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 A(2)B(2)-GAPDH constitutes the major photosynthetic GAPDH isoform of higher plants chloroplasts and coexists with CP12 and A(4)-GAPDH. GapB subunits of A(2)B(2)-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 A(2)B(2)-GAPDH into strongly inactivated A(8)B(8)-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 A(4)-GAPDH and the analysis of site-specific mutants, a model of the autonomous (CP12-independent) regulatory mechanism of A(2)B(2)-GAPDH is proposed. Both CP12 and CTE seem to regulate different photosynthetic GAPDH isoforms according to a common and ancient molecular mechanism.
Trost
P
P
Laboratory of Molecular Plant Physiology, Department of Evolutionary Experimental Biology, University of Bologna, Via Irnerio 42, Bologna, 40126, Italy. trost@alma.unibo.it
Fermani
S
S
Marri
L
L
Zaffagnini
M
M
Falini
G
G
Scagliarini
S
S
Pupillo
P
P
Sparla
F
F
D024602:Glyceraldehyde-3-Phosphate Dehydrogenase (NADP+)(Phosphorylating) / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology*; D010940:Plant Proteins / Q000378:metabolism*; D013879:Thioredoxins / Q000378:metabolism*
D010940:Plant Proteins; D013879:Thioredoxins; D024602:Glyceraldehyde-3-Phosphate Dehydrogenase (NADP+)(Phosphorylating)