FC0101
Calvin cycle protein CP12  -  GAPDH

Biological function
CP12 is a widespread regulatory protein of oxygenic photosynthetic organisms that contributes to the regulation of the Calvin cycle by forming a supra-molecular complex with at least two enzymes: glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK).

Structural evidence
The labeled variants C31S and C23S allowed the spin label dynamics grafted at Cys23 and Cys31 to be monitored. Addition of GAPDH in an equimolar ratio or in molar excess did not induce any significant change in the EPR spectral shape, the N terminal region of CP12 remains almost as mobile as in the unbound state. Although this N-terminal region can fold into an α-helical structure under oxidized conditions or upon adding TFE, EPR results show that the labeled CP12 variants remain fully disordered even after binding to GAPDH. The C-terminal region in the vicinity of the binding site is also dynamic, around the spin label at position Tyr78.

Biochemical evidence
K_D for CP12 for GAPDH is nM. In vitro reconstitution tests supported that the binary complex with the N-terminal regions was formed. Y78S and D80K abolished the regulation of GAPDH in the supra-molecular complex.

Mechanism category
tethering

Significance
Fuzziness is also important for dynamic assembly and disassembly processes controlled
 by dark/light transitions, for example it facilitates binding of PRK to CP12–GAPDH to form the ternary supra-molecular complex.