HicAB toxin-antitoxin complex from Escherichia coli: expression and crystallization

Toxin-antitoxin (TA) systems are widespread in both bacteria and archaea, where they enable cells to adapt to environmental cues. TA systems play crucial roles in various cellular processes, such as programmed cell death, cell growth, persistence and virulence. Here, two distinct forms of the type II toxin-antitoxin complex HicAB were identified and characterized in Escherichia coli K-12, and both were successfully overexpressed and purified. The two proposed forms, HicAB^L and HicAB^S, differed in the presence or absence of a seven-amino-acid segment at the N-terminus in the antitoxin HicB. The short form HicAB^S readily crystallized under the conditions 0.1 M Tris-HCl pH 8.0, 20%(w/v) PEG 6000, 0.2 M ammonium sulfate. The HicAB^S crystal diffracted and data were collected to 2.5 Å resolution. The crystal belonged to space group I222 or I2₁2₁2₁, with unit-cell parameters a = 67.04, b = 66.31, c = 120.78 Å. Matthews coefficient calculation suggested the presence of two molecules each of HicA and HicB^S in the asymmetric unit, with a solvent content of 55.28% and a Matthews coefficient (V_M) of 2.75 ų Da^-1.