Chloroplast chaperone required for RuBisCo complex biogenesis and translational regulation of the RuBisCo large subunit (RbcL) (PubMed:10330470, PubMed:12239405). 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). Involved in the differentiation of bundle sheath cells, especially chloroplast structure (PubMed:12239405). Interacts with the RuBisCo large subunit (RbcL) assembled as an intermediate complex made of eight RbcL and eight BSD2 subunits. Associates with chloroplastic polysomes. Expressed in shoot tissues, in both bundle sheath and mesophyll cells. Relatively abundant in etiolated, light-shifted, and young (plastochron 1 to 5) leaves. Mu-active bsd2-m1 mutants are specifically disrupted C4 differentiation in bundle sheath cells with reduced levels of bundle sheath cell-specific photosynthetic enzymes and aberrant bundle sheath chloroplast structure and altered RuBisCo complex formation (PubMed:12239405). Ectopic accumulation of RuBisCo large subunit (RbcL) in mesophyll cells (PubMed:12239405). Reduced accumulation in Mu-inactivated bsd2-m1 mutants exhibiting slightly pale green and grainy leaves (due to the random presence of altered bundle sheath cell chloroplasts) associated with the loss of RuBisCo complex formation, but accumulation of chloroplast-encoded RbcL transcripts associated with polysomes in both bundle sheath and mesophyll cells (PubMed:10330470). RbcL transcription is abnormally similar in both dark-grown and light-shifted seedlings (PubMed:10330470). Belongs to the BSD2 chaperone family.