HSP60: Family Members

The 60 kDa chaperonin family of proteins (HSP60, HSPD1, CPN60, CCT) consists of molecular chaperones of approximately 60 kDa in size comprising stress inducible and constitutively expressed members ⁶². The chaperonin family includes GroEL, 60-kDa heat shock protein (Hsp60), RuBisCO subunit-binding protein (RBP) and thermophilic factor 55 (TF55). The family members exhibit ATPase activity which is essential for the reaction cycle and the binding and release of partially or misfolded substrate proteins. The chaperonins assemble into multisubunit protein-folding complexes that fall into two major groups. Group I chaperonins (GroEL, Cpn60, Hsp60) are present in bacteria, mitochondria, hydrogenosomes, and chloroplasts, while group II chaperonins (CCTs) can be found in the cytosol of eukaryotes and in Archaea. Apart from E. coli which possesses one Hsp60 gene (groEL), several eubacteria contain up to seven HSP60/groEL homologs ^{63, 64}. Archaea possess one to three homologous chaperonin-encoding genes except for Methanosarcina acetivorans which expresses five genes ⁶⁵. Eight distinct CCT gene families (paralogs) have been described in eukaryotes (see also Table 2).

The human chaperonin HSP60/CCT gene family consists of at least ten members while in zebrafish (Danio rerio) and the yeast Saccharomyces cerevisiae nine isoforms of Hsp60/Cct can be found (Table 2). The genomes of the unicellular green alga Chlamydomonas reinhardtii and the fruit fly D. melanogaster are somewhat unusual among eukaryotes in that they contain four HSP60 genes only. A detailed description of pro- and eukaryotic HSP60 family members is given in Table 2 and the following paragraphs.