Evidence for peroxisomal redundancy among the glucose-6-phosphate dehydrogenase isoforms of Arabidopsis thaliana.

The oxidative pentose phosphate pathway (OPPP) plays an important role in the generation of reducing power in all eukaryotes. In plant cells, the OPPP operates in several cellular compartments, but as full cycle only in the plastid stroma where it is essential. As suggested by our recent results, OPPP reactions are also mandatory inside peroxisomes, at least during fertilization. For the first enzyme of the OPPP, glucose-6-phosphate dehydrogenase (G6PD), we previously showed that one Arabidopsis isoform (G6PD1) can be directed to peroxisomes under specific circumstances. Since g6pd1 knock-out plants are viable, we aimed at elucidating potential redundancy regarding peroxisomal targeting among the other G6PD isoforms. Localization studies of so far cytosolic annotated G6PD5 and G6PD6 (both ending -PTL>) using different reporter fusions of full-length versus the last 50 amino acids revealed that GFP-C-short versions are efficiently imported into peroxisomes. Modification of the final tripeptide to a canonical peroxisomal targeting signal type 1 (PTS1) also resulted in peroxisomal localization of the full-length versions and revealed that G6PD5/6 import may occur as homo- or heterodimer. Interestingly, the new noncanonical PTS1 motif is highly conserved among the cytosolic G6PD isoforms of the Angiosperms, whereas members of the Poaceae (rice and maize) possess two variants, one ending with an additional amino acid (-PTLA>) and the other one extended by a stronger PTS1 motif. From both evolutionary and physiological perspectives, we postulate that G6PD import as homo- and heterodimer restricted the acquisition of more efficient peroxisomal targeting motifs to leave some G6PDH activity in the cytosol.