Conformational dynamics and free energy of BHRF1 binding to Bim BH3.

The interaction between the Bim BH3 peptide and the viral protein BHRF1 is pivotal to understanding the fundamental molecular details of the mechanism used by the Epstein-Barr virus to trick the mammalian immune system. Here, we study the mechanism of binding/unbinding and compute the free energy for the association of the Bim peptide to the BHRF1 protein. Key elements of the binding mechanism are the conformational rearrangement together with a main free energy barrier of 11.5kcal/mol. The simulations show complete unbinding and rebinding of the Bim peptide to BHRF1. The peptide slowly dissociates, disrupting the hydrophobic contacts, then tilting to one side. The peptide then completely disrupts all the remaining interactions and moves into the bulk solvent. The rebinding of the peptide from the solvent to the receptor binding site occurs slowly. This is because the helix partially unfolds in the unbound state. Rebinding involves an intermediate state, in which the peptide interacts with the hydrophobic binding pocket, which mainly involves Leu 62, Arg 63, Ile 65, and Phe 69. This novel intermediate structure forms 65 contacts with the receptor before the peptide again reaches the bound state. The standard binding free energy value is close to the experimental Kd in the nanomolar range. Finally, we observe how the breathing motions of α3-α4 are coupled with the binding/unbinding of the Bim BH3 peptide. The structure of the intermediate can be used for designing novel peptide inhibitors of the BHRF1 protein.