Insights into the ligand-free structure of cyclic diguanosine monophosphate I riboswitch of Vibrio cholerae using molecular dynamics simulation

Abstract

Riboswitches are key cis regulatory elements present at 5' UTRs of mRNAs. They play a critical role in gene expression regulation at transcriptional and translational level by binding selectively to specific ligands followed by conformational changes. Ligands bind to the aptamer of riboswitches and their complex structures have been solved, but ligand-free riboswitches structures are not available which is important to understand specific ligand binding mechanism. In this paper, an all atom 150 nano-second (ns) molecular dynamics (MD) simulations of cyclic diguanosine monophosphate (c-di-GMP I) riboswitch aptamer domain from Vibrio cholerae were carried out to study ligand-free c-di-GMP I riboswitch aptamer structure and the binding mechanism. The Principle component analysis, cross correlation dynamics analysis and trajectory analyses revealed that the ligand-free structure has stable conformation with folded P2, P3 and an open P1 helix which opens the ligand binding helix-join-helix while the ligand-bound structure shows less deviation and remains as closed structure compared to the ligand-free structure. The junction residues significantly showed anti-correlated motions with each other elucidating the open conformation of the ligand-free aptamer of riboswitch. The identified key residues involved in binding are A18, G20, C46, A47 and C92.