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composed of two molecules of VP23 and one molecule of VP19c.
The lower portion of the triplex, which interacts with the floor of the
pentons and hexons, is three-fold symmetric, with all three subunits
roughly equivalent. This arrangement alters the middle of the triplex
such that the upper portion is composed mostly of VP23 in a dimeric
configuration. All three subunits of the triplex are required for the
correct tertiary structure to form. VP23 exists in isolation only as a
molten globule with no distinct tertiary structure.
HCMV Capsid
Structural studies of HCMV capsids have not reached as high a res-
olution as those of HSV-1, as it has been difficult to scale up and iso-
late an adequate amount of HCMV capsids from HCMV-infected
cells. The first structure of the HCMV capsid, to 35 Å, was obtained
by Butcher et al . 75 The hexons are slightly skewed with an oval,
rather than round, axial channel. The penton tips are more pointed
with a triangular cross-section than those of the hexons. Yu et al .
subsequently obtained high-resolution structures that confirmed those
features (Fig. 6e). 76,22
The HCMV capsid structure is very similar to HSV-1 in overall
organization, with four homologous structural proteins at the same
stoichiometries. The main difference is that the HCMV capsid has a
larger diameter (650 Å) than HSV-1 (620 Å), with a volume ratio of
1.17. The increased size of the HCMV capsid — despite the similar
molecular mass of its component proteins — results in a greater
center-to-center spacing of the capsomers and greater relative tilt
between adjacent hexons compared to HSV-1.
The major point of interest in this study was the question of how
HCMV packages its genome, which is 50% larger than that of HSV-1,
into a capsid with only 17% more internal volume. Bhella et al . dis-
covered that packing density is an important factor in genome encap-
sidation. 77 They compared radial density plots computed from cryoEM
structures of HCMV and HSV-1. The average spacing between DNA
layers in their HSV-1 reconstruction was 26 Å, whereas the interlayer
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