungal drugs. In the presence of drug, Scarv1 cells are unable to induce ERG gene expression, have an altered pleiotrophic drug response, and are defective in multi-drug resistance efflux pump expression. All phenotypes are remediated by ectopic expression of CaARV1 or CgARV1. The AHDs of these pathogenic fungi are required for specific drug tolerance, demonstrating conservation of function. In order to understand how Arv1 regulates antifungal susceptibility, we examined sterol trafficking. CaARV1/CgARV1 expression suppressed the sterol trafficking defect of Scarv1 cells. Finally, we show that C. albicansarv1/arv1 cells are avirulent using a BALB/c disseminated mouse model. We suggest that overall cell survival in response to antifungal treatment requires the lipid transporter function of Arv1.

Determination of the membrane topology of Arv1 and the requirement of the ER luminal region for Arv1 function in Saccharomyces cerevisiae.

In Saccharomyces cerevisiae, ARV1 encodes a 321 amino acid transmembrane protein localized to the endoplasmic reticulum (ER) and Golgi. It has been shown previously that arv1 cells harbor defects in sphingolipid and glycosylphosphatidylinositol biosyntheses, and may harbor sterol trafficking defects. Using C-terminal fusion to Suc2-His4, we determined the orientation of full-length Arv1 in the ER membrane. Once membrane topology was determined, we used this information and truncation analysis to establish the minimum protein length required for Arv1 function and phenotypic suppression. By understanding the topology of Arv1 we can now further analyze its putative lipid and glycosylphosphatidylinositol intermediate transport activities.

The putative lipid transporter, Arv1, is required for activating pheromone-induced MAP kinase signaling in Saccharomyces cerevisiae.

Saccharomyces cerevisiae haploid cells respond to extrinsic mating signals by forming polarized projections (shmoos), which are necessary for conjugation. We have examined the role of the putative lipid transporter, Arv1, in yeast mating, particularly the conserved Arv1 homology domain (AHD) within Arv1 and its role in this process. Previously it was shown that arv1 cells harbor defects in sphingolipid and glycosylphosphatidylinositol (GPI) biosyntheses and may harbor sterol trafficking defects. Here we demonstrate that arv1 cells are mating defective and cannot form shmoos. They lack the ability to initiate pheromone-induced G1 cell cycle arrest, due to failure to polarize PI(4,5)P(2) and the Ste5 scaffold, which results in weakened MAP kinase signaling activity. A mutant Ste5, Ste5(Q59L), which binds more tightly to the plasma membrane, suppresses the MAP kinase signaling defects of arv1 cells. Filipin staining shows arv1 cells contain altered levels of various sterol microdomains that persist throughout the mating process. Data suggest that the sterol trafficking defects of arv1 affect PI(4,5)P(2) polarization, which causes a mislocalization of Ste5, resulting in defective MAP kinase signaling and the inability to mate. Importantly, our studies show that the AHD of Arv1 is required for mating, pheromone-induced G1 cell cycle arrest, and for sterol trafficking.

Changes in fluorescence intensity of selected leukocyte surface markers following fixation.

BACKGROUND: Immunophenotyping of blood leukocytes often involves fixation with paraformaldehyde prior to cytometry analysis. However, the influence of cell type and marker specificity on the stability of fluorescence intensity after fixation has not been well studied. METHODS: Human whole blood was stained using a panel of fluorescein isothiocyanate-labeled antibodies to surface markers. Unfixed and fixed samples were analyzed by flow cytometry at 0, 2, 4, 6, 24, 48, and 96 h after staining. Fluorescence measurements were converted to molecules of equivalent soluble fluorochrome for comparison. RESULTS: Fixation caused a significant decrease in both forward and side scatter at 48 h which required gating adjustments to achieve resolution of cell populations. The autofluorescence increased progressively in fixed samples (ninefold at 96 h for monocytes). Variable decreases in marker-associated fluorescence became apparent after correction for autofluorescence. The magnitude of the decrease at 96 h varied with cell type and marker, from 5% for CD32 on monocytes to 39% for CD16 on neutrophils. CONCLUSION: The change in fluorescence intensity following staining and fixation of leukocytes varies with cell type and surface marker. Fluorescence stability should be determined for each cell type and marker used, and the confounding effects of fixation on cell autofluorescence should be considered.

Structure of the MTIP-MyoA complex, a key component of the malaria parasite invasion motor.

The causative agents of malaria have developed a sophisticated machinery for entering multiple cell types in the human and insect hosts. In this machinery, a critical interaction occurs between the unusual myosin motor MyoA and the MyoA-tail Interacting Protein (MTIP). Here we present one crystal structure that shows three different conformations of Plasmodium MTIP, one of these in complex with the MyoA-tail, which reveal major conformational changes in the C-terminal domain of MTIP upon binding the MyoA-tail helix, thereby creating several hydrophobic pockets in MTIP that are the recipients of key hydrophobic side chains of MyoA. Because we also show that the MyoA helix is able to block parasite growth, this provides avenues for designing antimalarials.