Dimeric Sfh3 has structural changes in its binding pocket that are associated with a dimer–monomer state transformation induced by substrate binding

Yeast Sec14-like lipid transfer proteins Pdr16 and Pdr17 bind and transfer the ergosterol precursor lanosterol in addition to phosphatidylinositol

Yeast Sec14-like phosphatidylinositol transfer proteins (PITPs) contain a hydrophobic cavity capable of accepting a single molecule of phosphatidylinositol (PI) or another molecule in a mutually exclusive manner. We report here that two yeast Sec14 family PITPs, Pdr16p (Sfh3p) and Pdr17p (Sfh4p), possess high-affinity binding and transfer towards lanosterol. To our knowledge, this is the first identification of lanosterol transfer proteins. In addition, a pdr16Δpdr17Δ double mutant had a significantly increased level of cellular lanosterol compared with the corresponding wild-type. Based on the lipid profiles of wild-type and pdr16Δpdr17Δ cells grown in aerobic and anaerobic conditions, we suggest that PI-lanosterol transfer proteins are important predominantly for the optimal functioning of the post-lanosterol part of sterol biosynthesis.

Structural basis of ligand recognition and transport by Sfh2, a yeast phosphatidylinositol transfer protein of the Sec14 superfamily

Sec14-like phosphatidylinositol transfer proteins (PITPs) are involved in lipid metabolism and phosphatidylinositol 4-phosphate signaling by transporting phosphatidylinositol (PI) and a secondary ligand between the organellar membranes in eukaryotes. Yeast Sfh2 is a PITP that transfers PI and squalene without phosphatidylcholine transfer activity. To investigate the structural determinants for ligand specificity and transport in Sfh2, crystal structures of Sfh2 in complex with PI and squalene were determined at 1.5 and 2.4 Å resolution, respectively. The inositol head group of PI is recognized by highly conserved residues around the pocket entrance. The acyl chains of PI bind into a large hydrophobic cavity. Squalene is accommodated in the bottom of the cavity entirely by hydrophobic interactions. The binding of PI and squalene are mutually exclusive due to their overlapping binding sites, correlating with the role in lipid exchange. The binding mode of PI is well conserved in Sfh family proteins. However, squalene binding is unique to the Sfh2 homolog due to the specific hydrophobic residues forming a shape-complementary binding pocket. Recombinant apo Sfh2 forms a homodimer in vitro by the hydrophobic interaction of the gating α10–α11 helices in an open conformation. Ligand binding closes the lid and dissociates the dimer into monomers. This study reveals the structural determinants for the recognition of the conserved PI and a secondary ligand, squalene, and provides implications for the lipid-transfer function of Sfh2.

Sec14 family of lipid transfer proteins in yeasts

The hydrophobicity of lipids prevents their free movement across the cytoplasm. To achieve highly heterogeneous and precisely regulated lipid distribution in different cellular membranes, lipids are transported by lipid transfer proteins (LTPs) in addition to their transport by vesicles. Sec14 family is one of the most extensively studied groups of LTPs. Here we provide an overview of Sec14 family of LTPs in the most studied yeast Saccharomyces cerevisiae as well as in other selected non-Saccharomyces yeasts-Schizosaccharomyces pombe, Kluyveromyces lactis, Candida albicans, Candida glabrata, Cryptococcus neoformans, and Yarrowia lipolytica. Discussed are specificities of Sec14-domain LTPs in various yeasts, their mode of action, subcellular localization, and physiological function. In addition, quite few Sec14 family LTPs are target of antifungal drugs, serve as modifiers of drug resistance or influence virulence of pathologic yeasts. Thus, they represent an important object of study from the perspective of human health.

Deletion of the PDR16 gene influences the plasma membrane properties of the yeast Kluyveromyces lactis

The plasma membrane is the first line of cell defense against changes in external environment, thus its integrity and functionality are of utmost importance. The plasma membrane properties depend on both its protein and lipid composition. The PDR16 gene is involved in the control of Kluyveromyces lactis susceptibility to drugs and alkali metal cations. It encodes the homologue of the major K. lactis phosphatidylinositol transfer protein Sec14p. Sec14p participates in protein secretion, regulation of lipid synthesis, and turnover in vivo. We report here that the plasma membrane of the Klpdr16Δ mutant is hyperpolarized and its fluidity is lower than that of the parental strain. In addition, protoplasts prepared from the Klpdr16Δ cells display decreased stability when subjected to hypo-osmotic conditions. These changes in membrane properties lead to an accumulation of radiolabeled fluconazole and lithium cations inside mutant cells. Our results point to the fact that the PDR16 gene of K. lactis (KlPDR16) influences the plasma membrane properties in K. lactis that lead to subsequent changes in susceptibility to a broad range of xenobiotics.

