MBE Advance Access originally published online on June 27, 2003
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Mol. Biol. Evol. 20(10):1588-1597. 2003
DOI: 10.1093/molbev/msg160
© 2003 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038
Positive Selection and Subfunctionalization of Duplicated CCT Chaperonin Subunits
Department of Genetics, Smurfit Institute, University of Dublin, Trinity College, Dublin, Ireland
E-mail: faresm{at}tcd.ie.
To reach a functional and energetically stable conformation, many proteins need molecular helpers called chaperonins. Among the group II chaperonins, CCT proteins provide crucial machinery for the stabilization and proper folding of several proteins in the cytosol of eukaryotic cells through interactions that are subunit-specific and geometry-dependent. CCT proteins are made up of eight different subunits, all with similar sequences, positioned in a precise arrangement. Each subunit has been proposed to have a specialized function during the binding and folding of the CCT protein substrate. Here, we demonstrate that functional divergence occurred after several CCT duplication events due to the fixation of amino acid substitutions by positive selection. Sites critical for ATP binding and substrate binding were found to have undergone positive selection and functional divergence predominantly in subunits that bind tubulin but not actin. Furthermore, we show clear functional divergence between CCT subunits that bind the C-terminal domains of actin and tubulin and those that bind the N-terminal domains. Phylogenetic analyses could not resolve the deep relationships between most subunits, except for the groups 
/ß/
and 
/
, suggesting several almost simultaneous ancient duplication events. Together, the results support the idea that, in contrast to homo-oligomeric chaperonins such as GroEL, the high divergence level between CCT subunits is the result of positive selection after each duplication event to provide a specialized role for each CCT subunit in the different steps of protein folding.
Key Words: CCT • duplication • positive selection • functional divergence • convergent evolution • protein folding
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