Evidence for Peroxisomal Redundancy among the Glucose-6-phosphate Dehydrogenase Isoforms of Arabidopsis thaliana.

The oxidative pentose phosphate pathway (OPPP) plays an important role for 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 fertilisation. For the first enzyme of the OPPP, glucose-6-phosphate dehydrogenase (G6PD), so far only one Arabidopsis isoform (G6PD1) was shown to 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. Localisation 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 localisation of the full-length versions, and revealed that G6PD5/6 import may occur as homo- or heterodimer. Interestingly, the new non-canonical PTS1 motif is highly conserved among the cytosolic G6PD isoforms of the Angiosperms, whereas members of the Poaceae (rice, maize) possess two variants, one ending with an additional amino acid (-PTLA>), and the other extended by a stronger PTS1 motif. From both evolutionary and physiological perspectives, we postulate that import as homo- and heterodimer restricted the acquisition of more efficient peroxisomal targeting motifs to leave some G6PDH activity to the cytosol.