Flavin-dependent histone demethylases catalyze the posttranslational oxidative demethylation of mono- and dimethylated lysine residues, producing formaldehyde and hydrogen peroxide in addition to the corresponding demethylated protein. In vivo, the histone demethylase LSD1 (KDM1; BCH110) is a component of the multiprotein complex that includes histone deacetylases (HDACs 1/2) and the scaffolding protein CoREST. Although little is known concerning the affinities of or the structural basis for the interaction between CoREST and HDACs, the structure of CoREST286–482 bound to an alpha helical coiled-coil tower domain within LSD1 has been recently reported. Given the significance of CoREST in directing demethylation to specific nucleosomal substrates, insight into the molecular basis of the interaction between CoREST and LSD1 may suggest a new means to inhibit LSD1 activity by misdirecting the enzyme away from nucleosomal substrates. Towards this end, isothermal titration calorimetry studies (ITC) were conducted to determine the affinity and thermodynamic parameters characterizing the binding interaction between LSD1 and CoREST286–482. The proteins tightly interact in a 1:1 stoichiometry with a dissociation constant (Kd) of 15.9 ± 2.07 nM, and their binding interaction is characterized by a favorable enthalpic contribution near room temperature with a smaller entropic penalty at pH 7.4. Additionally, one proton is transferred from the buffer to the heterodimeric complex at pH 7.4. From the temperature dependence of the enthalpy change of interaction, a constant pressure heat capacity change (ΔCp) of the interaction was determined to be −0.80 ± 0.01 kcal/mol·K. Notably, structure-driven truncation of CoREST revealed that the central binding determinant lies within residues 293–380, also known as the CoREST ‘linker’ region, which is a central isolated helix that interacts with the LSD1 coiled-coil tower domain to create a triple helical bundle. Thermodynamic parameters obtained from the binding between LSD1 and the linker region293–380 of CoREST are similar to those obtained from the interaction between LSD1 and CoREST286–482. These results provide a framework for understanding the molecular basis of protein-protein interactions that govern nucleosomal demethylation.

Ramoplanin is a lipoglycodepsipeptide antimicrobial active against clinically important Gram-positive bacteria including methicillin resistant Staphylococcus aureus. To proactively examine ramoplanin resistance, we subjected S. aureus NCTC 8325-4 to serial passage in the presence of increasing concentrations of ramoplanin, generating the markedly resistant strain RRSA16. Susceptibility testing of RRSA16 revealed the unanticipated acquisition of cross-resistance to vancomycin and nisin. RRSA16 displayed phenotypes, including a thickened cell wall and reduced susceptibility to Triton X-100 induced autolysis, which are associated with vancomycin intermediate resistant S. aureus strains. Passage of RRSA16 for 18 days in drug-free medium yielded strain R16-18d with restored antibiotic susceptibility. The RRSA16 isolate may be used to identify the genetic and biochemical basis for ramoplanin-resistance and further our understanding of the evolution of antibiotic cross-resistance mechanisms in S. aureus.