In Streptomyces lividans an extracytoplasmic copper-binding Sco protein plays a
role in two unlinked processes: (i) initiating a morphological development
switch and (ii) facilitating the co-factoring of the CuA domain of CcO
(cytochrome c oxidase). How Sco obtains copper once secreted to the
extracytoplasmic environment is unknown. In the present paper we report on a
protein possessing an HX6MX21HXM motif that binds a single cuprous ion with
subfemtomolar affinity. High-resolution X-ray structures of this
extracytoplasmic copper chaperone-like protein (ECuC) in the apo- and
Cu(I)-bound states reveal that the latter possesses a surface-accessible
cuprous-ion-binding site located in a dish-shaped region of β-sheet structure.
A cuprous ion is transferred under a favourable thermodynamic gradient from ECuC
to Sco with no back transfer occurring. The ionization properties of the
cysteine residues in the Cys86xxxCys90 copper-binding motif of Sco, together
with their positional locations identified from an X-ray structure of Sco,
suggests a role for Cys86 in initiating an inter-complex ligand-exchange
reaction with Cu(I)-ECuC. Generation of the genetic knockouts, Δsco, Δecuc and
Δsco/ecuc, and subsequent in vivo assays lend support to the existence of a
branched extracytoplasmic copper-trafficking pathway in S. lividans. One branch
requires both Sco and to a certain extent ECuC to cofactor the CuA domain,
whereas the other uses only Sco to deliver copper to a cuproenzyme to initiate
morphological development.
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