Overview Actin filaments
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text below the images if available! (Labelling is in German)
Actin filaments (Terminologia histologica: Filamenta actinia) are the microfilaments
of the cytoskeleton. They are present in
all cells and comprised of two fibrils helical winding around each other.
These fibrils form by polymerisation of free actin molecules, which
have a molecular wheight of 42 000 Dalton (Da).
|area densa + actin filaments
of a smooth muscle cell 1 (rat)
|actin filaments & area
densa, smooth muscle cell 2 (rat)
|detail of the previous image
||area densa + actin filaments
of a smooth muscle cell 3 (rat)
|actin filaments , synapse en pass-
ent smooth muscle cell (rat)
|cross-section of striated
muscle, actin filaments (rat)
About 10% of all proteins in a cell are actin in muscle
cells, in other cells the actin proportion ranges from ~1 to ~5%. Roughly
parallel actin filaments are responsible for stability of microvilli.
Regarding actin molecules, there are 3 different alpha-actins, one
of which is exclusively present in one kind of muscle. In all other (non-muscle)
cells beta- or gamma-actin is seen. One kind of gamma acin
is found in smooth muscle cells of the gut besides the alpha actin typical
for smooth muscle cells. Actin may be present as globular monomere (G-actin)
or as polymerised, filamentous F-actin.
Microfilaments are formed by this F-actin with linked proteins. All actin
molecules have a magnesium ion binding adenosin-di-phosphate (ADP) or adenosin-tri-phosphate
(ATP). Plate-like beta-actin monomeres are about 5.5 x 5.5 x 3 nanometers
(nm) in size and have 4 subdomains. The helical structure of actin filaments
may be seen under ultra-high resolution in the electron microscope. The
diameter of a single filament is 7 nm in thinner and 9
nm in thick portions, i.e. the mean diameter is 8
nm. The sequence of the 28 subunits of an actin filament helix which
consists of two F-actin fibrils helical winding around each other is repeated
all 72 nm. Actin-cross-linking proteins connect actin filaments
in all bundles or meshworks formed by actin in the cytoskeleton. These
coss-linking proteins are classified according to the character of their
actin binding domains in 3 groups:
group 1: 30 KD protein (33.000 Da; pseudopods,
stress-fibres), EF-1a (50.000 Da; pseudopods),
Fascin (55.000 Da; pseudopods, stress-fibres,
microvilli, acrosomal processes), Scruin (102.000
Da; acrosomal processes).
group 2: Villin (92.000 Da; microvilli
of enterocytes in gut and renal tubules), Dematin
(48.000 Da; red blood cells).
group 3: Fimbrin (68.000 Da; microvilli,
adhesive strips), alpha-Actinin (102.000 Da;
smooth muscle cells, stress-fibres, pseudopods),
Spectrin (alpha subunit: 280.000 + beta subunit: 246.000-275.000 Da; terminal
web), Dystrophin (427.000 Da; cell membrane
close meshwork in striated muscle cells),
ABP 120 (92.000 Da; pseudopods), Filamin
(280.000; pseudopods, stress-fibres).
The term contractile filaments is used for both, actin and myosin
filaments together, in muscle cells. These
contractile filaments cause contraction of these cells which finally bases
on a knicking of myosin heads. Almost perfect parallel order of actin filaments
is seen in the I-band of striated
muscle cells (images above). These actin filaments
are linked to each other at the Z-stripe
with help of the capZ-protein. In the A-band
myosin filaments are located between actin filaments.
In general, the three dimensional shape of any cell mainly is caused
by the organisation of cell membrane-close
actin filament bundles and meshworks and their connection to the cell
membrane which is realised by the previously mentioned linking proteins.
The latter also have connections to cell
membrane proteins. Thus these connections also stabilise the cell
--> smooth muscle cells, Area
--> Electron microscopic atlas Overview
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Five images were kindly provided by Prof. H. Wartenberg;
other images, page & copyright H. Jastrow.