Chaperonins are involved in protein-folding. The rice genome encodes six plastid chaperonin subunits (Cpn60) - three ? and three ?. Our study showed that they were differentially expressed during normal plant development. Moreover, five were induced by heat stress (42?C) but not by cold (10°C). The oscpn60?1 mutant had a pale-green phenotype at the seedling stage and development ceased after the fourth leaf appeared. Transiently expressed OsCpn60?1:GFP fusion protein was localized to the chloroplast stroma. Immuno-blot analysis indicated that the level of Rubisco large subunit (rbcL) was severely reduced in the mutant while levels were unchanged for some imported proteins, e.g., stromal heat shock protein 70 (Hsp70) and chlorophyll a/b binding protein 1 (Lhcb1). This demonstrated that OsCpn60?1 is required for the folding of rbcL and that failure of that process is seedling-lethal.

Keywords: chaperonin, Cpn60, plastid, rice, Rubisco