Backbone assignment of CcdB_G100T toxin from E.coli in complex with the toxin binding C-terminal domain of its cognate antitoxin CcdA.

The CcdAB system expressed in the E.coli cells is a prototypical example of the bacterial toxin-antitoxin (TA) systems that ensure the survival of the bacterial population under adverse environmental conditions. The solution and crystal structures of CcdA, CcdB and of CcdB in complex with the toxin-binding C-terminal domain of CcdA have been reported. Our interest lies in the dynamics of CcdB-CcdA complex formation. Solution NMR studies have shown that CcdB_G100T, in presence of saturating concentrations of CcdA-c, a truncated C-terminal fragment of CcdA exists in equilibrium between two major populations. Sequence specific backbone resonance assignments of both equilibrium forms of the ~ 27 kDa complex, have been obtained from a suite of triple resonance NMR experiments acquired on 2 H, 13 C, 15 N enriched samples of CcdB_G100T. Analysis of 1 H, 13 C α , 13 C β secondary chemical shifts, shows that both equilibrium forms of CcdB_G100T have five beta-strands and one alpha-helix as the major secondary structural elements in the tertiary structure. The results of these studies are presented below.