SilE is an intrinsically disordered periplasmic “molecular sponge” involved in bacterial silver resistance

Abstract

SummaryAg+ resistance was initially found on the Salmonella enetrica serovar Typhimurium multi‐resistance plasmid pMG101 from burns patients in 1975. The putative model of Ag+ resistance, encoded by the sil operon from pMG101, involves export of Ag+ via an ATPase (SilP), an effluxer complex (SilCFBA) and a periplasmic chaperon of Ag+ (SilE). SilE is predicted to be intrinsically disordered. We tested this hypothesis using structural and biophysical studies and show that SilE is an intrinsically disordered protein in its free apo‐form but folds to a compact structure upon optimal binding to six Ag+ ions in its holo‐form. Sequence analyses and site‐directed mutagenesis established the importance of histidine and methionine containing motifs for Ag+‐binding, and identified a nucleation core that initiates Ag+‐mediated folding of SilE. We conclude that SilE is a molecular sponge for absorbing metal ions.

Model peptide studies of Ag+binding sites from the silver resistance protein SilE

“…26 Furthermore, two ''core-motifs'' have been previously identified to be the first nucleation sites of SilE for the Ag + -mediated folding. 14 The HEFM motif, which shows the highest affinity for silver ions (log K ass = 6.6), is a part of one of these segments (M108-M121 /peptide stoichiometry of the nine herein described models is closer to this second assumption, as it suggests a maximum of 9 Ag + bound to the protein.…”

Spontaneous mutation in the AgrRS two-component regulatory system ofCupriavidus metalliduransresults in enhanced silver resistance

“…21,22 Although the function of PsrQ 1 and PsrQ 2 still needs to be determined, they do not contain conserved histidine and methionine residues within specific sequence motifs that are involved in Ag + binding like SilE and PcoE. 22,64 As the increased silver resistance is not mediated by efflux, we hypothesize that the formation of silver nanoparticles prevents ionic silver to exert its action in the cytoplasm. The latter provides C. metallidurans the ability to withstand much higher silver concentrations than efflux-mediated resistance.…”

“…Such disordered proteins are common in nature and include SilE and PcoE, which have been shown to be involved in silver resistance. 22,64 Both SilE and PcoE act as a 'metal sponge', fold upon binding to Ag + and transport Ag + to the associated HME-RND efflux pump. 21,22 Although the function of PsrQ 1 and PsrQ 2 still needs to be determined, they do not contain conserved histidine and methionine residues within specific sequence motifs that are involved in Ag + binding like SilE and PcoE.…”

Bacteria and nanosilver: the quest for optimal production

“…It has been postulated that it is the aforementioned SilE protein that chelates and then presents Ag + to a reductase, (15,52) as supported by the discovery that a number of bacteria, which are able to produce nanoparticles, carry the sil genes. (38 These mechanisms remain to be understood before they can be fully exploited.…”