Walkmycin B targets WalK (YycG), a histidine kinase essential for bacterial cell growth

The WalK(a histidine kinase)/WalR (a response regulator, akaYycG/YycF) two-component system is indispensable in the signal transduction pathway for the cell-wall metabolism of Bacillus subtilis and Staphylococcus aureus. The inhibitors directed against WalKwould be expected to have a bactericidal effect. After we screened 1368 culture broths of Streptomyces sp. by a differential growth assay, walkmycin A, B and C, which were produced by strain MK632-100F11, were purified using silica-gel column chromatography and HPLC. In this paper, the chemical structure of the major product (walkmycin B) was determined to be di-anthracenone (C 44 H 44 Cl 2 O 14 ), which was very similar to BE40665A. MICs of walkmycin B against B. subtilis and S. aureus were 0.39 and 0.20 μg ml -1 , and IC 50 measurements against WalKwere 1.6 and 5.7 μm, respectively. To clarify the affinity between WalKand walkmycin B, surface plasmon resonance was measured to obtain the equilibrium dissociation constant, K D1 , of 7.63 μM at the higher affinity site of B. subtilis WalK. These results suggest that walkmycin B inhibits WalK autophosphorylationby binding to the WalKcytoplasmic domain.