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self-interaction, which in turn potentiates or stabilizes pAP binding through its
carboxyl conserved domain (CCD) to the major capsid protein (MCP, pUL86) (Wood
et al. 1997; see Figs. 3 and 4, step 1). This interaction enables MCP, which lacks its
own nuclear localization sequence (NLS), to be translocated into the nucleus (Fig. 4,
step 4) as part of the pAP-MCP complex via two NLSs present in pAP (Wood et al.
1997; Plafker and Gibson 1998). Because its carboxyl end is identical to pAP (Figs.
2, 3), the proteinase precursor can interact with itself and pAP through its ACD, and
with MCP through its CCD. Mimicking these pAP interactions ensures incorporation
of pPR into the capsid cavity, where its enzymatic function is required. The composi-
tion and variety of the complexes formed (e.g., pAP-MCP; pAP-pPR-MCP;
pPR-MCP) and the extent to which they progress in the cytoplasm toward completely
preassembled protocapsomers (Fig. 4, step 2) is unknown.
The biological relevance of ACD-mediated pAP self-interaction was established
by using mutant viruses, which were found to replicate slowly, assemble nucleo-
capsids inefficiently, and yield roughly 20-fold less virus than wild type (Loveland
et al. 2007). Mutant viruses have also been used to verify the biological requirement
for pAP NLS, by showing that loss of both is lethal, loss of either one alone slows
nuclear translocation of MCP, and loss of NLS2 inhibits virus replication more
profoundly than loss of NLS1 (Nguyen et al. 2007). Restriction of NLS2 to the
β-herpesvirus pAP homologs and its comparatively greater impact on virus replication
suggests it may have a different or additional group-specific function.
Through a similar subunit assembly process, the triplex proteins associate in the
cytoplasm before translocation into the nucleus (Fig. 4, step 3) (Baxter and Gibson
1997; Spencer et al. 1998). Like MCP, the minor capsid protein (mCP, pUL85) does
not enter the nucleus when expressed alone in transfected cells, even though its size
is small enough to be allowed entry by diffusion. The similar-size mCBP, in contrast,
does enter the nucleus on its own and when mCP and the mCP-binding protein
(mCBP, pUL46) are expressed together, they co-localize to the nucleus (Baxter and
Gibson 1997). This is attributed to the rapid formation of approximately 70-kDa mCP
homodimers that require interaction with an NLS-bearing mCBP for nuclear translo-
cation. These sequential cytoplasmic interactions of pAP and pPR with each other
and with MCP, and of mCP with itself and with mCBP, initiate and direct the assem-
bly process and consequently have the potential to help modulate it. Cytoplasmic
preassembly may also increase the fidelity and efficiency of procapsid formation by
ensuring delivery into the nucleus of correctly organized capsid substructures.
The HCMV portal protein (pUL104, 78 kDa) may also interact with pAP, as it
does in HSV (Singer et al. 2005). However, little is known about when and where its
self-interaction and interaction with pAP occur. Unlike MCP, the portal protein con-
tains its own NLS (Patel and MacLean 1995; Patel et al. 1996) and would not seem
to require interaction with pAP as an NLS-bearing nuclear-translocation escort.
Following translocation into the nucleus, the pAP-MCP, pPR-MCP, and pAP-
pPR-MCP complexes or protocapsomers interact more extensively with one
another and associate with the triplexes and portal protein complex to form
procapsids (Fig. 4, step 5). These unstable particles (Newcomb et al. 1999; Rixon
and McNab 1999), first evidenced in HSV (Newcomb et al. 1999; Rixon and
