Biology Reference
In-Depth Information
Location and Morphology of the SCP
As shown in Table 3, the most obvious difference among herpesvirus
capsids is in the size of their small capsid protein (SCP). The SCPs are of
great interest as they share the least sequence homology of herpesvirus
capsid proteins.
78
The major and minor capsid proteins of herpesviruses
share relatively high sequence similarity, so the overall capsid structure is
very similar among the species, but the conformations, locations, and
functions of the SCPs may be more diverse. The SCP of HCMV has
been shown to be essential for HCMV infection
in vivo
,
84
whereas its
counterpart in HSV-1, VP26, is dispensable for HSV-1 infection.
85,86
In KSHV, although the presence of SCP (pORF65) is used as a hallmark
of KSHV infection for diagnostic purposes, it remains unknown whether
it is essential for KSHV infection. However, HSV-1 VP26 associates with
ribosomes and may regulate host protein translation.
87
A similar role for
pORF65 is possible, as evidenced by another study in which pORF65
was shown to relocate from the nucleus to the cytoplasm of primary
effusion lymphoma cells following induction of lytic replication.
88
The SCPs have been difficult to study because of their small size
and the relative low resolutions of the capsid KSHV and HCMV. At
such resolutions, protein boundaries cannot be resolved, and it is dif-
ficult to recognize structural features of proteins as small as the SCP.
In fact, even at 8.5 Å resolution, at which
-helices can be resolved,
it has proven difficult to clearly delineate VP26 in HSV-1 hexons
because of its extensive and interdigitated interactions across a large
area of the MCP upper domain,
34
and because its secondary structure
consists mainly of
α
-sheets.
89
The 12 kDa SCP of HSV-1, VP26, binds the MCP subunits of
the hexons, but not those of the pentons.
90
Its exact shape and loca-
tion on the capsid — on the tips of the hexon subunits — have been
determined by difference mapping
90
and observed in detail in the
8.5 Å structure.
34
However, a comparative study of wild-type and
VP26-minus virions has shown that VP26 is not involved in the dif-
ferential binding of tegument proteins to the pentons of the HSV-1
capsid.
β
