cell news 2/2013
18
research news
towards a better understanding of actin dynamics in vivo –
a high resolution view of the fy actin cytoskeleton
sven bogdan
insitute for neurobiology, university of Münster, badestraße 9, d-48149 Münster
Introduction
The actin cytoskeleton provides mechanical forces to drive cell
shape changes and cell migration during morphogenesis. For
initiation of actin polymerization, a nucleation core (nucleus)
composed of three actin monomers is required. Once this actin
nucleus is formed, actin polymerizes spontaneously into long
flaments (Figure 1). In vivo, the actin polymerization is strictly
controlled in time and space. Eukaryotic cells have evolved a
multitude of actin binding proteins that maintain the pool of
actin monomers, promote actin nucleation, restrict the length of
actin flaments and cross-link flaments into networks or bund-
les (Figure 1; Pollard & Borisy, 2003, Pollard & Cooper, 2009).
Thus, these accessory proteins also defne the mechanical and
dynamic properties of actin flaments and differentially control
the organization of actin flaments into higher-order structures
adapted to fulfll distinct cellular functions. New superresoluti-
on microscopy techniques such structured illumination micro-
scopy (SIM) allow to visualize actin structures as well as protein
localization at high spatial and temporal resolution, a perquisite
for a better mechanistic understanding how actin regulatory
proteins act in concert in vivo.
Actin nucleation – a key step in actin polymerization
The rate-limiting step in actin polymerization is the de novo nu-
cleation of actin flaments from actin monomers. To overcome
this high kinetic barrier, cells have evolved a variety of actin
nucleators that catalyze the nucleation reaction. The Arp2/3
protein complex is one of the key players initiating de novo for-
F-actin
elongation
G-actin
nucleation
Phagocytic structures
Endocytic structures
Lamellipodial
protrusion
Filopodia and
microspikes
Figure 1: Actin binding
proteins controlling
distinct aspects of actin
cytoskeleton architecture
and dynamics.
Schematic drawing of an
eukaryotic cell, depicting
the different cellular roles
of actin binding prote-
ins in lamellipodial and
flopodial protrusion, endo-
cytosis and phagocytosis
(adapted from Lodish et al.,
Molecular Cell Biology, 6th
ed., 2008). For initiation
of actin polymerization, a
nucleation core (nucleus)
composed of three actin
monomers is required.
Once the actin nucleus is
formed, additional actin
monomers bind to the
nucleus spontaneously re-
sulting into a fast flament
elongation.
