Crystal structure and site-directed mutagenesis of a bleomycin resistance protein and their significance for drug sequestering.

The antibiotic bleomycin, a strong DNA cutting agent, is naturally produced by actinomycetes which have developed a resistance mechanism against such a lethal compound. The crystal structure, at 2.3 A resolution, of a bleomycin resistance protein of 14 kDa reveals a structure in two halves with the same alpha/beta fold despite no sequence similarity. The crystal packing shows compact dimers with a hydrophobic interface and involved in mutual chain exchange. Two independent solution studies (analytical centrifugation and light scattering) showed that this dimeric form is not a packing artefact but is indeed the functional one. Furthermore, light scattering also showed that one dimer binds two antibiotic molecules as expected. A crevice located at the dimer interface, as well as the results of a site-directed mutagenesis study, led to a model wherein two bleomycin molecules are completely sequestered by one dimer. This provides a novel insight into antibiotic resistance due to drug sequestering, and probably also into drug transport and excretion.