Mechanisms of aurothiomalate-Cys2His2 zinc finger interactions

Zinc finger motifs are present in a wide variety of regulatory proteins and generally function as interaction modules between macromolecules. These functional interactions are controlled by mechanisms of zinc (Zn2+)-binding and release. Besides Zn2+ certain electrophilic metals can potentially react with zinc finger domains and lead to changes in the structure and function of those domains. In these studies, the Cys2His2 zinc finger was chosen as a model for understanding how the gold (I) (Au1+) drug, aurothiomalate (AuTM), interacts mechanistically with the Zn2+ coordination sphere. DNA binding assays were used to analyze functional interactions between AuTM and two model Cys2His2 zinc finger transcription factors, TFIIIA and Sp1; inhibition in the micromolar range of AuTM was observed in both cases. Electrospray ionization mass spectrometry (ESI-MS) was utilized to examine molecular interactions between AuTM and a zinc finger peptide modeled after the third finger of Sp1 (Sp1-3). These experiments demonstrated Au1+ ions can bind the zinc finger structure and trigger the release of the Zn2+ ion. Quantifying the ESI-MS data allowed for a relative affinity value between Zn2+ and Au1+ ions to be calculated and shows Au1+ has a 4-fold higher affinity for Sp1-3 than Zn2+. Mechanistic differences between Zn2+ and Au1+ binding to the model Sp1-3 zinc finger were analyzed at isotopic resolution, and the metal-coordination spheres were probed with small molecules (H+, hydrogen peroxide, glutathione disulfide, and iodoacetamide). Natural isotope cluster analysis suggested the presence of a metal-thiol bond in the Cys2His2 zinc finger structure. Metal exchange reactions between zinc fingers demonstrated Zn2+ ions exchanged more rapidly than Au1+ ions. Circular dichroism (CD) exhibited differences in the secondary structure of the Sp1-3 model peptide when binding Zn2+ or Au1+ ions.