Validating fragment-based drug discovery for biological RNAs: lead fragments bind and remodel the TPP riboswitch specifically.
Publication Type:Journal Article
Source:Chem Biol, Volume 21, Issue 5, p.591-5 (2014)
Keywords:Binding Sites, Crystallography, X-Ray, Drug Discovery, Ligands, Models, Molecular, Molecular Structure, Reproducibility of Results, Riboswitch, RNA, Substrate Specificity, Thiamine Pyrophosphate
<p>Thiamine pyrophosphate (TPP) riboswitches regulate essential genes in bacteria by changing conformation upon binding intracellular TPP. Previous studies using fragment-based approaches identified small molecule "fragments" that bind this gene-regulatory mRNA domain. Crystallographic studies now show that, despite having micromolar Kds, four different fragments bind the TPP riboswitch site-specifically, occupying the pocket that recognizes the aminopyrimidine of TPP. Unexpectedly, the unoccupied site that would recognize the pyrophosphate of TPP rearranges into a structure distinct from that of the cognate complex. This idiosyncratic fragment-induced conformation, also characterized by small-angle X-ray scattering and chemical probing, represents a possible mechanism for adventitious ligand discrimination by the riboswitch, and suggests that off-pathway conformations of RNAs can be targeted for drug development. Our structures, together with previous screening studies, demonstrate the feasibility of fragment-based drug discovery against RNA targets.</p>