A significant proportion of bacteria express two or more chaperonin genes.
Chaperonins are a group of molecular chaperones, defined by sequence similarity,
required for the folding of some cellular proteins. Chaperoninmonomers have amass
of c. 60 kDa, and are typically found as large protein complexes containing 14
subunits arranged in two rings. The mechanism of action of the Escherichia coli
GroEL protein has been studied in great detail. It acts by binding to unfolded proteins
and enabling them to fold in a protected environment where they do not interact
with any other proteins. GroEL can assist the folding of many proteins of different
sizes, sequences, and structures, and homologues from many different bacteria can
functionally replace GroEL in E. coli. What then are the functions of multiple
chaperonins? Do they provide a mechanism for cells to increase their general
chaperoning ability, or have they become specialized to take on specific novel cellular
roles? Here I will review the genetic, biochemical, and phylogenetic evidence that has
a bearing on this question, and show that there is good evidence for at least some
specificity of function in multiple chaperonin genes.