Required for assembly or stability of RuBisCO. Acts at a postchaperonin step to fold and/or assemble the large subunit (LS) into RuBisCO. Homotrimer. Expressed in bundle sheath. Has 3 domains, the N-terminal alpha-helical domain, an extended flexible linker and the C-terminal beta-sheet domain. The N-terminal alpha-helical domain stabilizes RbcL dimers and RbcL dimer-dimer interactions, facilitating RbcL(8) formation. The C-terminal beta-sheet domain probably dimerizes Raf1 (By similarity). The 2 C-terminal beta-sheet domains are swapped and pack against each other to form the dimer interface (By similarity). Seedling lethal due to a lack of RuBisCO. Belongs to the RAF family.