Page 213 - Color Atlas of Biochemistry

P. 213

204 Organelles

Cytoskeleton: components to other cell components (villin, D–actinin,
spectrin), or disrupt the helical structure of F
The cytoplasm of eukaryotic cells is traversed actin (gelsolin). The activity of these proteins
by three–dimensional scaffolding structures is regulated by protein kinases via Ca 2+ and
consisting of filaments (long protein fibers), other second messengers (see p. 386).
which together form the cytoskeleton.These
filaments are divided into three groups, based
B. Intermediate filaments
on their diameters: microfilaments (6–8 nm),
intermediate filaments (ca. 10 nm), and mi- The components of the intermediate fila-
crotubules (ca. 25 nm). All of these filaments ments belong to five related protein families.
are polymers assembled from protein compo- They are specific for particular cell types. Typ-
nents. ical representatives include the cytokeratins,
desmin, vimentin, glial fibrillary acidic protein
(GFAP), and neurofilament. These proteins all
A. Actin
have a rod–shaped basic structure in the cen-
Actin, the most abundant protein in eukary- ter, which is known as a superhelix (“coiled
otic cells, is the protein component of the coil”; see keratin, p. 70). The dimers are ar-
microfilaments (actin filaments). Actin occurs ranged in an antiparallel fashion to form tet-
in two forms—a monomolecular form (Gactin, ramers. A staggered head-to–head arrange-
globular actin) and a polymer (Factin,fila- ment produces protofilaments. Eight protofi-
mentous actin). G actin is an asymmetrical laments ultimately form an intermediary fil-
molecule with a mass of 42 kDa, consisting ament.
of two domains. As the ionic strength in- Free protein monomers of intermediate fil-
creases, G actin aggregates reversibly to aments rarely occur in the cytoplasm, in con-
form F actin, a helical homopolymer. G actin trast to microfilaments and microtubules.
carries a firmly bound ATP molecule that is Their polymerization leads to stable polymers
slowly hydrolyzed in F actin to form ADP. that have no polarity.
Actin therefore also has enzyme properties
(ATPase activity).
As individual G actin molecules are always C. Tubulins
oriented in the same direction relative to one The basic components of the tube-shaped mi-
another, F actin consequently has polarity. It crotubules are α–and β–tubulin (53 and
has two different ends, at which polymeriza- 55 kDa). These form α,β-heterodimers, which
tion takes place at different rates. If the ends in turn polymerize to form linear protofila-
are not stabilized by special proteins (as in ments. Thirteen protofilaments form a ring-
muscle cells), then at a critical concentration shaped complex, which then grows into a
of G actin the (+) end of F actin will constantly long tube as a result of further polymeriza-
grow, while the (–) end simultaneously de- tion.
cays. These partial processes can be blocked Like microfilaments, microtubules are dy-
by fungal toxins experimentally. Phalloidin,a namic structures with (+) and (–) ends. The
toxin contained in the Amanita phalloides (–) end is usually stabilized by bonding to the
mushroom, inhibits decay by binding to the centrosome. The (+) end shows dynamic
(–) end. By contrast, cytochalasins,moldtox- instability. It can either grow slowly or
ins with cytostatic effects, block polymeriza- shorten rapidly. GTP, which is bound by the
tion by binding to the (+) end. microtubules and gradually hydrolyzed into
Actin–associated proteins. The cytoplasm GDP, plays a role in this. Various proteins can
contains more than 50 different proteins also be associated with microtubules.
that bind specifically to G actin and F actin.
Their actin uptake has various different func-
tions. This type of bonding can serve to regu-
late the G actin pool (example: profilin), influ-
ence the polymerization rate of G actin (vil-
lin), stabilize the chain ends of F actin (fragin,
b–actinin), attach filaments to one another or

208 209 210 211 212 213 214 215 216 217 218