Asymmetric functional interaction between chaperonin and its plastidic cofactors.

The specific cochaperonin, chloroplast chaperonin (Cpn)20, consisting of two tandem Gro ES-like domains, is present abundantly in plant and algal chloroplasts, in addition to Cpn10, which is similar in size to Gro ES. How Cpn20 oligomers, containing six or eight 10-kDa domains, cooperate with the heptameric ring of chaperonin at the same time as encountering symmetry mismatch is unclear. In the present study, we characterized the functional cooperation of cochaperonins, including two plastidic Cpn20 homo-oligomers from Arabidopsis (AtCpn20) and Chlamydomonas (Cr CPN20), and one algal Cr CPNs hetero-oligomer, consisting of three cochaperonins, Cr CPN11, Cr CPN20 and Cr CPN23, with two chaperonins, Escherichia coli Gro EL and Chlamydomonas Cr CPN60. AtCpn20 and Cr CPNs were functional for assisting both chaperonins in folding model substrates ribulose bisphosphate carboxylase oxygenase from Rhodospirillum rubrum (RrRubisco) in vitro and complementing Gro ES function in E. coli. Cr CPN20 cooperated only with Cr CPN60 (and not Gro EL) to refold RrRubisco in vitro and showed differential complementation with the two chaperonins in E. coli. Cochaperonin concatamers, consisting of six to eight covalently linked 10-kDa domains, were functionally similar to their respective native forms. Our results indicate that symmetrical match between chaperonin and cochaperonin is not an absolute requisite for functional cooperation. [ABSTRACT FROM AUTHOR]