Biology Reference
In-Depth Information
two types of particles are enveloped and recovered from the culture medium of
CMV-infected cells; neither contains DNA. Noninfectious enveloped particles
(NIEPs) are enveloped B-capsids that closely resemble virions in structure and
composition, but retain the internal B-capsid scaffolding proteins. Dense bodies
(DBs), which differ from NIEPs and virions by their larger and more heterogeneous
size (~250-600 nm) and by the absence of all nucleocapsid constituents, are solid
spheroidal aggregates of a single predominant tegument protein species (i.e.,
pUL83), surrounded by an envelope so far undistinguished from that of the virion.
More detailed descriptions of these particles are presented in earlier reports and
reviews (Irmiere and Gibson 1983, 1985; Gibson and Irmiere 1984; Gibson 1996).
Formation of the Nucleocapsid
Capsid assembly is coordinated by the assembly protein precursor (pAP, pUL80.5,
38 kDa) and the genetically related protease precursor (pPR, pUL80a, 74 kDa),
both of which are ultimately eliminated from the maturing particle. These proteins
are encoded by 3′-coterminal in-frame genes, with the consequence that the car-
boxyl approximately 60% of pPR is identical to pAP (Fig. 2). Two smaller proteins
encoded by the same set of genes (pUL80.4 and pUL80.3) have unknown functions
that are dispensable for growth in cell culture (N. Nguyen and W. Gibson, unpub-
lished data) from mutant viruses having one or both translational start methionines
replaced with isoleucines. Key amino acid sequences within these proteins that
enable their function are illustrated in Fig. 3.
A working model of CMV capsid formation is illustrated in Fig. 4. The earliest
steps in the assembly process begin in the cytoplasm. One pathway leads to proto-
capsomers and is initiated when the amino conserved domain (ACD) promotes pAP
Fig. 3
Landmarks on the assembly protein and protease precursors. The assembly protein precur-
sor (
pAP
) has the same amino acid sequence as the carboxyl half of the protease precursor (
pPR
)
and includes the following sequences of interest: the amino conserved domain (
ACD
), which pro-
motes self-interaction of pAP and pPR; the carboxyl conserved domain (
CCD
), which promotes
