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. L ike GroEL and GroES of Escherichia coli, the chloroplast chaperonins c an physically interact with each other. The asymmetric complexes that form in the presence of ADP are ''bullet-shaped'' particles that likel y consist of 1 mol each of ch-cpn60 and ch-cpn10. The purified ch-cpn6 0 is a functional molecular chaperone. Under ''nonpermissive'' conditi ons, where spontaneous folding was not observed, it was able to assist in the refolding of two different target proteins. In both cases, suc cessful partitioning to the native state also required ATP hydrolysis and chaperonin 10. Surprisingly, how ever, the ''double-domain'' ch-cp n10, comprised of unique 21-kDa subunits, was not an obligatory co-cha peronin. 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 g roEL homologs is not a universal property of all chaperonin 60s.