Chaperonin produced by an intracellular symbiont is an energy-coupling protein with phosphotransferase activity.

Symbionin, a GroEL homologous molecular chaperone produced by an intracellular symbiont of the pea aphid, is autocatalytically phosphorylated in vitro at elevated temperatures. The phosphorylated symbionin showed a potent suppressive activity in spontaneous refolding of chemically denatured dihydrofolate reductase. When the 32P-labeled autophosphorylated symbionin was incubated with ADP, a portion of the radioactivity was transferred to ADP, suggesting that the autocatalytically phosphorylated symbionin contains high-energy phosphate bonds. It was also shown that when symbionin was incubated with [gamma-32P]ATP and GDP, a large amount of the radioactivity was found in GTP, indicating that phosphate transfer between ATP and GDP is catalyzed by symbionin. These results suggested that in the endosymbiotic system symbionin functions as not only a molecular chaperone but also an energy-coupling protein.