All known co-chaperonin protein 10 (cpn10) molecules are heptamers of seven identical subunits that are linked together by beta-strand interactions. Here, we report the first characterization of a cpn10 protein from a thermophilic organism: Aquifex aeolicus. Primary-structure alignment of A. aeolicus cpn10 (Aaecpn10) shows high homology with mesophilic cpn10 sequences, except for a unique 25-residue C-terminal extension not found in any other cpnl0. Recombinant Aaecpn10 adopts a heptameric structure in solution at pH values above 4 (20degreesC). Both monomers and heptamers are folded at 20degreesC, although the thermal stability of the monomers (pH 3; T-m similar to 58 degreesC) is lower than that of the heptamers (pH 7; T-m similar to 115 degreesC). Aaecpn 10 functions in a GroEL-dependent in vitro activity assay. Taken together, Aaecpn 10 appears similar in secondary, tertiary, and quaternary structure, as well as in many biophysical features, to its mesophilic counterparts despite a functional temperature of 90degreesC. (C) 2004 Elsevier Inc. All rights reserved.