Chaperonins, type

Proteasomes are sfrikingly similar in architecture, though not in peptide sequences, to another particle found in both bacteria and eukaryotes a molecular "chaperone" or chaperonin. The chaperonins, of which there are several types, protect proteins while they fold or undergo franslocation within cells. 

Llorca, O., E. A. McCormack, G. Hynes, J. Grantham, J. Cordell, J. L. Carrascosa, K. R. Willison, J. J. Fernandez, and J. M. Valpuesta. 1999. Eukaryotic type II chaperonin CCT interacts with actin through specific subunits. Nature 402 693-6. 

TYPE II CHAPERONINS, PREFOLDIN, AND THE TUBULIN-SPECIFIC CHAPERONES... 

II. Discovery of Type II Chaperonins and Early Functional Studies. 74... 

In common with the polypeptides of Type I chaperonins, each subunit of Type II chaperonins is organized into three recognizable domains an equatorial domain that contains the ATP-binding site and forms the inter-ring contacts, an apical domain that contains the sites of interaction with target protein, and an intermediate domain that forms a hingelike connection between the two (Ditzel et al, 1998 Klumpp et al., 1997 ... 

Waldmann et al., 1995). The extent of amino acid sequence identity between Type I and Type II chaperonins is only 15—25%, implying a very early divergence from a common ancestor occurring more than two billion years ago (Gutsche et al., 1999 Kubota et al., 1994 Kubota etal., 1995a). 

The target range of the archaeal Type II chaperonins remains to be investigated. No natural in vivo targets have been identified to date, although there are reports of facilitated folding in vitro of several thermophilic enzymes by the thermosomes from Sulfolobus solfataricus (Guagliardi et al., 1994) and Methanococcus thermolithotrophicus (Furutani etal., 1998). 

Whether or not target proteins bound to Type II chaperonins undergo similar cycles of binding and release has been a matter of controversy. On the one hand, CCT-generated tubulin intermediates can be efficiently trapped by the inclusion in the folding reaction of a molar excess of... 

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