Chaperone proteins double as enzymes in TB bacteria

Protein-folding machines called GroEL/Hsp60 chaperonins are already known as central players in cell metabolism, stress adaptation and survival. These proteins typically assemble into a tetradecameric structure and, coupled to ATP hydrolysis, assist the folding of about 10% of all cellular proteins. In work led by Véronique Fontaine, researchers set out to look beyond that classic role. Using recombinant proteins from three sources—E. coli, human mitochondria and M. tuberculosis—the team tested whether GroEL/Hsp60 family members might carry out other chemical reactions. Rather than acting only as passive folding helpers, the chaperonins showed enzymatic activities not usually attributed to them: thioesterase and esterase reactions, and in some cases auto-acyltransferase activity. The discoveries were most striking when the researchers compared different assembly states: smaller oligomers of Hsp60 and M. tuberculosis GroEL1 were more likely to use a long acyl carbon chain substrate, palmitoyl-CoA, than the fully assembled tetradecameric E. coli GroEL and Hsp60. These findings expand the functional picture of GroEL/Hsp60 proteins beyond their textbook role in protein folding.

The experimental approach, described in the abstract, relied on recombinant E. coli, human mitochondrial and M. tuberculosis chaperonins to probe novel enzymatic functions. By testing activity against substrates such as palmitoyl-CoA, the researchers detected thioesterase and esterase activities and, for some variants, an ability to transfer acyl groups in an auto-acyltransferase reaction. Comparing oligomeric states revealed that smaller oligomers of Hsp60 and M. tuberculosis GroEL1 preferentially processed the long acyl chain palmitoyl-CoA, while tetradecameric E. coli GroEL and Hsp60 were less prone to do so. To pinpoint parts of the protein responsible for these enzymatic behaviors, the team used enzymatic competition experiments and replacement of residues in M. tuberculosis GroEL1. Those manipulations identified Asp86 and Thr89 in the ATP-binding pocket as important and highlighted an additional residue, Ser393, that influences thioesterase activity. The work also provided evidence that M. tuberculosis GroEL1 might enhance palmitoylation of the PpsE protein, a factor involved in phthiocerol dimycocerosate (PDIM) biosynthesis.

The implications of these findings are twofold. First, they reveal that GroEL/Hsp60 chaperonins can act as a kind of molecular Swiss army knife, combining conventional folding support with chemical activities that modify other molecules. Second, the specific connection between M. tuberculosis GroEL1, palmitoyl-CoA processing and enhanced palmitoylation of PpsE offers a plausible mechanistic link to PDIM biosynthesis. PDIM lipids have been associated with aspects of M. tuberculosis biology, and the authors suggest that GroEL1’s enzymatic activities could at least partly explain its involvement in PDIM biosynthesis and antibiotic resistance. Taken together, the study encourages researchers to think of chaperonins not only as folding machines but also as active metabolic participants, which may reshape how scientists investigate bacterial survival strategies and their contributions to drug resistance.