Activation of an anti-bacterial toxin by the biosynthetic enzyme CysK: mechanism of binding, interaction specificity and competition with cysteine synthase

作者：Benoni Roberto; Beck Christina M; Garza Sanchez Fernando; Bettati Stefano; Mozzarelli Andrea; Hayes Christopher S^*; Campanini Barbara^*

来源：Scientific Reports, 2017, 7(1): 8817.

DOI：10.1038/s41598-017-09022-6

Contact-dependent growth inhibition (CDI) is a wide-spread mechanism of inter-bacterial competition. CDI+ bacteria deliver CdiA-CT toxins into neighboring bacteria and produce specific immunity proteins that protect against self-intoxication. The CdiA-CT toxin from uropathogenic Escherichia coli 536 is a latent tRNase that is only active when bound to the cysteine biosynthetic enzyme CysK. Remarkably, the CysK: CdiA-CT binding interaction mimics the 'cysteine synthase' complex of CysK: CysE. The C-terminal tails of CysE and CdiA-CT each insert into the CysK active-site cleft to anchor the respective complexes. The dissociation constant for CysK: CdiA-CT (K-d similar to 11 nM) is comparable to that of the E. coli cysteine synthase complex (K-d similar to 6 nM), and both complexes bind through a two-step mechanism with a slow isomerization phase after the initial encounter. However, the second-order rate constant for CysK: CdiA-CT binding is two orders of magnitude slower than that of the cysteine synthase complex, suggesting that CysE should outcompete the toxin for CysK occupancy. However, we find that CdiA-CT can effectively displace CysE from pre-formed cysteine synthase complexes, enabling toxin activation even in the presence of excess competing CysE. This adventitious binding, coupled with the very slow rate of CysK: CdiA-CT dissociation, ensures robust nuclease activity in target bacteria.

• 出版日期2017-8-18