The solvent at antigen-binding site regulated C3d–CR2 interactions through the C-terminal tail of C3d at different ion strengths: insights from molecular dynamics simulation

Background The interactions of complement receptor 2 (CR2) and the degradation fragment C3d of complement component C3 play important links between the innate and adaptive immune systems. Due to the importance of C3d–CR2 interaction in the design of vaccines and inhibitors, a number of studies have been performed to investigate C3d–CR2 interaction. Many studies have indicated C3d–CR2 interactions are ionic strength-dependent. Methods To investigate the molecular mechanism of C3d–CR2 interaction and the origin of effects of ionic strength, molecular dynamics simulations for C3d–CR2 complex together with the energetic and structural analysis were performed. Results Our results revealed the increased interactions between charged protein and ions weaken C3d–CR2 association, as ionic strengths increase. Moreover, ion strengths have similar effects on antigen-binding site and CR2 binding site. Meanwhile, Ala17 and Gln20 will transform between the activated and non-activated states mediated by His133 and Glu135 at different ion strengths. Conclusions Our results reveal the origins of the effects of ionic strengths on C3d–CR2 interactions are due to the changes of water, ion occupancies and distributions. General significance This study uncovers the origin of the effect of ionic strength on C3d–CR2 interaction and deepens the understanding of the molecular mechanism of their interaction, which is valuable for the design of vaccines and small molecule inhibitors.