414
Cell Nucleus Biogenesis, Structure and Function
No
Fig. 8
Nuclear architecture and the nucleoskeleton. To
reveal the nucleoskeleton, HeLa cells encapsulated in agarose
microbeads were permeabilized and chromatin cut with
nucleases so that
90% DNA could be removed before
preparing a resinless electron micrograph. In this
250-nm
thick section, note that a diffuse agarose mesh surrounds the
densely stained cell. The spherical, central nucleus is
demarcated by a dense nuclear lamina (L). Critically, in this
example, nuclear architecture is preserved even though most
chromatin – half of the nuclear mass – is removed. The
residual nucleoskeleton can be visualized as a diffuse network
of coated Flaments that pervade the nuclear volume and
provide a ‘‘solid phase’’ upon which nuclear compartments
such as nucleoli (No), replication and transcription factories
and interchromatin granule clusters are assembled. Note that
the distribution of transcription and replication centers is
preserved even though almost all chromatin has been
removed. The bar is 2
µ
m. (Reproduced from Hozak, P.,
Jackson, D.A., Cook, P.R. (1994) Replication factories and
nuclear bodies: the ultrastructural characterization of
replication sites during the cell cycle,
J. Cell Sci.
107
,
2191–2202. With permission of the Company of Biologists.)
a silenced chromatin state. Another pro-
tein, SAF-B, (scaffold attachment factor B)
speci±cally binds to S/MAR regions, in-
teracts with RNA polymerase II (RNA pol
II) and a subset of serine-/arginine-rich
RNA processing factors (SR proteins). It
was proposed that these interactions allow
SAF-B to provide a surface for the assem-
bly of the transcription apparatus. As with
the vast majority of characterized matrix
proteins, these are involved in different
aspects of chromatin function. Extending
this idea implies that the nucleoskeleton
and nuclear matrix (that forms during salt
extraction) are an expression of different
processes that are performed within the
interchromatin space.
Recent studies on the nuclear matrix
protein SATB1 (special AT-rich sequence
binding 1) emphasize this view. SATB1 is
a nuclear matrix component that is found
predominantly in thymocytes. In these
cells, a nuclear network of SATB1 binds
chromatin to form chromatin domains
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