Origin and evolution of eukaryotic chaperonins: Phylogenetic evidence for ancient duplications in CCT genes

Chaperonins are oligomeric protein-folding complexes which an divided into two distantly related structural classes. Group I chaperonins (called GroEL /cpn60/hsp60) are found in bacteria and eukaryotic organelles, while group II chaperonins are present in archaea and the cytoplasm of eukaryotes (call ed CCT/TriC). While archaea possess one to three chaperonin subunit-encodin g genes, eight distinct CCT gene families (paralogs) have been characterize d in eukaryotes. We are interested in determining when during eukaryotic ev olution the multiple gene duplications producing the CCT subunits occurred. We describe the sequence and phylogenetic analysis of five CCT genes from Trichomonas vaginalis and seven from Giardia lamblia, representatives of am itochondriate protist lineages thought to have diverged early from other eu karyotes. Our data show that the gene duplications producing the eight CCT paralogs took place prior to the organismal divergence of Trichomonas and G iardia from other eukaryotes. Thus, these divergent protists likely possess completely hetero-oligomeric CCT complexes like those in yeast and mammali an cells. No close phylogenetic relationship between the archaeal chaperoni ns and specific CCT subunits was observed, suggesting that none of the CCT gene duplications predate the divergence of archaea and eukaryotes. The dup lications producing the CCT delta and CCT epsilon subunits, as well as CCT alpha, CCT beta, and CCT eta, are the most recent in the CCT gene family. O ur analyses show significant differences in the rates of evolution of archa eal chaperonins compared with the eukaryotic CCTs, as well as among the dif ferent CCT subunits themselves. We discuss these results in light of curren t views on the origin, evolution, and function of CCT complexes.