Unravelling the shape and structural assembly of the photosynthetic GAPDH–CP12–PRK complex fromArabidopsis thalianaby small-angle X-ray scattering analysis

Abstract

Oxygenic photosynthetic organisms produce sugars through the Calvin–Benson cycle, a metabolism that is tightly linked to the light reactions of photosynthesis and is regulated by different mechanisms, including the formation of protein complexes. Two enzymes of the cycle, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK), form a supramolecular complex with the regulatory protein CP12 with the formula (GAPDH–CP122–PRK)2, in which both enzyme activities are transiently inhibited during the night. Small-angle X-ray scattering analysis performed on both the GAPDH–CP12–PRK complex and its components, GAPDH–CP12 and PRK, fromArabidopsis thalianashowed that (i) PRK has an elongated, bent and screwed shape, (ii) the oxidized N-terminal region of CP12 that is not embedded in the GAPDH–CP12 complex prefers a compact conformation and (iii) the interaction of PRK with the N-terminal region of CP12 favours the approach of two GAPDH tetramers. The interaction between the GAPDH tetramers may contribute to the overall stabilization of the GAPDH–CP12–PRK complex, the structure of which is presented here for the first time.