AN ENDOSYMBIONT CHAPERONIN IS A NOVEL TYPE OF HISTIDINE PROTEIN-KINASE

Symbionin, a GroEL homologous molecular chaperone produced by an intra cellular symbiont of the pea aphid, is able to transfer its high-energ y phosphate bond to other compounds through its autophosphorylation. W hen the urea-dissociated monomeric symbionin fixed onto a polyvinylide ne difluoride membrane was incubated with [gamma-P-32] ATP, it was eff iciently phosphorylated at elevated temperatures. The autophosphorylat ed monomeric P-32-labeled symbionin, when incubated with ADP, transfer red a significant portion of its radioactivity to ADP, suggesting that the autocatalytically phosphorylated monomeric symbionin contains hig h-energy phosphate bonds. It was also shown that when symbiotic protei ns were electrophoretically separated, blotted onto a polyvinylidene d isulfide membrane and incubated with P-32-labeled symbionin, radioacti vity was found on several kinds of polypeptides, indicating that the p hosphoryl group was transferred from symbionin to other symbiotic prot eins. Peptide sequence analysis and thin-layer chromatographic analysi s of the P-32-labeled tryptic fragment of the phosphorylated symbionin revealed that the site of phosphorylation is His-133. These results s uggested that symbionin functions as a histidine protein kinase, or a sensor molecule, of the two-component pathway known in other organisms . However, symbionin is not similar in amino acid sequence to any know n histidine protein kinase.