Coxsackievirus A9 VP1 mutants with enhanced or hindered A particle formation and decreased infectivity

We have studied coxsackievirus A9 (CAV9) mutants that each have a single am ino acid substitution in the conserved 29-PALTAVETGHT-39 moth of VPI and a reduced capacity to produce infectious progeny virus. After uncoating, ail steps in the infection cycle occurred according to the same kinetics as and similar efficiency to the wild-type virus. However, the particle/infectiou s unit ratio in the progeny,vas significantly increased. The differences we re apparently due to altered stability of the capsid: there were mutant vir uses with enhanced or hindered uncoating, and both of these characteristics were found to reduce fitness under standard passaging conditions. At 32 de greesC the instable mutants had an advantage, while the wild-type and the m ost stable mutant grew poorly. When comparing the newly published CAV9 stru cture and the other enterovirus structures, we found that the PALTAVETGHT m otif is always in exactly the same position, in a cavity formed by the 3 ot her capsid proteins, with the C terminus of VP4 between this motif and the RNA. In the 7 enterovirus structures determined to date, the most conserved residues of the studied motif have identical contacts to neighboring resid ues of VP2, VP3, and VP4. We conclude that (i) the mutations affect the unc oating step necessary for infection, resulting in an untimely or hindered e xternalization of the VP1 N terminus together with the VP4, and (ii) the re ason for the studied moth being evolutionarily conserved is its role in mai ntaining an optimal balance between the protective stability and the functi onal flexibility of the capsid.