Biology Reference
In-Depth Information
Maturational proteolysis converts at least some procapsids to B-capsids (Fig. 6b),
which differ by having an angular appearance (instead of round), and containing
cleaved forms of the internal scaffolding proteins (instead of precursors). B-capsids are
depicted in Fig. 4 as intermediates in the assembly pathway, but it has been difficult to
demonstrate their maturation to DNA-containing nucleocapsids (O'Callaghan and
Randall 1976; Ladin et al. 1982; Lee et al. 1988; Sherman and Bachenheimer 1988;
Church and Wilson 1997). One explanation is that only a small percentage of the rela-
tively large B-capsid pool incorporates DNA, making their loss from the pool difficult
to detect. Moreover, once DNA packaging begins, the particles involved may become
compositionally heterogeneous (e.g., a decreasing amount of scaffolding proteins and
an increasing amount of DNA) and, consequently, escape detection by methods rou-
tinely used to recover and characterize capsids (e.g., sedimentation and equilibrium
centrifugation). An alternate and plausible interpretation is that B-capsids are formed
when early steps in the procapsid maturation process fail, such as timely cleavage and
elimination of the scaffolding proteins or successful initiation of DNA packaging.
Fig. 6
Capsids in nucleus of CMV-infected cells. These selected particles from electron micrographs
of CMV-infected human foreskin fibroblasts show features consistent with the DNA packaging
scheme illustrated in Fig. 4.
a
Capsid at top appears to be at comparatively early stage of DNA
packaging. The elongated putative nascent core is smaller and less electron dense than those at the
top of panel
b and bottom of panel
c, and is off center to the extent of appearing to touch the inner
wall of the capsid and to be continuous with more filamentous material extending through the
capsid (presumably via portal) into the nucleoplasm (
arrow
).
b
Capsid at lower left appears to
contain scaffolding proteins absent in the particle just above it. Capsid at top contains DNA core
with filamentous extension through the capsid shell and into the nucleoplasm (
arrow
).
c
Lower
capsid appears to contain scaffolding proteins and upper capsid is interpreted to be nearly finished
packaging its DNA, the end of which may extend out to the left of the capsid (
arrow
)
