Identification of a gene encoding a homocitrate synthase isoenzyme of Saccharomyces cerevisiae

The Expanding Landscape of Moonlighting Proteins in Yeasts

Moonlighting proteins are multifunctional proteins that participate in unrelated biological processes and that are not the result of gene fusion. A certain number of these proteins have been characterized in yeasts, and the easy genetic manipulation of these microorganisms has been useful for a thorough analysis of some cases of moonlighting. As the awareness of the moonlighting phenomenon has increased, a growing number of these proteins are being uncovered. In this review, we present a crop of newly identified moonlighting proteins from yeasts and discuss the experimental evidence that qualifies them to be classified as such. The variety of moonlighting functions encompassed by the proteins considered extends from control of transcription to DNA repair or binding to plasminogen. We also discuss several questions pertaining to the moonlighting condition in general. The cases presented show that yeasts are important organisms to be used as tools to understand different aspects of moonlighting proteins.

Evaluation of lysine biosynthesis as an antifungal drug target: biochemical characterization of Aspergillus fumigatus homocitrate synthase and virulence studies

Aspergillus fumigatus is the main cause of severe invasive aspergillosis. To combat this life-threatening infection, only limited numbers of antifungals are available. The fungal alpha-aminoadipate pathway, which is essential for lysine biosynthesis, has been suggested as a potential antifungal drug target. Here we reanalyzed the role of this pathway for establishment of invasive aspergillosis in murine models. We selected the first pathway-specific enzyme, homocitrate synthase (HcsA), for biochemical characterization and for study of its role in virulence. A. fumigatus HcsA was specific for the substrates acetyl-coenzyme A (acetyl-CoA) and alpha-ketoglutarate, and its activity was independent of any metal ions. In contrast to the case for other homocitrate synthases, enzymatic activity was hardly affected by lysine and gene expression increased under conditions of lysine supplementation. An hcsA deletion mutant was lysine auxotrophic and unable to germinate on unhydrolyzed proteins given as a sole nutrient source. However, the addition of partially purified A. fumigatus proteases restored growth, confirming the importance of free lysine to complement auxotrophy. In contrast to lysine-auxotrophic mutants from other fungal species, the mutant grew on blood and serum, indicating the existence of high-affinity lysine uptake systems. In agreement, although the virulence of the mutant was strongly attenuated in murine models of bronchopulmonary aspergillosis, virulence was partially restored by lysine supplementation via the drinking water. Additionally, in contrast to the case for attenuated pulmonary infections, the mutant retained full virulence when injected intravenously. Therefore, we concluded that inhibition of fungal lysine biosynthesis, at least for disseminating invasive aspergillosis, does not appear to provide a suitable target for new antifungals.

Lysine-overproducing mutants of Saccharomyces cerevisiae baker's yeast isolated in continuous culture

Saccharomyces cerevisiae baker's yeast mutants which produce 3 to 17 times as much lysine as the wild type, depending on the nitrogen source, have been selected. The baker's yeast strain was growth in a pH-regulated chemostat in minimal medium with proline as the nitrogen source, supplemented with increasing concentrations of the toxic analog of the lysine S-2-aminoethyl-L-cysteine (AEC). The lysine-overproducing mutants, which were isolated as AEC-resistant mutants, were also resistant to high external concentrations of lysine and to alpha-aminoadipate and seemed to be affected in the lysine biosynthetic pathway but not in the biosynthetic pathways of other amino acids. Lysine overproduction by one of the mutants seemed to be due to, at least, the loss of repression of the homocitrate synthase encoded by the LYS20 gene. The mutant grew slower than the wild type, and its dough-raising capacity was reduced in in vitro assays, probably due to the toxic effects of lysine accumulation or of an intermediate produced in the pathway. This mutant can be added as a food supplement to enrich the nutritive qualities of bakery products, and its resistance to alpha-aminoadipate, AEC, and lysine can be used as a dominant marker.

Purification of the Azotobacter vinelandii nifV-encoded homocitrate synthase

The nifV gene product (NifV) from Azotobacter vinelandii was recombinantly expressed at high levels in Escherichia coli and purified. NifV is a homodimer that catalyzes the condensation of acetyl coenzyme A (acetyl-CoA) and alpha-ketoglutarate. Although alpha-ketoglutarate supports the highest level of activity, NifV will also catalyze the condensation of acetyl-CoA and certain other keto acids. E. coli cells in which a high level of nifV expression is induced excrete homocitrate into the growth medium.

Homocitrate synthase is located in the nucleus in the yeast Saccharomyces cerevisiae

We have generated monoclonal antibodies against nuclear proteins from the yeast Saccharomyces cerevisiae. The monoclonal antibodies react with proteins of 47 and 49 kDa on immunoblots and with partially overlapping sets of proteins on two-dimensional nonequilibrium pH gradient electrophoresis-SDS blots. Immunofluorescence localization shows a nuclear staining pattern. Immunoscreening a yeast expression library yielded five independent full-length clones of two open reading frames from chromosome IV, corresponding to YDL182w (LYS20) and YDL131w in the Saccharomyces genome data base. These two open reading frames are predicted to encode homocitrate synthase isozymes of 47 and 49 kDa, respectively. A clone carrying YDL182w was sequenced in its entirety and directs the expression of a 47-kDa protein in Escherichia coli. A clone carrying YDL131w expresses a 49-kDa protein in E. coli. Yeast grown in minimal medium plus lysine show significant reductions in nuclear immunofluorescence staining. Cell fractionation studies localize the 47- and 49-kDa proteins to the nucleus. Nuclear fractionation studies reveal that a portion of the 47- and 49-kDa proteins can only be extracted with DNase digestion and high salt. The localization of homocitrate synthase to the nucleus is unexpected given previous reports that homocitrate synthase is present in mitochondria and the cytoplasm in S. cerevisiae.