Thioredoxin-dependent regulation of photosynthetic glyceraldehyde-3-phosphate dehydrogenase: autonomous vs. CP12-dependent mechanisms

Regulation of the Calvin–Benson cycle under varying light/dark conditions is a common property of oxygenic photosynthetic organisms and photosynthetic glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is one of the targets of this complex regulatory system. In cyanobacteria and most algae, photosynthetic GAPDH is a homotetramer of GapA subunits which do not contain regulatory domains. In these organisms, dark-inhibition of the Calvin–Benson cycle involves the formation of a kinetically inhibited supramolecular complex between GAPDH, the regulatory peptide CP12 and phosphoribulokinase. Conditions prevailing in the dark, i.e. oxidation of thioredoxins and low NADP(H)/NAD(H) ratio promote aggregation. Although this regulatory system has been inherited in higher plants, these phototrophs contain in addition a second type of GAPDH subunits (GapB) resulting from the fusion of GapA with the C-terminal half of CP12. Heterotetrameric A₂B₂-GAPDH constitutes the major photosynthetic GAPDH isoform of higher plants chloroplasts and coexists with CP12 and A₄-GAPDH. GapB subunits of A₂B₂-GAPDH have inherited from CP12 a regulatory domain (CTE for C-terminal extension) which makes the enzyme sensitive to thioredoxins and pyridine nucleotides, resembling the GAPDH/CP12/PRK system. The two systems are similar in other respects: oxidizing conditions and low NADP(H)/NAD(H) ratios promote aggregation of A₂B₂-GAPDH into strongly inactivated A₈B₈-GAPDH hexadecamers, and both CP12 and CTE specifically affect the NADPH-dependent activity of GAPDH. The alternative, lower activity with NADH is always unaffected. Based on the crystal structure of spinach A₄-GAPDH and the analysis of site-specific mutants, a model of the autonomous (CP12-independent) regulatory mechanism of A₂B₂-GAPDH is proposed. Both CP12 and CTE seem to regulate different photosynthetic GAPDH isoforms according to a common and ancient molecular mechanism.