Analysis of homo- and hetero-dimerization among the three 6-phosphogluconate dehydrogenase isoforms of Arabidopsis

We recently described that all three 6-phosphogluconate dehydrogenase (6PGDH) isoforms of Arabidopsis (PGD) with similar length show dual localization: PGD1 and PGD3 in the cytosol and in plastids, and PGD2 in the cytosol and in peroxisomes. We set out to investigate heterodimer formation, however due to only weak homodimerization of all Arabidopsis PGD isoforms in yeast cells, we conducted further protein-protein interaction studies in planta to investigate homomer versus heteromer formation and their sub-cellular localization. Bimolecular fluorescence complementation (BiFC) analyses in co-transfected Arabidopsis protoplasts demonstrated that all PGD isoforms may form homo- and heterodimers. Notably, with free N-terminal ends, PGD1-PGD3 heterodimers were detected both in the cytosol and in the plastid stroma, but heterodimers with PGD2 only in the cytosol. This independently confirmed that PGD2 cannot enter plastids. On the other hand, with free C-terminal ends, PGD1-PGD2 and PGD3-PGD2 heterodimers were confined to the cytosol, indicating that only PGD2 homodimers are imported by peroxisomes. Together these findings suggest functional redundancy of PGD1 and PGD3 inside plastids, and relevance of PGD1-PGD2 or PGD3-PGD2 heterodimer formation in the cytosol: this could retain sufficient 6PGDH activity needed for NADPH provision, especially during stress defense and initiation of developmental responses.