Structural characterization of the PCV2d genotype at 3.3 Å resolution reveals differences to PCV2a and PCV2b genotypes, a tetranucleotide, and an N-terminus near the icosahedral 3-fold axes

Porcine circovirus 2 (PCV2) is a T=1 non-enveloped icosahedral virus that has a major impact on the swine industry as an agent of porcine circovirus associate disease. PCV2 capsid protein sequences have been employed by others to provide a temporal description of the emerging genotypes. PCV2a is believed to be the earliest genotype and responsible for giving rise to PCV2b, which gives rise to PCV2d. The underlying mechanism responsible for the emerging genotypes is not understood. To determine if a change in the PCV2d capsid accompanies the emergence of this genotype, we determined the cryo-electron microscopy image reconstruction of PCV2d VLP at 3.3 A resolution and compared it to the previously reported PCV2a and PCV2b structures. Differences between the CD and GH loops identify structural changes that accompany the emergence of PCV2b from PCV2a, and PCV2d from PCV2b. We also model additional amino acids for the N-terminus near the icosahedral 3-fold axes of symmetry and a tetranucleotide between the 5- and 2-fold axes of symmetry. To interpret the sequence diversity that defines the PCV2 genotypes on a structural platform we have performed structure-based sequence comparison. Our analysis demonstrates that each genotype possesses a unique set of amino acids located on the surface of the capsid that experience a high degree of substitution. These substitutions may be a response to the PCV2 vaccination program. The structural difference between PCV2a, b and d genotypes indicate that it is important to determine the PCV2 capsid structure as the virus evolves into different genotypes.