Structural and mechanistic insights into an extracytoplasmic copper trafficking pathway in Streptomyces lividans

In Streptomyces lividans an extracytoplasmic copper binding Sco protein plays a role in two unlinked processes; 1) initiating a morphological development switch and 2) facilitating the co-factoring of the CuA domain of cytochrome c oxidase (CcO). How Sco obtains copper once secreted to the extracytoplasmic environment is unknown. Here we report on a protein possessing a 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 ionisation properties of the cysteine residues in the C86XXXC90 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 lends 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 co-factor the CuA domain, while the other utilises only Sco to deliver copper to a cuproenzyme to...
Source: BJ Cell - Category: Biochemistry Authors: Tags: BJ Biomolecules Source Type: research