Identification and characterization of major lipid particle proteins of the yeast Saccharomyces cerevisiae

Conservation of eukaryotic sterol homeostasis: new insights from studies in budding yeast

The model eukaryote Saccharomyces cerevisiae (budding yeast) has provided significant insight into sterol homeostasis. The study of sterol metabolism in a genetically amenable model organism such as yeast is likely to have an even greater impact and relevance to human disease with the advent of the complete human genome sequence. In addition to definition of the sterol biosynthetic pathway, almost to completion, the remarkable conservation of other components of sterol homeostasis are described in this review.

Functional morphology of the secretory pathway organelles in yeast

The glycoprotein secretory pathway of yeast serves mainly for cell surface growth and cell division. It involves a centrifugal transport of transit macromolecules among organelles, whose membranes contain resident proteins needed for driving the transport. These resident membrane proteins return by retrograde vesicular transport. Apart from this, the pathway involves endocytosis. The model yeast Saccharomyces cerevisiae and vertebrate cells were found to contain very similar gene products regulating the molecular mechanism of glycoprotein transport, and the cellular mechanism of their secretion pathways was therefore also presumed to be identical. Biochemists have postulated that, in S. cerevisiae, the translocation of peptides through the endoplasmic reticulum membranes into the lumen of ER cisternae and the core glycosylation is followed by a vector-mediated transport into the functional cascade of the Golgi system cisternae and between them. This is the site of maturation and sorting of glycoproteins, before the ultimate transport by other vectors involving either secretion from the cells (exocytosis across the plasmalemma into the cell wall) or transport into the lysosome-like vacuole via a prevacuolar compartment, which serves at the same time as a primary endosome. The established cellular model of secretion deals with budding yeast; interphase yeast cells, in which the secretion is limited and which predominate in exponential cultures, have not been taken into consideration. The quality of organelle imaging in S. cerevisiae ultra-thin sections depends on the fixation technique used and on specimen contrasting by metals. The results achieved by combinations of different techniques differ mostly in the imaging of bilayers of membrane interfaces and the transparence of the matrix phase. Fixation procedures are decisive for the results of topochemical localisations of cellular antigenic components or enzyme activities, which form the basis of the following survey of functional morphology of organelles involved in the yeast secretory pathway. The existing results of these studies do not confirm all aspects of the vertebrate model of the Golgi apparatus proposed by molecular geneticists to hold for S. cerevisiae, and alternative models of the cellular mechanism of secretion in this yeast are, therefore, also discussed.

Perilipin A increases triacylglycerol storage by decreasing the rate of triacylglycerol hydrolysis

The perilipins are the most abundant proteins at the surfaces of lipid droplets in adipocytes and are also found in steroidogenic cells. To investigate perilipin function, perilipin A, the predominant isoform, was ectopically expressed in fibroblastic 3T3-L1 pre-adipocytes that normally lack the perilipins. In control cells, fluorescent staining of neutral lipids with Bodipy 493/503 showed a few minute and widely dispersed lipid droplets, while in cells stably expressing perilipin A, the lipid droplets were more numerous and tightly clustered in one or two regions of the cytoplasm. Immunofluorescence microscopy revealed that the ectopic perilipin A localized to the surfaces of the tiny clustered lipid droplets; subcellular fractionation of the cells using sucrose gradients confirmed that the perilipin A localized exclusively to lipid droplets. Cells expressing perilipin A stored 6-30-fold more triacylglycerol than control cells due to reduced lipolysis of triacylglycerol stores. The lipolysis of stored triacylglycerol was 5 times slower in lipid-loaded cells expressing perilipin A than in lipid-loaded control cells, when triacylglycerol synthesis was blocked with 6 microm triacsin C. This stabilization of triacylglycerol was not due to the suppression of triacylglycerol lipase activity by the expression of perilipin A. We conclude that perilipin A increases the triacylglycerol content of cells by forming a barrier that reduces the access of soluble lipases to stored lipids, thus inhibiting triacylglycerol hydrolysis. These studies suggest that perilipin A plays a major role in the regulation of triacylglycerol storage and lipolysis in adipocytes.

