title: Alternative zinc-binding sites explain the redox sensitivity of zinc-containing anti-sigma factors.
abstract: Certain bacterial zinc-containing anti-sigma (ZAS) factors respond sensitively to thiol-induced oxidative stress by undergoing conformational changes, which in turn reduce binding affinities for their cognate sigma factors. This redox sensitivity provides a mechanism for coping with oxidative stress by activating the transcription of antioxidant genes. Not all ZAS proteins are redox-sensitive, but the mechanism of redox sensitivity is not fully understood. Here we propose that alternative zinc-binding sites determine redox sensitivity. To support this proposal, we performed protein modeling and zinc docking on redox-sensitive and redox-insensitive ZAS proteins complexed with their cognate sigma factors. At least one strong alternative zinc-binding pocket was detected for all known redox-sensitive ZAS factors in actinomycetes, while no strong alternative zinc-binding pocket was identified in redox-insensitive ZAS factors, except for one controversial case. This hypothesis of alternative zinc-binding sites can also explain residue-specific contributions to the redox sensitivity of RsrA, a redox-sensing ZAS protein from Streptomyces coelicolor, for which alanine mutagenesis experiments are available. Our results suggest a mechanistic model for redox sensitivity as follows: zinc ion can probabilistically occupy multiple sites in redox-sensitive ZAS proteins, increasing the susceptibility of zinc-coordinating cysteine residues to oxidation. This picture of probabilistic zinc occupation agrees with a previous structure and energy analysis on zinc finger proteins, and thus it may be more widely applicable to other classes of reactive zinc-binding proteins.
authors.affiliation: Department of Chemistry, Seoul National University, Seoul, 151-747, Republic of Korea.
authors.forename: Lim
authors.initials: L
authors.lastname: Heo
authors.forename: Yoo-Bok
authors.initials: YB
authors.lastname: Cho
authors.forename: Myeong Sup
authors.initials: MS
authors.lastname: Lee
authors.forename: Jung-Hye
authors.initials: JH
authors.lastname: Roe
authors.forename: Chaok
authors.initials: C
authors.lastname: Seok
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites; D056004:Molecular Dynamics Simulation; D010084:Oxidation-Reduction; D012680:Sensitivity and Specificity; D012808:Sigma Factor / Q000037:antagonists & inhibitors*; D014157:Transcription Factors / Q000737:chemistry* / Q000378:metabolism*; D015032:Zinc / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; C121147:RsrA protein, Streptomyces coelicolor; D012808:Sigma Factor; D014157:Transcription Factors; D015032:Zinc
keywords: ZAS; anti-sigma factor; redox sensitivity; zinc binding proteins; zinc binding site