Self-Assembly of Symbionin, a Chaperonin of Intracellular Symbiont1

Abstract

Symbionin, a homologue of Escherichia coli GroEL, which functions as a molecular chaperone in the aphid endosymbiont, exists as a double-doughnut structure, consisting of two rings of seven 63-kDa subunits. Symbionin had a more labile oligomeric structure than GroEL and completely disassembled into monomeric (1-mer) components in 3 M urea. The urea-dissociated symbionin self-assembled into 14-mer symbionin in a Mg-ATP dependent manner. When 1-mer symbionin was incubated in the presence of Mg-ATP, its reassembly proceeded hyperbolically with time. The yield of reassembled symbionin increased in response to increase in the initial concentration of 1-mer symbionin. The reassembled symbionin not only had the same molecular mass as native symbionin but also exhibited the same ATPase and phosphotransferase activity, suggesting that the correct three-dimensional structure was restored in vitro. While the self-assembly of symbionin was markedly stimulated by the presence of reassembled symbionin, it was not affected by the presence of native symbionin, which may suggest that native symbionin contains component(s) inhibitory to its chaperoning activity. As in the self-assembly of GroEL, that of symbionin was stimulated by the presence of GroES.