The VapBC1 toxin-antitoxin complex from Mycobacterium tuberculosis: purification, crystallization and X-ray diffraction analysis

Mycobacterium tuberculosis, a major human pathogen, encodes at least 88 toxin-antitoxin (TA) systems. Remarkably, more than half of these modules belong to the VapBC family. Under normal growth conditions, the toxicity of the toxin VapC is neutralized by the protein antitoxin VapB. When bacteria face an unfavourable environment, the antitoxin is degraded and the free toxin VapC targets important cellular processes in order to inhibit cell growth. TA systems function in many biological processes, such as in the stringent response, in biofilm formation and in drug tolerance. To explore the structure of the VapBC1 complex, the toxin VapC1 and the antitoxin VapB1 were separately cloned, co-expressed and crystallized. The best crystal was obtained using a crystallization solution consisting of optimized solution with commercial sparse-matrix screen solutions as additives. The crystal diffracted to a resolution of 2.7 angstrom and belonged to space group P2(1), with unit-cell parameters a = 59.3, b = 106.7, c = 250.0 angstrom, beta = 93.75 degrees.