Herpes simplex virus 1 Us3 deletion mutant is infective despite impaired capsid translocation to the cytoplasm

Herpes simplex virus 1 (HSV-1) capsids are assembled in the nucleus bud at the inner nuclear membrane into the perinuclear space, acquiring envelope and tegument. In theory, these virions are de-enveloped by fusion of the envelope with the outer nuclear membrane and re-enveloped by Golgi membranes to become infective. Us3 enables the nucleus to cytoplasm capsid translocation. Nevertheless, Us3 is not essential for the production of infective progeny viruses. Determination of phenotype distribution by quantitative electron microscopy, and calculation per mean nuclear or cell volume revealed the following: (i) The number of R7041(ΔU_S3) capsids budding at the inner nuclear membrane was significantly higher than that of wild type HSV-1; (ii) The mean number of R7041(ΔU_S3) virions per mean cell volume was 2726, that of HSV-1 virions 1460 by 24 h post inoculation; (iii) 98% of R7041(ΔU_S3) virions were in the perinuclear space; (iv) The number of R7041(ΔU_S3) capsids in the cytoplasm, including those budding at Golgi membranes, was significantly reduced. Cell associated R7041(ΔU_S3) yields were 2.37 × 10⁸ and HSV-1 yields 1.57 × 10⁸ PFU/mL by 24 h post inoculation. We thus conclude that R7041(ΔU_S3) virions, which acquire envelope and tegument by budding at the inner nuclear membrane into the perinuclear space, are infective.