Three Dimensional Structure of the MqsR:MqsA Complex: A Novel TA Pair Comprised of a Toxin Homologous to RelE and an Antitoxin with Unique Properties
Figure 3
MqsA is a structured antitoxin.
(A) The MqsA-F monomer can be visualized as a ‘leg’: MqsA-C (the HTH-XRE domain; light blue) is the thigh, the flexible linker (centered on T68) is the knee and MqsA-N (the zinc binding domain; dark blue) is the ‘calf’ and ‘foot’; the zinc (teal sphere) is bound by the ‘toes’. (B) The zinc is coordinated by Cys3, Cys6, Cys37 and Cys40 (blue/yellow sticks). Sulfur-zinc distances are commensurate with values typical for structural zinc binding sites. Residues near the zinc binding pocket are also shown (grey sticks). (C) The MqsA zinc binding domain adopts a novel fold characterized by a long, twisted β-sheet buttressed by a five-turn α-helix with the loops coordinated by zinc. It is stabilized by two hydrophobic cores (core 1, sidechains shown as sticks in beige; core 2, sidechains shown as sticks in orange). (D) The MqsA-C dimerization interface is composed of the MqsA-C α-helices 3, 5, and 6, with the two monomers (chain A, grey; chain B, light blue) related by local two-fold symmetry. The sequence most highly conserved among MqsA proteins (residues 98–105, green) is predicted to bind DNA (see text). (E) The MqsA-C dimerization interface rotated by 90° with one monomer represented in surface representation (1600 Å2 of buried SASA illustrated in beige) and the second in ribbon representation. (F) Residues buried at the MqsA-C dimer interface are shown as sticks (chain B) and the charge distribution is shown in surface representation (chain A; positive charge, blue; negative charge, red).