CH16 Cytoskeleton.pdf - finedrafts

1006 Chapter 16:The Cytoskeleton
u.5 um
functional plectin gene die within a few days of birth, with blistered skin and
abnormal skeletal and heart muscles. Thus, although plectin may not be necessary
for the initial formation and assembly of intermediate filaments, its crosslinking
action is required to provide cells with the strength they need to withstand
the mechanical stresses
inherent to vertebrate life.
cross-linking Proteins with Distinct properties organize Different
Assemblies of Actin Filaments
Actin filaments in animal cells are organized into two types of arrays: bundles
and weblike (gel-like) networks (Figure lHz). As described earlier, these different
structures are initiated by the action of distinct nucleating proteins: the
long straight filaments produced by formins make bundles and the ARp complex
makes webs. The actin filament cross-linking proteins that help to stabilize
and maintain these distinct structures are divided into tvvo classes: bundling
proteins and gel-forming proteins. Bundling proteins cross-link actin filaments
into a parallel array, while gel-forming proteins hold two actin filaments
together at a large angle to each other, thereby creating a looser meshwork. Both
Each type of bundling protein also determines which other molecules can
interact with an actin filament. Myosin II (discussed later) is the motor protein
in stress fibers and other contractile arrays that enables them to contract. The
contractile bundle
gel-like network
100 nm
Figure 16-46 Plectin cross-linking of
diverse cytoskeletal elements. Plectin
(green) is seen here making cross-links
from intermediate filaments (blue) to
microtubules ( red). ln this electron
micrograph, the dots (yellow) are gold
particles linked to anti-plectin antibodies.
The entire actin filament network was
removed to reveal these proteins. (From
T.M. Svitkina and G.G.Borisy, J. Cell Biol.
1 35:991-1 007, 1 996. With permission
from The Rockefeller University Press.)
Figure 16-47 Actin arrays in a cell.
A fibroblast crawling in a tissue culture dish
is shown with three areas enlarged to show
the arrangement of actin filaments. The
actin fifaments are shown in red, with
arrowheads pointing toward the minus
end. Stress fibers are contractile and exert
tension. Filopodia are spike-like projections
of the plasma membrane that allow a cell
to explore its environment. The cortex
underlies the plasma membrane.