Depletion of Virion-Associated Divalent Cations Induces Parvovirus Minute Virus of Mice To Eject Its Genome in a 3′-to-5′ Direction from an Otherwise Intact Viral Particle

Abstract

We describe a structural rearrangement that can occur in parvovirus minute virus of mice (MVMp) virions following prolonged exposure to buffers containing 0.5 mM EDTA. Such particles remain stable at 4°C but undergo a conformational shift upon heating to 37°C at pH 7.2 that leads to the ejection of much of the viral genome in a 3′-to-5′ direction, leaving the DNA tightly associated with the otherwise intact capsid. This rearrangement can be prevented by the addition of 1 mM CaCl ₂ or MgCl ₂ prior to incubation at 37°C, suggesting that readily accessible divalent cation binding sites in the particle are critical for genome retention. Uncoating was not seen following the incubation of virions at pH 5.5 and 37°C or at pH 7.2 and 37°C in particles with subgenomic DNA, suggesting that pressure exerted by the full-length genome may influence this process. Uncoated genomes support complementary-strand synthesis by T7 DNA polymerase, but synthesis aborts upstream of the right-hand end, which remains capsid associated. We conclude that viral genomes are positioned so that their 3′ termini and coding sequences can be released from intact particles at physiological temperatures by a limited conformational rearrangement. In the presence of divalent cations, incremental heating between 45°C and 65°C induces structural transitions that first lead to the extrusion of VP1 N termini, followed by genome exposure. However, in cation-depleted virions, the sequence of these shifts is blurred. Moreover, cation-depleted particles that have been induced to eject their genomes at 37°C continue to sequester their VP1 N termini within the intact capsid, suggesting that these two extrusion events represent separable processes.