Evolution of the chaperonin families (HSP60, HSP 10 and TCP‐1) of proteins and the origin of eukaryotic cells
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SummaryThe members of the 10 kDa and 60 kDa heat‐shock chaperonin proteins (Hsp10 and Hsp60 or Cpn10 and Cpn60), which form an operon in bacteria, are present in all eubacteria and eukaryotic ceil organelles such as mitochondria and chloroplasts. In archaebacteria and eukaryotic cell cytosol, no close homologues of Hsp10 or Hsp60 have been identified. However, these species (or ceil compartments) contain the Tcp‐1 family of proteins (distant homologues of Hsp60). Phylogenetic analysis based on global alignments of Hsp60 and Hsp10 sequences presented here provide some evidence regarding the evolution of mitochondria from a member of the α‐subdivision of Gram‐negative bacteria and chloroplasts from cyanobacterial species, respectively. This inference is strengthened by the presence of sequence signatures that are uniquely shared between Hsp60 homologues from α‐purple bacteria and mitochondria on one hand, and the chloroplasts and cyanobacterial hsp60s on the other. Within the α‐purple subdivision, species such as Rickettsia and Ehrlichia, which live intracellularly within eukaryotic cells, are indicated to be the closest relatives of mitochondrial Homologues, In the Hsp60 evolutionary tree, rooted using the Tcp‐1 homologue, the order of branching of the major groups was as follows: Gram‐positive bacteria — cyanobacteria and chloroplasts — chlamydiae and spirochaetes —β and γ‐Gram‐negative purple bacteria —α‐purple bacteria — mitochondria. A similar branching order was observed independently in the Hsp10 tree. Multiple Hsp60 homologues, when present in a group of species, were found to be clustered together in the trees, indicating that they evolved by independent gene‐duplication events. This review also considers in detail the evolutionary relationship between Hsp50 and Tcp‐1 families of proteins based on two different models (viz. archaebacterial and chimeric) for the origin of eukaryotic cell nucleus. Some predictions of the chimeric model are also discussed.
