Evidence for a cytochrome bcc–aa 3 interaction in the respiratory chain of Mycobacterium smegmatis

Abstract

Spectroscopic analysis of membranes isolated fromMycobacterium smegmatis, along with analysis of its genome, indicates that the cytochromecbranch of its respiratory pathway consists of a modifiedbc1complex that contains two cytochromescin itsc1subunit, similar to other acid-fast bacteria, and anaa3-type cytochromecoxidase. A functional association of the cytochromebccandaa3complexes was indicated by the findings that levels of detergent sufficient to completely disrupt isolated membranes failed to inhibit quinol-driven O2reduction, but known inhibitors of thebc1complex did inhibit quinol-driven O2reduction. The gene for subunit II of theaa3-type oxidase indicates the presence of additional charged residues in a predicted extramembrane domain, which could mediate an intercomplex association. However, high concentrations of monovalent salts had no effect on O2reduction, suggesting that ionic interactions between extramembrane domains do not play the major role in stabilizing thebcc–aa3interaction. Divalent cations did inhibit electron transfer, likely by distorting the electron-transfer interface between cytochromec1and subunit II. Soluble cytochromeccannot donate electrons to theaa3-type oxidase, even though key cytochromec-binding residues are conserved, probably because the additional residues of subunit II prevent the binding of soluble cytochromec. The results indicate that hydrophobic interactions are the primary forces maintaining thebcc–aa3interaction, but ionic interactions may assist in aligning the two complexes for efficient electron transfer.