Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

950 Chapter 16: The Cytoskeleton
(A) bud
(B) (C)
neck
filaments
mother cell
0.5 µm 10 µm 2 µm
Figure 16–73 Cell compartmentalization
by septins. (A) Septins form filaments in
the neck region between a mother yeast
cell and bud. (B) In this photomicrograph
of human cultured cells, the DNA is
stained blue and septins are labeled in
green. The microtubules of primary cilia are
labeled with an antibody that recognizes a
modified (acetylated) form of tubulin (red)
that is enriched in the axoneme. (C) A
magnified image reveals a collar of septin
at the base of the cilium. (A, from B. Byers
and L. Goetsch, J. Cell Biol. 69:717–721,
1976. With permission from Rockefeller
University Press. B and C, from Q. Hu et
al., Science 329:436–439, 2010. With
permission from AAAS.)
form nonpolar paired filaments (Figure 16–74). GTP binding is required for the
folding of septin polypeptides, but the role of GTP hydrolysis in septin function is
not understood. Septin structures assemble and disassemble inside cells, but they
are not as dynamic as actin filaments and microtubules.
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Summary
Whereas tubulin and actin have been highly conserved in evolution, intermediate
filament proteins are very diverse. There are many tissue-specific forms of intermediate
filaments in the cytoplasm of animal cells, including keratin filaments in epithelial
cells, neurofilaments in nerve cells, and desmin filaments in muscle cells. The
primary function of these filaments is to provide mechanical strength. Septins comprise
an additional system of filaments that organize compartments inside cells.
(A) (B) (C)
Cdc11
Cdc12
Cdc3
Cdc10
Cdc10
Cdc3
Cdc12
Cdc11
100 nm
Figure 16–74 Septins polymerize to form paired filaments and sheets.
(A) Electron micrograph of a septin rod assembled by combining two copies
each of the four yeast septins illustrated at the right. The eight-subunit rod is
nonpolar because the central pair of subunits (Cdc10) creates a symmetrical
dimer. (B) Electron micrograph of paired septin filaments and sheets, assembled
from purified septins in the presence of high salt concentrations. (C) Paired septin
filaments may assemble by lateral association between filaments, mediated by
coiled-coils formed between the paired C-terminal extensions of Cdc3 and Cdc12
that project from each filament. (Images and schematics adapted from A. Bertin
et al., Proc. Natl Acad. Sci. USA 105:8274–8279, 2008. With permission from the
National Academy of Sciences.)
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