Hydrophobic domains of mouse polyomavirus minor capsid proteins promote membrane association and virus exit from the ER

The minor structural protein VP2 and its shorter variant, VP3, of mouse polyomavirus (MPyV) are essential for virus exit from the endoplasmic reticulum (ER) during viral trafficking to the nucleus. Here, we followed the role of putative hydrophobic domains (HD) of the minor proteins in membrane affinity and viral infectivity. We prepared variants of VP2, each mutated to decrease hydrophobicity of one of three predicted hydrophobic domains: VP2‐mHD1, VP2‐mHD2 or VP2‐mHD3 mutated in HD1 (amino acids (aa) 60–101), HD2 (aa 125–165) or HD3 (aa 287–307), respectively. Transient production of the mutated proteins revealed that only VP2‐mHD2 lost the affinity for intracellular membranes. Cytotoxicity connected with the ability of VP2/VP3 to perforate membranes decreased markedly for VP2‐mHD2, but only slightly for VP2‐mHD1. The mutant VP2‐mHD3 exhibited properties similar to the wild‐type protein. MPyV genomes, each carrying one of the mutations, were prepared for virus production. MPyV‐mHD1 and MPyV‐mHD2 viruses could be isolated, while the HD3 mutation in VP2/VP3 prevented virus assembly. We found that both MPyV‐mHD1 and MPyV‐mHD2 viruses arrived at the ER without delay and were processed by ER residential enzymes. However, the ability to associate with ER membranes was decreased in the case of MPyV‐mHD1 and practically abolished in the case of MPyV‐mHD2. Interestingly, while MPyV‐mHD2 was not infectious, infection of MPyV‐mHD1 virus was delayed. These findings reveal that HD2, common to both VP2 and VP3, is responsible for the membrane binding properties of the minor proteins, while HD1 of VP2 is likely required to stabilize VP2–membrane association and to enhance viral exit from the ER. Mouse polyomavirus is a non‐enveloped DNA virus that is internalised into a cell after binding to its receptor on the cell surface. For successful infection, viral capsids must penetrate intracellular membranes before reaching the host nucleus; however, the location at which mouse polyomavirus capsids breach cellular membranes and the mechanisms by which this occurs remained enigmatic. Forstová and colleagues now report that the mouse polyomavirus minor structural protein VP2 and its shorter variant, VP3, are essential for virus exit from the endoplasmic reticulum (ER) during viral trafficking to the nucleus. They also identify roles for hydrophobic domains within VP2 and VP3 that promote virus‐ER membrane association and ER exit.