title: Thioredoxin-dependent regulation of photosynthetic glyceraldehyde-3-phosphate dehydrogenase: autonomous vs. CP12-dependent mechanisms.
abstract: 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.
authors.affiliation: Laboratory of Molecular Plant Physiology, Department of Evolutionary Experimental Biology, University of Bologna, Via Irnerio 42, Bologna, 40126, Italy. trost@alma.unibo.it
authors.forename: P
authors.initials: P
authors.lastname: Trost
authors.forename: S
authors.initials: S
authors.lastname: Fermani
authors.forename: L
authors.initials: L
authors.lastname: Marri
authors.forename: M
authors.initials: M
authors.lastname: Zaffagnini
authors.forename: G
authors.initials: G
authors.lastname: Falini
authors.forename: S
authors.initials: S
authors.lastname: Scagliarini
authors.forename: P
authors.initials: P
authors.lastname: Pupillo
authors.forename: F
authors.initials: F
authors.lastname: Sparla
mesh_terms: D024602:Glyceraldehyde-3-Phosphate Dehydrogenase (NADP+)(Phosphorylating) / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology*; D010940:Plant Proteins / Q000378:metabolism*; D013879:Thioredoxins / Q000378:metabolism*
chemical_list: D010940:Plant Proteins; D013879:Thioredoxins; D024602:Glyceraldehyde-3-Phosphate Dehydrogenase (NADP+)(Phosphorylating)