Abstract
Type I chaperonins are large double-ring assemblies present in bacteria (GroEL), mitochondria and chloroplasts (Cpn60) and they are involved in the efficient protein folding. In E. coli, GroEL consists of 14 identical subunits and 85 substrates exhibit an obligate dependence on GroEL for folding, suggesting a broad range of GroEL substrates. In contrast, chloroplasts comprise two different Cpn60 isoforms, α and β. Two Cpn60α and four Cpn60β subunits were found in Arabidopsis chloroplasts and they were assembled into mixed Cpn60α7β7 oligomer. While cpn60α and cpn60β1β2 mutants are lethal, cpn60β4 was only defective in NDH activity. Consistent with the phenotype of cpn60β4, an NDH subunit, NdhH, was detected in the chaperonin complex containing Cpn60β4 as a substrate by mass analysis. Cpn60β4 has an additional C-terminal extension compared with other 3 Cpn60β subunits and this extension was shown to be critical for the specific function of Cpn60β4 on the NDH assembly. These facts imply that Cpn60β4 has acquired some structure features during the evolution to accomplish the folding of the specific substrate NdhH.
