The activity of the glyoxylate cycle in peroxisomes of Candida albicans depends on a functional β-oxidation pathway: evidence for reduced metabolite transport across the peroxisomal membrane

摘要

The glyoxylate cycle, a metabolic pathway required for generating C4 units from C2 compounds, is an important factor in virulence, in both animal and plant pathogens. Here, we report the localization of the key enzymes of this cycle, isocitrate lyase (Icl1; EC 4.1.3.1) and malate synthase (Mls1; EC 2.3.3.9), in the human fungal pathogen Candida albicans. Immunocytochemistry in combination with subcellular fractionation showed that both Icl1 and Mls1 are localized to peroxisomes, independent of the carbon source used. Although Icl1 and Mls1 lack a consensus type I peroxisomal targeting signal (PTS1), their import into peroxisomes was dependent on the PTS1 receptor Pex5p, suggesting the presence of non-canonical targeting signals in both proteins. Peroxisomal compartmentalization of the glyoxylate cycle is not essential for proper functioning of this metabolic pathway because a pex5Δ/Δ strain, in which Icl1 and Mls1 were localized to the cytosol, grew equally as well as the wild-type strain on acetate and ethanol. Previously, we reported that a fox2Δ/Δ strain that is completely deficient in fatty acid β-oxidation, but has no peroxisomal protein import defect, displayed strongly reduced growth on non-fermentable carbon sources such as acetate and ethanol. Here, we show that growth of the fox2Δ/Δ strain on these carbon compounds can be restored when Icl1 and Mls1 are relocated to the cytosol by deleting the PEX5 gene. We hypothesize that the fox2Δ/Δ strain is disturbed in the transport of glyoxylate cycle products and/or acetyl-CoA across the peroxisomal membrane and discuss the possible relationship between such a transport defect and the presence of giant peroxisomes in the fox2Δ/Δ mutant.