Cell Nucleus Biogenesis, Structure and Function
403
marker), various transcription factors, as
well as small ribonucleoproteins. These
bodies are not transcriptionally active, and
are thought to play a role in the biogenesis
of small nuclear RNA–protein complexes
(snRNPs) and in the nuclear trafFcking
of snRNPs and snoRNPs. RNP complexes
that are essential for mRNA splicing (U1,
U2, U4/U6, and U5 snRNPs), rRNA pro-
cessing (U3 and U8 snoRNPs) and histone
mRNA end formation (U7 snRNPs), all lo-
calize to the Cajal bodies. Possible roles
for these structures are suggested by the
fact that they are often seen to associate
with speciFc internuclear sites, such as the
histone loci and U1/U2/U3 gene cluster.
PML bodies are deFned by their content
of p
rom
yelocytic l
eukemia (PML) protein.
They are similar in size to coiled bodies but
are usually more abundant – mammalian
cells commonly have 10 to 20 PML
bodies. PML bodies have also been referred
to as the ND10 compartment, PODs
(PML oncogenic domains) and Kr bodies.
In addition to the PML protein, these
structures
also
contain
proteins
such
as: Sp100; SUMO1; HAUSP; and CBP.
Though the precise role of PML bodies
is unclear, it has been suggested that
they will play some role in transcriptional
regulation. Interestingly, these domains
are common targets of viral genomes
during the early infectious stages in
mammalian cells.
Pc-G bodies that contain polycomb
group proteins such as RING1, BMI1,
and hPc2 have been observed to as-
sociate with pericentromeric heterchro-
matin. Gems (gemini of Cajal bodies)
generally lie adjacent to the Cajal bodies
and are characterized by the presence of
the survival of motor neurons gene product
(SMN) and an associated factor Gemin2.
Other sites have been described that might
be classiFed as transcription superfacto-
ries. The OPT (O
ct1/P
T±/t
ranscription)
domain provides a good example. Im-
munofluorescence shows that in HeLa
cells, the transcription factors Oct1 and
PT±a
refoundin1to3l
a
rg
edom
a
in
s
(diameter
1
.
5
µ
m) that appear during
G1 phase and disappear in S-phase. Each
OPT domain typically contains 2 to 3
transcription sites where BrUTP is incor-
porated into nascent transcripts, as well as
RNA polymerase II, TBP, and Sp1. Some
chromosomes, notably chromosome 6, are
found to be closely associated with the OPT
domains. But while nuclear bodies have
raised much interest, the extent to which
they deFne essential functional compart-
ments remains a matter of debate. The
Cajal bodies exemplify this, as these struc-
tures can be disrupted in living cells or
transgenic animals without obvious detri-
ment to the hosts. Hence, the bodies, as
structures, might not be absolutely cru-
cial to the functions that are performed
by components that are normally found
within them.
3.5.4
Nuclear Speckles
Nuclear compartments such as nuclear
bodies are not membrane-bound. In-
stead, they appear to persist because
of the ‘‘self-assembly’’ characteristics of
their components and architectural con-
straints imposed by nuclear and chro-
mosome structure (see below). Nuclear
speckles/interchromatin granule clusters
(IGCs) provide an excellent example of
this architectural theme. (Speckles were
deFned using light microscopy and im-
munofluorescence and were later found to
be equivalent to the IGC that had been vi-
sualized using electron microscopy.) This
compartment is especially rich in pro-
teins required for RNA splicing – such
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