TCB Publications - Abstract

Chang Sun, Samir Benlekbir, Padmaja Venkatakrishnan, Yuhang Wang, Emad Tajkhorshid, John L. Rubinstein, and Robert B. Gennis. Structure of the alternative complex III in a supercomplex with cytochrome oxidase. Nature, 557:123-126, 2018. (PMC: PMC6004266)

SUN2018-ET Alternative Complex III (ACIII) is a key component of the respiratory and/or photosynthetic electron transport chains of many bacteria. Like Complex III (bc$_1$ complex), ACIII catalyzes the oxidation of membrane- bound quinol and reduction of cytochrome c or an equivalent electron carrier. However, the two complexes have no structural similarity. Although ACIII has eluded structural characterization, several of its subunits are homologous to members of the CISM (Complex Iron-Sulfur Molybdoenzyme) superfamily, including the proton pump polysulfide reductase. We isolated the ACIII from Flavobacterium johnsoniae with some of its native lipids using styrene maleic acid (SMA) copolymer, both as an independent enzyme and as a functional 1:1 supercomplex with an aa$_3$ -type cytochrome c oxidase (cyt aa$_3$). We determined the structure of ACIII to 3.4 Å resolution by cryo-EM and constructed an atomic model for its six subunits. The structure, which contains a [3Fe-4S] cluster, a [4Fe-4S] cluster, and six hemes c, shows that ACIII carries out its catalytic reaction employing known elements from other electron transport complexes arranged in a previously unknown manner. Modeling of the cyt aa$_3$ component of the supercomplex revealed that it is structurally modified to facilitate association with ACIII, illustrating the importance of the supercomplex in this electron transport chain. The structure also resolves two of the subunits of ACIII that are anchored to the lipid bilayer with N-terminal triacylated cysteine residues, an important post-translational modification found in numerous prokaryotic membrane proteins that has not previously been observed structurally in a lipid bilayer.

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