Mycobacterial chaperonins: the tail wags the dog.

Molecular chaperones are defined as proteins that assist the noncovalent assembly of other protein-containing structures in vivo, but which are not components of these structures when they are carrying out their normal biological functions. There are numerous families of protein that fit this definition of molecular chaperones, the most ubiquitous of which are the chaperonins and the Hsp70 families, both of which are required for the correct folding of nascent polypeptide chains and thus essential genes for cell viability. The groE genes of Escherichia coli were the first chaperonin genes to be discovered, within an operon comprising two genes, groEL and groES, that function together in the correct folding of nascent polypeptide chains. The identification of multiple groEL genes in mycobacteria, only one of which is operon-encoded with a groES gene, has led to debate about the functions of their encoded proteins, especially as the essential copies are surprisingly often not the operon-encoded genes. Comparisons of these protein sequences reveals a consistent functional homology and identifies an actinomycete-specific chaperonin family, which may chaperone the folding of enzymes involved in mycolic acid synthesis and thus provide a unique target for the development of a new class of broad-spectrum antimycobacterial drugs.