Chloroplast chaperone required for RuBisCo biogenesis and translational regulation of the RuBisCo large subunit (RbcL) (PubMed:25124725). Stabilizes an end-state assembly intermediate of eight RbcL subunits until the small subunits (RBCSs) become available to produce a complete stable RuBisCo complex containing eight small and eight large subunits (By similarity). Interacts with the RuBisCo large subunit (RbcL) assembled as an intermediate complex made of eight RbcL and eight BSD2 subunits. Associates with chloroplastic polysomes. Belongs to the BSD2 chaperone family.