Biology Reference
In-Depth Information
virion with an intact envelope and tegument (Fig. 4a).
2
The overall
morphology of the virion is similar to that of the capsid, except for the
presence of capsid-bound tegument densities. These appear as convo-
luted, ribbon-like structures that are anchored at one end to the pen-
tons and extend to neighboring triplexes at the other end. The bulk
of the attachment tegument density is centered on the pentons and
makes only minimal contact with adjacent hexons.
In contrast, the tegument proteins of HCMV form a thin, net-like
shell that surrounds the entire capsid (Fig. 4b).
60
The filamentous
tegument densities contact the pentons, hexons and triplexes of the
HCMV capsid. The structure of the human gammaherpesvirus tegu-
ment has not been reported because of difficulty in purifying intact
virions. However, gammaherpesvirus tegument differs from those of
HSV-1 and HCMV in that it does not make extensive symmetric con-
tact with the capsid proteins.
61
Some portions of the inner tegument
layer appeared to be tethered to the tips of the capsomers, but these
contacts did not appear to be regular or to fill every possible binding
site on the capsid. Also, in contrast to HSV-1, the majority of tegu-
ment molecules in virions of MHV-68, a mouse gammaherpesvirus,
seem to be loosely organized, allowing the capsid to “float” inside the
envelope and vary in its location within the tegument.
61
Herpesvirus tegument proteins have essential functions in virion
assembly and maturation through their interactions with the underly-
ing capsid proteins and the overlying envelope glycoproteins. These
proteins may also modulate the cellular machinery during the early
stages of infection, as they are the first to contact the cellular environ-
ment after the viral envelope fuses with the cell membrane or the endo-
cytic vesicle membrane and releases the partially tegumented
nucleocapsid into the cytoplasm. The formation of tegument repre-
sents a subsequent evolutionary adaptation to infection of these nucle-
oplasms enclosed by a cytoplasm, when considering that herpesvirus
capsids and icosahedral bacteriophages share a common ancestor
that existed prior to the divergence of eubacteria and a primordial
nucleoplasm.
64,65
With later divergence of the host range of her-
pesviruses, protein functions differentiated as well and have been incor-
porated into the tegument structures, as indicated by the structural
