Uncoupling Uncoating of Herpes Simplex Virus Genomes from Their Nuclear Import and Gene Expression

Incoming capsids of herpes simplex virus type 1 (HSV-1) enter the cytosol by fusion of the viral envelopes with host cell membranes and use microtubules and microtubule motors for transport to the nucleus. Upon docking to the nuclear pores, capsids release their genomes into the nucleoplasm. Progeny genomes are replicated in the nucleoplasm and subsequently packaged into newly assembled capsids. The minor capsid protein pUL25 of alphaherpesviruses is required for capsid stabilization after genome packaging and for nuclear targeting of incoming genomes. Here, we show that HSV-1 pUL25 bound to mature capsids within the nucleus and remained capsid associated during assembly and nuclear targeting. Furthermore, we tested potential interactions between parental pUL25 bound to incoming HSV-1 capsids and host factors by competing for such interactions with an experimental excess of cytosolic pUL25. Overexpression of pUL25, GFPUL25, or UL25GFP prior to infection reduced gene expression of HSV-1. Electron microscopy and in situ hybridization studies revealed that an excess of GFPUL25 or UL25GFP prevented efficient nuclear import and/or transcription of parental HSV-1 genomes, but not nuclear targeting of capsids or the uncoating of the incoming genomes at the nuclear pore. Thus, the uncoating of HSV-1 genomes could be uncoupled from their nuclear import and gene expression. Most likely, surplus pUL25 competed with important interactions between the parental capsids, and possibly between authentic capsid-associated pUL25, and cytosolic or nuclear host factors required for functional interaction of the incoming genomes with the nuclear machinery.