Acts as a linker essential in the assembly of a core complex of PRK/GAPDH. Coordinates the reversible inactivation of chloroplast enzymes GAPDH and PRK during darkness in photosynthetic tissues. E(0) are -332 mV and -373 mV for the disulfide bonds at pH 7.9. Monomer (By similarity). Component of a complex that contains two dimers of PRK, two tetramers of GAPDH and CP12. CP12 associates with GAPDH, causing its conformation to change. This GAPDH/CP12 complex binds PRK to form a half-complex (one unit). This unit probably dimerizes due partially to interactions between the enzymes of each unit. Mostly expressed, at low levels, in stems and, to a lesser extent, in leaves and roots. In flowers, restricted to the stigma and anthers. Insensitive to light/darkness. Slightly repressed by heat. Induced by anaerobic treatment. Contains two disulfide bonds; only the oxidized protein, with two disulfide bonds, is active in complex formation. The C-terminal disulfide is involved in the interaction with GAPDH and the N-terminal disulfide mediates the binding of PRK with this binary complex. Binds copper and nickel ions. Copper ions catalyze the oxidation of reduced thiol groups and thus promote formation of the disulfide bonds required for linker activity (By similarity). Belongs to the CP12 family.