Intra- and Inter-Complex Cross-Linking of Subunits in the Quinol Oxidase Super-Complex from Thermophilic Bacillus PS3

Abstract

Gram-positive thermophilic Bacilli contain quinol-cytochrome c reductase and cytochrome c oxidase as two major respiratory complexes of the electron transfer chain, and these enzymes can be extracted with mild detergents as an associated quinol oxidase supercomplex. The reductase is composed of three subunits; cytochrome b₆, cytochrome c₁, and FeS protein, whereas cytochrome c oxidase consists of four subunits numbered 1 through 4. In order to clarify the interactions between the subunits, the super-complex isolated from Bacillus PS3 was cross-linked with three bifunctional cross-linkers; disuccinimidyl tartrate, 3, 3'-dithiobis(succinimidylpropionate), and ethylene glycolbis(sulfosuccinimidylsuccinate). The most prominent cross-linking was observed for the combination of subunit 1 plus 2 in cytochrome c oxidase, and for that of cytochrome b₆ plus cytochrome c₁ in the reductase. In addition to these intra-complex cross-linkings, inter-complex linking was observed for the combination of cytochrome b₆ plus subunit 1 with ethylene glycolbis(sulfosuccinimidylsuccinate), and for the combinations of cytochrome b₆ plus subunit 1 and cytochrome b₆ plus subunit 2 with 3, 3'-dithiobis(succinimidylpropionate). Incubation in the presence of Triton X-100, which was confirmed to cleave the two enzyme complexes, selectively reduced the inter-complex cross-linking, suggesting that the chemical crosslinking reflect the spatial arrangement of subunits in the super-complex.