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)
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
-type cytochrome c
oxidase
(cyt aa). 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 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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