394
Cell Nucleus Biogenesis, Structure and Function
Cytoplasmic ring
Lumen domain
Inner membrane
Outer membrane
Basic
framework
Terminal
ring
Nuclear
basket
Cytoplasmic filaments
Nuclear ring
Central plug
~40 nm
50
100 nm
50 nm
Fig. 3
The nuclear pore. The structure of the nuclear pore complex, shown
here in cartoon form, was assimilated using a variety of electron microscopy
(em) techniques. Details of the structure and the location of the constituent
components are given in the text and in Table 3. The left inset (courtesy
Francisco Iborra) shows a classical em thin section prepared in an epoxy
resin, the bar is 100 nm. The nucleus occupies the upper half of the image.
Note the densely stained heterochromatin lying against the nuclear
membrane, the structure of the double membrane and the density of
material at the pore. The right inset shows the inner nuclear face of an
isolated nuclear membrane, visualized using a high-voltage scanning
electron microscope. This shows the structure of the nuclear baskets and
emphasizes the eightfold symmetry of the pore complex. The central image
isreproducedfromIzaurra
lde
,E
.
,Kann,M.
,Pante
,N
.
,Sode
ik,B
.
,Hohn,T
.
(1999) Viruses, microorganisms and scientists meet the nuclear pore,
EMBO J.
18
, 289–296. With permission of Oxford University Press and the
right inset is from Goldberg, M.W., Allen, T.D. (1992) High-resolution
electron microscopy of the nuclear envelope – demonstration of a new,
regular ±brous lattice attached to the baskets of the nucleoplasmic face of
nuclear pores,
J. Cell Biol.
119
, 1429–1440. With permission of Rockefeller
University Press.
Th
el
am
in
sa
r
ec
l
a
s
s
if
eda
st
yp
eVin
-
termediate flaments (IFs), and like all
intermediate flament proteins have a spe-
cifc domain with an
α
-helical coiled-coil
region flanked by nonhelical domains.
The central coiled-coil domain is essen-
tial ±or assembly o± the flaments. This
process occurs initially through dimers
and then through higher multimers, such
that a mature IF o± 10 nm in cross-section
has
32 monomers in its cross-section.
During the assembly process, specifc
alignment o± the IF protein monomers
gives a repeat structure with
23 nm
periodicity. Major IF proteins in the cy-
toplasm o± mammalian cells, such as
vimentin, ±orm very regular 10 nm (di-
ameter) flaments that rarely branch. The
nuclear lamins, in contrast, are highly
branched and branches o± smaller cross-
section (i.e.
suggesting
flaments
with
±ewer monomers in cross-section) are o±-
ten seen. In addition to the peripheral
nuclear lamina, nuclear lamin proteins
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