A novel strategy for constructing N-terminal chromosomal fusions to green fluorescent protein in the yeast Saccharomyces cerevisiae

A novel rapid polymerase chain reaction (PCR)-based technique for N-terminal attachment of green fluorescent protein (GFP) to a yeast protein is described. Genomic integration of a PCR-generated loxPkanMX4loxP-yEGFP fusion cassette immediately upstream of the open reading frame (ORF) allows for selection of G418 resistant transformants carrying GFP fused N-terminally to the protein of interest. In a subsequent step, the loxPkanMX4loxP selection marker that is inserted between the tagged ORF and the endogenous promoter is excised upon site-specific recombination between the loxP sites by Cre recombinase, leaving behind in the promoter one loxP site, immediately upstream of the GFP start codon. The essential protein Ydl193wp of unknown function and the oleate-inducible fatty acid activation protein, encoded by FAA2, were N-terminally tagged using the novel technique. Both experiments yielded viable haploid strains with growth phenotypes indistinguishable from the wild type strain. The subcellular localization pattern for the chromosomally expressed GFP-Ydl193wp to the endoplasmic reticulum and lipid particles was identical to the pattern observed for a plasmid-borne GFP construct expressed under control of the MET25(p) promoter, albeit at a lower level and with a more homogeneous distribution among the cell population. GFP-FAA2 was inducible by oleate, as is the wild type gene, demonstrating that specific expression patterns are not grossly affected by the promoter manipulation. In agreement with previous reports, GFP-Faa2p was found to localize to peroxisomes.

Intracellular lipid particles of eukaryotic cells

In this review article we describe characterization of intracellular lipid particles of three different eukaryotic species, namely mammalian cells, plants and yeast. Lipid particles of all types of cells share a general structure. A hydrophobic core of neutral lipids is surrounded by a membrane monolayer of phospholipids which contains a minor amount of proteins. Whereas lipid particles from mammalian cells and plants harbor specific classes of polypeptides, mainly perilipins and oleosins, respectively, yeast lipid particles contain a more complex set of enzymes which are involved in lipid biosynthesis. Function of lipid particles as storage compartment and metabolic organelle, and their interaction with other subcellular fractions are discussed. Furthermore, models for the biogenesis of lipid particles are presented and compared among the different species.

Murine FATP alleviates growth and biochemical deficiencies of yeast fat1Delta strains

Saccharomyces cerevisiae is an ideal model eukaryote for studying fatty-acid transport. Yeast are auxotrophic for unsaturated fatty acids when grown under hypoxic conditions or when the fatty-acid synthase inhibitor cerulenin is included in the growth media. The FAT1 gene encodes a protein, Fat1p, which is required for maximal levels of fatty-acid import and has an acyl CoA synthetase activity specific for very-long-chain fatty acids suggesting this protein plays a pivotal role in fatty-acid trafficking. In the present work, we present evidence that Fat1p and the murine fatty-acid transport protein (FATP) are functional homologues. FAT1 is essential for growth under hypoxic conditions and when cerulenin was included in the culture media in the presence or absence of unsaturated fatty acids. FAT1 disruptants (fat1Delta) fail to accumulate the fluorescent long-chain fatty acid fatty-acid analogue 4, 4-difluoro-5-methyl-4-bora-3a,4a-diaza-s-indacene-3-do decanoic acid (C1-BODIPY-C12), have a greatly diminished capacity to transport exogenous long-chain fatty acids, and have very long-chain acyl CoA synthetase activities that were 40% wild-type. The depression in very long-chain acyl CoA synthetase activities were not apparent in cells grown in the presence of oleate. Additionally, beta-oxidation of exogenous long-chain fatty acids is depressed to 30% wild-type levels. The reduction of beta-oxidation was correlated with a depression of intracellular oleoyl CoA levels in the fat1Delta strain following incubation of the cells with exogenous oleate. Expression of either Fat1p or murine FATP from a plasmid in a fat1Delta strain restored these phenotypic and biochemical deficiencies. Fat1p and FATP restored growth of fat1Delta cells in the presence of cerulenin and under hypoxic conditions. Furthermore, fatty-acid transport was restored and was found to be chain length specific: octanoate, a medium-chain fatty acid was transported in a Fat1p- and FATP-independent manner while the long-chain fatty acids myristate, palmitate, and oleate required either Fat1p or FATP for maximal levels of transport. Lignoceryl CoA synthetase activities were restored to wild-type levels in fat1Delta strains expressing either Fat1p or FATP. Fat1p or FATP also restored wild-type levels of beta-oxidation of exogenous long-chain fatty acids. These data show that Fat1p and FATP are functionally equivalent when expressed in yeast and play a central role in fatty-acid trafficking.

