Chloroplasts have a novel Cpn10 in addition to Cpn20 as co-chaperonins in Arabidopsis thaliana

Previously, we characterized a mitochondrial co-chaperonin (Cpn10) and a chloroplast co-chaperonin (Cpn20) from Arabidopsis thaliana (Koumoto, Y., Tsugeki, R., Shimada, T., Mori, H, Kondo, M, Hara-Nishimura, I, and Nishimura, M. (1996) Plant J.. 10, 1119-1125; Koumoto, Y., Shimada, T., Kondo, M., Taka, T., Shimonishi, Y., Hara-Nishimura, I, and Nishimura, M (1999) Plant J. 17, 467-477). Here, we report a third co-chaperonin. The cDNA was 603 base pairs long, encoding a protein of 139 amino acids. From a sequence analysis, the protein was predicted to have one Cpn10 domain with an amino-terminal extension that might work as a chloroplast transit peptide. This novel Cpn10 was confirmed to be localized in chloroplasts, and we refer to it as chloroplast Cpn10 (chI-Cpn10). The phylogenic tree that was generated with amino acid sequences of other co-chaperonins indicates that chl-Cpn10 is highly divergent from the others. In the GroEL-assisted protein folding assay,. about 30% of the substrates were refolded with chI-Cpn10. indicating that chI-Cpn10 works as a co-chaperonin. A Northern blot analysis revealed that mRNA for chI-Cpn10 is accumulated in the leaves and stems, but not in the roots. In germinating cotyledons, the accumulation of chI-Cpn10 was similar to that of chloroplastic proteins and accelerated by light. It was proposed that two kinds of co-chaperonins, Cpn20 and chI-Cpn10, work independently in the chloroplast.