Functional characterization of the higher plant chloroplast chaperonins

The higher plant chloroplast chaperonins (ch-cpn60 and ch-cpn10) have been purified and their structural/functional properties examined. In all plants surveyed, both proteins were constitutively expressed, and only modest increases in their levels were detected upon heat shock. Like GroEL and GroES of Escherichia coli, the chloroplast chaperonins can physically interact with each other. The asymmetric complexes that form in the presence of ADP are 'bullet-shaped' particles that likely consist of 1 mol each of ch-cpn60 and ch-cpn10. The purified ch-cpn60 is a functional molecular chaperone. Under 'nonpermissive' conditions, where spontaneous folding was not observed, it was able to assist in the refolding of two different target proteins. In both cases, successful partitioning to the native state also required ATP hydrolysis and chaperonin 10. Surprisingly, however, the 'double-domain' ch- cpn10, comprised of unique 21-kDa subunits, was not an obligatory co- chaperonin. Both GroES and a mammalian mitochondrial homolog were equally compatible with the ch-cpn60. Finally, the assisted-folding reaction mediated by the chloroplast chaperonins does not require K⁺ ions. Thus, the K⁺- dependent ATPase activity that is observed with other known groEL homologs is not a universal property of all chaperonin 60s.