Biochemical characterization and subcellular localization of the sterol C-24(28) reductase, erg4p, from the yeast saccharomyces cerevisiae

The yeast ERG4 gene encodes sterol C-24(28) reductase which catalyzes the final step in the biosynthesis of ergosterol. Deletion of ERG4 resulted in a complete lack of ergosterol and accumulation of the precursor ergosta-5,7,22,24(28)-tetraen-3beta-ol. An erg4 mutant strain exhibited pleiotropic defects such as hypersensitivity to divalent cations and a number of drugs such as cycloheximide, miconazole, 4-nitroquinoline, fluconazole, and sodium dodecyl sulfate. Similar to erg6 mutants, erg4 mutants are sensitive to the Golgi-destabilizing drug brefeldin A. Enzyme activity measurements with isolated subcellular fractions revealed that Erg4p is localized to the endoplasmic reticulum. This view was confirmed in vivo by fluorescence microscopy of a strain expressing a functional fusion of Erg4p to enhanced green fluorescent protein. We conclude that ergosterol biosynthesis is completed in the endoplasmic reticulum, and the final product is supplied from there to its membranous destinations.

1-Acyldihydroxyacetone-phosphate reductase (Ayr1p) of the yeast Saccharomyces cerevisiae encoded by the open reading frame YIL124w is a major component of lipid particles

Biosynthesis of phosphatidic acid through the dihydroxyacetone phosphate pathway requires NADPH-dependent reduction of the intermediate 1-acyldihydroxyacetone phosphate before the second step of acylation. Studies with isolated subcellular fractions of the yeast Saccharomyces cerevisiae revealed that lipid particles and the endoplasmic reticulum harbor 1-acyldihydroxyacetone-phosphate reductase (ADR) activity. Deletion of the open reading frame YIL124w (in the following named AYR1) abolished reduction of 1-acyldihydroxyacetone phosphate in lipid particles, whereas ADR activity in microsomes of the deletion strain was decreased approximately 3-fold as compared with the wild-type level. This result indicates that (i) both lipid particles and microsomes harbor Ayr1p, which was confirmed by immunological detection of the protein in these two cellular compartments, and (ii) microsomes contain at least one additional ADR activity. As a consequence of this redundancy, deletion of AYR1 neither results in an obvious growth phenotype nor affects the lipid composition of a haploid deletion strain. When a heterozygous AYR1(+)/ayr1(-) diploid strain was subjected to sporulation; however, spores bearing the ayr1 defect failed to germinate, suggesting that Ayr1p plays an essential role at this stage. Overexpression of Ayr1p at a 5- to 10-fold level of wild type caused growth arrest. Heterologous expression of Ayr1p in Escherichia coli resulted in gain of ADR activity in the prokaryote, confirming that YIL124w is the structural gene of the enzyme and does not encode a regulatory or auxiliary component required for reduction of 1-acyldihydroxyacetone phosphate. Taken together, these results identified Ayr1p of the yeast as the first ADR from any organism at the molecular level.