Steered molecular dynamics simulation of the binding of the β2 and β3 regions in domain-swapped human cystatin C dimer

The crystal structure of the human cystatin C (hCC) dimer revealed that a stable twofold-symmetric dimer was formed via 3D domain swapping. Domain swapping with the need for near-complete unfolding has been proposed as a possible route for amyloid fibril initiation. Thus, the interesting interactions that occur between the two molecules may be important for the further aggregation of the protein. In this work, we performed steered molecular dynamics (SMD) simulations to investigate the dissociation of the beta 2 and beta 3 strands in the hCC dimer. The energy changes observed during the SMD simulations showed that electrostatic interactions were the dominant interactions involved in stabilizing the two parts of the dimer during the early stages of SMD simulation, whereas van der Waals (VDW) interactions and electrostatic interactions were equally matched during the latter stages. Furthermore, our data indicated that the two parts of the dimer are stabilized by intermolecular hydrogen bonds among the residues Arg51 (beta 2), Gln48 (beta 2), Asp65 (beta 3), and Glu67 (beta 3), salt bridges among the residues Arg53 (beta 2), Arg51 (beta 2), and Asp65 (beta 3), and VDW interactions among the residues Gln48 (beta 2), Arg51 (beta 2), Glu67 (beta 3), Asp65 (beta 3), Phe63 (beta 3), and Asn61 (beta 3). The residues Gln48 (beta 2), Arg51 (beta 2), Asp65 (beta 3) and Glu67 (beta 3) appear to be crucial, as they play important roles in both electrostatic and VDW interactions. Thus, the present study determined the key residues involved in the stabilization of the domain-swapped dimer structure, and also provided molecular-level insights into the dissociation process of the hCC dimer.