Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

958 Chapter 16: The Cytoskeleton
(A)
WASp
family
Rac-GTP
Arp 2/3
(branching
nucleator)
PAK
filamin
(web
cross-linker)
branched actin web
in lamellipodia
MHC
MLCK
(decreased)
myosin
activity
less stress
fiber formation
LIM
kinase
cofilin
MLC(P)
(increased)
myosin
activity
Rho-GTP
MLC
phosphatase
more stress fibers
actin
bundle
growth
integrin clustering and
focal adhesion formation
As we will explore in more detail below, the communication between the Rac and
Rho pathways also facilitates maintenance of the large-scale differences between
the cell front and the cell rear during migration.
Extracellular Signals Can Activate the Three Rho Protein Family
Members
MBoC6 m16.98/16.88
The activation of the monomeric GTPases Rho, Rac, and Cdc42 occurs through an
exchange of GTP for a tightly bound GDP molecule, catalyzed by guanine nucleotide
exchange factors (GEFs). Of the many GEFs that have been identified in the
human genome, some are specific for an individual Rho family GTPase, whereas
others seem to act on multiple family members. Different GEFs are restricted to
specific tissues and even specific subcellular locations, and they are sensitive to
distinct kinds of regulatory inputs. GEFs can be activated by extracellular cues
through cell-surface receptors, or in response to intracellular signals. GEFs may
also act as scaffolds that direct GTPases to downstream effectors. Interestingly,
several of the Rho family GEFs associate with the growing ends of microtubules
by binding to one of the +TIPs. This provides a connection between the dynamics
of the microtubule cytoskeleton and the large-scale organization of the actin cytoskeleton;
such a connection is important for the overall integration of cell shape
and movement.
(B)
Rho-dependent
kinase (Rock)
formins
Figure 16–85 The contrasting effects
of Rac and Rho activation on actin
organization. (A) Activation of the small
GTPase Rac leads to alterations in actin
accessory proteins that tend to favor
the formation of actin networks, as in
lamellipodia. Several different pathways
contribute independently. Rac-GTP
activates members of the WASp protein
family, which in turn activate actin
nucleation and branched web formation by
the Arp 2/3 complex. In a parallel pathway,
Rac-GTP activates a protein kinase, PAK,
which has several targets including the
web-forming cross-linker filamin, which
is activated by phosphorylation, and the
myosin light chain kinase (MLCK), which is
inhibited by phosphorylation. Inhibition of
MLCK results in decreased phosphorylation
of the myosin regulatory light chain and
leads to myosin II filament disassembly and
a decrease in contractile activity. In some
cells, PAK also directly inhibits myosin II
activity by phosphorylation of the myosin
heavy chain (MHC). (B) Activation of the
related GTPase Rho leads to nucleation of
actin filaments by formins and increases
contraction by myosin II, promoting the
formation of contractile actin bundles
such as stress fibers. Activation of myosin
II by Rho requires a Rho-dependent
protein kinase called Rock. This kinase
inhibits the phosphatase that removes the
activating phosphate groups from myosin
II light chains (MLC); it may also directly
phosphorylate the myosin light chains in
some cell types. Rock also activates other
protein kinases, such as LIM kinase, which
in turn contributes to the formation of
stable contractile actin filament bundles by
inhibiting the actin depolymerizing factor
cofilin. A similar signaling pathway
is important for forming the contractile
ring necessary for cytokinesis (see
Figure 17–44).
External Signals Can Dictate the Direction of Cell Migration
Chemotaxis is the movement of a cell toward or away from a source of some diffusible
chemical. These external signals act through Rho family proteins to set up
large-scale cell polarity by influencing the organization of the cell motility apparatus.
One well-studied example is the chemotactic movement of a class of white
blood cells, called neutrophils, toward a source of bacterial infection. Receptor
proteins on the surface of neutrophils enable them to detect very low concentrations
of N-formylated peptides that are derived from bacterial proteins (only
prokaryotes begin protein synthesis with N-formylmethionine). Using these
receptors, neutrophils are guided to bacterial targets by their ability to detect a
difference of only 1% in the concentration of these diffusible peptides on one side
of the cell versus the other (Figure 16–86A).
In this case, and in the chemotaxis of Dictyostelium amoebae toward a source
of cyclic AMP, binding of the chemoattractant to its G-protein-coupled receptor
activates phosphoinositide 3-kinases (PI3Ks) (see Figure 15–52), which generate
a signaling molecule [PI(3,4,5)P 3 ] that in turn activates the Rac GTPase. Rac