Biology Reference
In-Depth Information
Actin Filament Assembly:
The Search for a Barbed End
Craig F. Stovold, Stewart J. Sharp
and
Laura M. Machesky
Protrusion of the leading edge of a motile cell is thought to be largely driven
by actin assembly near the plasma membrane. Cells initially extend filopodia,
which are long thin processes composed of bundled actin or lamellipodia,
which are thin sheets containing highly branched arrays of actin filaments. It is
becoming clear that cells use a variety of mechanisms to initiate the assembly
of actin-based structures, centring around the need for the creation of free
actin filament ends for rapid polymerization. The Arp2/3 complex is a main
source of regulation for the availability of free barbed ends in cells and is
thought to nucleate branched actin filaments under the regulation of the
Wiskott-Aldrich Syndrome Family (WASp family) proteins. Additionally,
cells seem to be able to use other proteins, such as the formins in yeast, to
nucleate unbranched filaments, leading to the possibility that formation of
specific actin structures could be mediated by different sets of nucleating
proteins. Finally, the Ena/VASP proteins modulate the balance between
branching and growth of filaments by forming a complex that competes with
capping protein at the free growing end of the filament. Together, a variety of
actin modulating proteins are now thought to be responsible for the
complexity of actin-based protrusion and we are starting to understand how
they work together and in parallel to provide diversity in different cells under
various conditions.
