Ricin A-chain structural determinant for binding substrate analogues: a molecular dynamics simulation analysis.

Ricin A-chain is a cytotoxic protein that attacks ribosomes by hydrolyzing a specific adenine base from a highly conserved, single-stranded rRNA hairpin containing the tetraloop sequence GAGA. Molecular-dynamics simulation methods are used to analyze the structural determinant for three substrate analogues bound to the ricin A-chain molecule. Simulations were applied to the binding of the dinucleotide adenyl-3',5'-guanosine employing the x-ray crystal structure of the ricin complex and a modeled CGAGAG hexanucleotide loop taken from the NMR solution structure of a 29-mer oligonucleotide hairpin. A third simulation model is also presented describing a conformational search of the docked 29-mer structure by using a simulated-annealing method. Analysis of the structural interaction energies for each model shows the overall binding dominated by nonspecific interactions, which are mediated by specific arginine contracts from the highly basic region on the protein surface. The tetraloop conformation of the 29-mer was found to make specific interactions with conserved protein residues, in a manner that favored the GAGA sequence. A comparison of the two docked loop conformations with the NMR structure revealed significant positional deviations, suggesting that ricin may use an induced fit mechanism to recognize and bind the rRNA substrate. The conserved Tyr-80 may play an important conformational entropic role in the binding and release of the target adenine in the active site.