Stress response and expression of fluconazole resistance associated genes in the pathogenic yeast Candida glabrata deleted in the CgPDR16 gene

In yeasts, the PDR16 gene encodes a phosphatidylinositol transfer protein which belongs to the Sec14 homologue (SFH) family and localizes to lipid droplets, microsomes and at the cell periphery. The loss of its function alters the lipid droplet metabolism and plasma membrane properties, and renders yeast cells more sensitive to azole antimycotics. In this study, the entire chromosomal CgPDR16 ORF was replaced by the ScURA3 gene both in azole sensitive and azole resistant strains of Candida glabrata bearing a gain-of-function mutation in the CgPDR1 gene, and their responses to different stresses were assessed. The CgPDR16 deletion was found to sensitize the mutant strains to azole antifungals without changes in their osmo- and halotolerance. Fluconazole treated pdr16Δ mutant strains displayed a reduced expression of several genes involved in azole tolerance. The gain-of-function CgPDR1 allele as well as the cycloheximide and hydrogen peroxide treatments of cells enhanced the expression of the CgPDR16 gene. The results indicate that CgPDR16 belongs to genes whose expression is induced by chemical and oxidative stresses. The loss of its function can attenuate the expression of drug efflux pump encoding genes that might also contribute to the decreased azole tolerance in pdr16Δ mutant cells.

Phosphatidylinositol binding of Saccharomyces cerevisiae Pdr16p represents an essential feature of this lipid transfer protein to provide protection against azole antifungals

Pdr16p is considered a factor of clinical azole resistance in fungal pathogens. The most distinct phenotype of yeast cells lacking Pdr16p is their increased susceptibility to azole and morpholine antifungals. Pdr16p (also known as Sfh3p) of Saccharomyces cerevisiae belongs to the Sec14 family of phosphatidylinositol transfer proteins. It facilitates transfer of phosphatidylinositol (PI) between membrane compartments in in vitro systems. We generated Pdr16p^{E235A, K267A} mutant defective in PI binding. This PI binding deficient mutant is not able to fulfill the role of Pdr16p in protection against azole and morpholine antifungals, providing evidence that PI binding is critical for Pdr16 function in modulation of sterol metabolism in response to these two types of antifungal drugs. A novel feature of Pdr16p, and especially of Pdr16p^{E235A, K267A} mutant, to bind sterol molecules, is observed.

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Yeast Pdr16p binds phosphatidylinositol (PI) and cholesterol in lipid binding assay.

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Pdr16p^{E235A, K267A} is defective in PI binding, it binds sterols instead of PI.

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Pdr16p defective in PI binding does not fulfill Pdr16p role in azole protection.

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PI binding of Pdr16p is critical for its function.

Mutation of the CgPDR16 gene attenuates azole tolerance and biofilm production in pathogenic Candida glabrata

The PDR16 gene encodes the homologue of Sec14p, participating in protein secretion, regulation of lipid synthesis and turnover in vivo and acting as a phosphatidylinositol transfer protein in vitro. This gene is also involved in the regulation of multidrug resistance in Saccharomyces cerevisiae and pathogenic yeasts. Here we report the results of functional analysis of the CgPDR16 gene, whose mutation has been previously shown to enhance fluconazole sensitivity in Candida glabrata mutant cells. We have cloned the CgPDR16 gene, which was able to complement the pdr16Δ mutation in both C. glabrata and S. cerevisiae. Along with fluconazole, the pdr16Δ mutation resulted in increased susceptibility of mutant cells to several azole antifungals without changes in sensitivity to polyene antibiotics, cycloheximide, NQO, 5-fluorocytosine and oxidants inducing the intracellular formation of reactive oxygen species. The susceptibility of the pdr16Δ mutant strain to itraconazole and 5-fluorocytosine was enhanced by CTBT [7-chlorotetrazolo(5,1-c)benzo(1,2,4)triazine] inducing oxidative stress. The pdr16Δ mutation increased the accumulation of rhodamine 6G in mutant cells, decreased the level of itraconazole resistance caused by gain-of-function mutations in the CgPDR1 gene, and reduced cell surface hydrophobicity and biofilm production. These results point to the pleiotropic phenotype of the pdr16Δ mutant and support the role of the CgPDR16 gene in the control of drug susceptibility and virulence in the pathogenic C. glabrata.