Alternative splicing directs dual localization of Candida albicans 6-phosphogluconate dehydrogenase to cytosol and peroxisomes

The pentose phosphate pathway (PPP) is the main source of NADPH in the cell and therefore essential for the maintenance of the redox balance and anabolic reactions. NADPH is produced by the two dehydrogenases in the oxidative branch of the PPP: glucose-6-phosphate dehydrogenase (Zwf1) and 6-phosphogluconate dehydrogenase (Gnd1). We observed that in the commensal fungus Candida albicans these two enzymes contain putative peroxisomal targeting signals (PTSs): Zwf1 has a putative PTS1, while the annotated intron of GND1 encodes a PTS2. By subcellular fractionation and fluorescence microscopy, we show that both enzymes have a dual localization in which the majority is cytosolic, but a small fraction is peroxisome associated. Analysis of GND1 transcripts revealed that dual targeting of Gnd1 is directed by alternative splicing resulting in two Gnd1 isoforms, one without targeting signals localized to the cytosol and one with an N-terminal PTS2 targeted to peroxisomes. To our knowledge, Gnd1 is the first example of dual targeting of a protein by alternative splicing in C. albicans. In silico analysis suggests that PTS-mediated peroxisomal targeting of Zwf1 and Gnd1 is conserved across closely related Candida species. We discuss putative functions of the peroxisomal oxidative PPP in these organisms.