Figure 2. Percentage of viral genome ejected from HSV-1 capsids as a function of the external osmotic pressure.

DNA ejection from the capsids in vitro is triggered by mild trypsin treatment, which cleaves the portal protein (UL6) without degrading the major capsid protein (VP5) or causing morphological damage to capsids (Bauer et al., 2013). Figure shows that DNA ejection is progressively suppressed with increasing PEG concentration at 37°C with PEG 8 kDa. DNA ejection is completely suppressed at 18 atm external osmotic pressure, which matches and therefore ‘turns off’ the DNA pressure in the capsid. PEG is added to CBB buffer (capsid binding buffer: 20 mM HEPES-KOH with pH of 7.3, 80 mM K-acetate, 2 mM DTT, 1 mM EGTA, 2 mM Mg-acetate, 1 mM PMSF, and 1X CLAP cocktail), required for capsid binding to nuclei. (PEG concentration was converted to osmotic pressure using the relation in ref Evilevitch et al., 2003). Vertical error bars represent the standard error of the gel band intensity profile (see Materials and methods Section). Horizonal error bars representing standard error in weighted PEG concentration are negligibly small. Dashed line is drawn to guide the eye.

Figure 2—figure supplement 1. Osmotic suppression of HSV-1 genome ejection.

DNA ejection from HSV-1 capsids is initiated by trypsin digestion in the presence (or absence) of PEG 8 kDa and DNase I at 37°C in CBB buffer. Nonejected DNA was extracted from capsids by sodium dodecyl sulfate (SDS) and protease K treatment and analyzed by pulse field gel electrophoresis (PFGE). PFGE of osmotically suppressed DNA remaining inside viral capsids in the presence of varying concentrations of PEG shown in lanes 2–7. Lane M shows DNA ladder. For all experiments, DNA within unopened capsids resulted in an additional band of approximately 151 kbp corresponding to full length HSV-1 genome. Similar results were obtained in three independent experiments and a representative experiment is shown.