Glyceraldehyde-3-phosphate dehydrogenase is regulated by ferredoxin-NADP reductase in the diatom Asterionella formosa

Diatoms are a widespread and ecologically important group of heterokont algae that contribute about 20% to global productivity. Previous work has shown that regulation of key Calvin cycle enzymes in diatoms differs from that of the Plantae, and that in crude extracts, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) can be inhibited by NADPH under oxidizing conditions. Here, chromatography, mass spectrometry and sequence analysis showed that in the freshwater diatom, Asterionella formosa, GAPDH can interact with ferredoxin-NADP reductase (FNR) from the primary phase of photosynthesis, and the small chloroplast protein, CP12. In contrast, the ternary complex between GAPDH, phosphoribulokinase (PRK) and CP12, that is widespread in Plantae and cyanobacteria, was absent. Surface plasmon resonance measurements confirmed that GAPDH and FNR are able to interact. Activity measurements under oxidizing conditions, showed that NADPH can inhibit GAPDH-CP12 in the presence of FNR from A. formosa or Spinacia oleracea, explaining the earlier observed inhibition within crude extracts. Diatom plastids have distinctive attributes including the lack of the oxidative pentose phosphate pathway and so cannot produce NADPH in the dark. The observed down-regulation of GAPDH may allow NADPH to be re-routed towards other reductive processes contributing to their ecological success.