CONSTANT PH MOLECULAR DYNAMICS OF PORCINE CIRCOVIRUS 2 CAPSID PROTEIN REVEALS A MECHANISM FOR CAPSID ASSEMBLY

Spatiotemporal regulation of viral capsid assembly ensures selection of the viral genome for encapsidation. The porcine circovirus 2 is the smallest autonomously replicating pathogenic virus, yet it remains unknown how PCV2 capsid assembly is regulated to occur within the nucleus. We report that pure PCV2 capsid proteins, in the absence of nucleic acids, require acidic conditions to assemble into empty capsids in vitro. By employing Constant pH Replica Exchange Molecular Dynamics, we unveil the atomistic mechanism of pH-dependency for capsid assembly. The results show that the appropriate protonation configuration for a cluster of acidic amino acids is necessary to properly position the GH-loop for driving capsid assembly. We demonstrate that assembly is prohibited at neutral pH because deprotonation of these residues results in their electrostatic repulsion, shifting the GH-loop to a position incompetent with capsid assembly. We propose that encapsulation of nucleic acid overcomes this repulsion to properly position the GH-loop. Our findings provide the first atomic resolution mechanism of capsid assembly regulation. The findings are useful for development of therapeutics that inhibit PCV2 self-assembly.Graphical Abstract