Chloroplast chaperonins:: Evidence for heterogeneous assembly of α and β Cpn60 polypeptides into a chaperonin oligomer

Higher plant chloroplasts contain two chaperonin 60 family proteins, Cpn60 alpha and Cpn60 beta, which are known to have divergent amino acid sequences. Although Cpn60 alpha and Cpn60 beta are present in roughly equal amounts and copurify in their native states, a heterogeneous ensemble of the chaperonin oligomer has not yet been demonstrated. We separately purified Cpn60 alpha and Cpn60 beta proteins from spinach leaves as the monomeric form, Either antibody raised against one chaperonin 60 protein could coimmunoprecipitate the other chaperonin 60 protein in their oligomeric state but mot in its monomeric state, suggesting that the chloroplast Cpn60 alpha and Cpn60 beta polypeptides actually reside in the same chaperonin oligomer in the stroma. Moreover, the chaperonin oligomers migrated as at least two distinct bands on the native gel electrophoresis, each of which contained both chaperonin 60 proteins. These results suggest that chloroplast chaperonin oligomers might be composed of at least two distinct sets of two chaperonin proteins. (C) 1999 Academic Press.