342
Cell Junctions, Structure, Function, and Regulation
Fig. 7
Tyrosine phosphorylated
proteins at focal adhesions. Shown is a
rat embryo Fbroblast stained for actin in
stress Fbers (green) and with antibodies
that recognize phosphotyrosine (red).
Phosphotyrosine staining is seen in
focal adhesions at the ends of stress
Fbers. Regions of overlapping red and
green staining are yellow. (Reprinted
with permission from Burridge, K.,
Chrzanowska-Wodnicka, M., Zong, C.
(1997) ±ocal adhesion assembly,
Trends
Cell Biol.
7
, 342–347.) (see color plate
p. xxvii).
interactions involving integrins and other
focal adhesion proteins, complex linkage
maps have been proposed connecting inte-
grins to the activation of various signaling
pathways (Figs. 3 and 6). Surprisingly, new
focal adhesion proteins are still being
identi±ed every year. The most recently
identi±ed focal adhesion protein is the
Mig±lin, which is a ±lamin-binding pro-
tein that regulates cell spreading.
1.4.2
Regulation of Focal Adhesion
Assembly
Focal adhesions are formed by most cell
types when cultured on two-dimensional
substrates. The assembly of focal adhe-
sions has been studied in spreading and
migrating cells. In these instances, the ac-
tivation of Rac or Cdc42, either by integrin
or growth factor signaling, promotes the
formation of ±lopodia and lamellipodia
and new integrin-mediated cell–matrix
junctions. These nascent adhesions, re-
ferred to as focal complexes, contain inte-
grins, as well as a subset of focal adhesions
proteins. Following the activation of Rho
and actinomyosin contractility, focal com-
plexes mature into focal adhesions with
their associated stress ±bers. The forma-
tion of focal adhesion has also been studied
in quiescent cells, where the activation Rho
and
downstream
myosin
II-dependent
contractility by extracellular factors in-
duces focal adhesion formation. Focal
adhesion formation can also be promoted
when external force is applied to integrin-
mediated adhesions. When this force is
increased, focal adhesions become larger,
whereas a reduction in the applied force
causes disassembly of focal adhesions. The
stimulation of focal adhesion formation by
external force also requires Rho signaling,
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