Gene duplication and the evolution of group II chaperonins: Implications for structure and function

Chaperonins are multisubunit protein-folding assemblies. They are composed of two distinct structural classes, which also have a characteristic phylog enetic distribution. Group I chaperonins (called GroEL/cpn60/hsp60) are pre sent in Bacteria and eukaryotic organelles while group II chaperonins are f ound in Archaea (called the thermosome or TF55) and the cytoplasm of eukary otes (called CCT or TriC). Gene duplication has been an important force in the evolution of group II chaperonins: Archaea possess one, two, or three h omologous chaperonin subunit-encoding genes, and eight distinct CCT gene fa milies (paralogs) have been described in eukaryotes. Phylogenetic analyses indicate that while the duplications in archaeal chaperonin genes have occu rred numerous times independently in a lineage-specific fashion, the eight different CCT subunits found in eukaryotes are the products of duplications that occurred early and very likely only once in the evolution of the euka ryotic nuclear genome. Analyses of CCT sequences from diverse eukaryotic sp ecies reveal that each of the CCT subunits possesses a suite of invariant s ubunit-specific amino acid residues ("signatures"). When mapped onto the cr ystal structure of the archaeal chaperonin from Thermoplasma acidophilum, t hese signatures are located in the apical, intermediate, and equatorial dom ains. Regions that were found to be variable in length and/or amino acid se quence were localized primarily to the exterior of the molecule and, signif icantly, to the extreme tip of the apical. domain (the "helical protrusion" ). In light of recent biochemical and electron microscopic data describing specific CCT-substrate interactions, our results have implications for the evolution of subunit-specific functions in CCT. (C) 2001 Academic Press.