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 -326 mV and -350 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 in flowers, hypocotyl, cotyledons, leaves, stems, and flower stalks. Barely detectable in roots and siliques. Present in root tips and lateral roots. Accumulates in the cotyledons of etiolated seedlings. In flowers, expressed in the sepals and the style. In siliques, present in the tip and the base, in funiculus and in mature seeds. Present in both dried and imbibed seeds, especially in the seed coat and micropyle. Insensitive to light/darkness, anaerobic treatment and heat, but repressed by sucrose. 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.