Copper trafficking in the CsoR regulon of Streptomyces lividans

In the actinobacterium Streptomyces lividans copper homeostasis is controlled through the action of the metalloregulator CsoR. Under copper stress, cuprous ions bind to apo-CsoR resulting in the transcriptional derepression of genes encoding for copper efflux systems involving CopZ-like copper chaperones and CopA-like P-type ATPases. Whether CsoR obtains copper via a protein-protein mediated trafficking mechanism is unknown. In this study we have characterised the copper trafficking properties of two S. lividans CopZ proteins (SLI_1317 and SLI_3079) under the transcriptional control of a CsoR (SLI_4375). Our findings indicate that both CopZ-proteins have cysteine residues in the Cu(i) binding MX1CX2X3C motif with acid-base properties that are modulated for a high cuprous ion affinity and favourable Cu(i)-exchange with a target. Using electrophoretic mobility shift assays transfer of Cu(i) is shown to occur in a unidirectional manner from the CopZ to the CsoR. This transfer proceeds via a shallow thermodynamic affinity gradient and is also kinetically favoured through the modulation of the acid-base properties of the cysteine residues in the Cys2His cuprous ion binding motif of CsoR. Using RNA-seq coupled with the mechanistic insights of Cu(i) transfer between CopZ and CsoR in vitro, we propose a copper trafficking pathway for the CsoR regulon that initially involves the buffering of cytosolic copper by three CopZ chaperones followed by transfer of Cu(i) to CsoR to illicit a transcriptional response.