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myosin II-mediated contraction of the posterior part of the cell body.
Chemoattractants, such as formyl-Met-Leu-Phe (f-MLP) in neutrophils and
cyclic adenosine monophosphate (cAMP) in Dictyostelium, trigger signalling
by activating specific GPCRs leading to the dissociation of the Ga and Gbg
subunits of the coupled heterotrimeric G protein.
The first insights into the establishment of an intracellular signalling
gradient came from experiments in Dictyostelium in which a subfamily of PH
domains that preferentially bind PI(3,4)P 2 and PI(3,4,5)P 3 , the lipid products
of Class I PI3Ks, rapidly and transiently translocate to the plasma membrane
in response to a global stimulation of cAMP (a rapid non-spatial rise in
chemoattractant so all receptors are uniformly activated) and to the leading
edge in chemotaxing cells (Figure 17.1A,B; Parent et al., 1998; Meili et al.,
1999; Funamoto et al., 2001). These studies were subsequently extended to
neutrophils (Servant et al., 2000). This translocation is not dependent on the
actin cytoskeleton, as it occurs in the presence of the actin polymerization
inhibitor latrunculin A (Parent et al., 1998).
Mechanisms controlling PH domain localization and the role
of PI3K in chemotaxis
Binding of a chemoattractant to a GPCR results in recruitment of PH
domain-containing proteins to the plasma membrane. These proteins include
CRAC, a cytosolic regulator of adenylyl cyclase, the protein kinase Akt/PKB,
and the protein PhdA (Parent et al., 1998; Meili et al., 1999; Funamoto et al.,
2001). As these PH domains bind to the D3-PIs (phosphatidyl inositols
phosphorylated on the 3' position of the inositol ring) PI(3,4)P 2 and
PI(3,4,5)P 3 , PH domain translocation in living cells suggests rapid and
transient accumulation of PI(3,4)P 2 /PI(3,4,5)P 3 on the membrane, while the
asymmetric localization of the PH domains in chemotaxing cells reflects the
preferential accumulation of D3-PIs at the leading edge.
These results suggested that PH domain localization is dependent on the
activation of PI3Ks. This was confirmed by demonstrating that PH domain
localization in Dictyostelium cells is abrogated in cells in which the genes
encoding two of the three Class I PI3Ks, PI3K1 and PI3K2, are disrupted
(pi3k1/2 null cells) or by treating wild-type cells with the PI3K inhibitor
LY294002 (Chung et al., 2001b, Funamoto et al., 2001). In mammalian cells,
the Class 1B PI3Kg (p110g) is activated downstream of the fMLP and C5a
chemoattractant receptors (Stephens et al., 1994; Stoyanov et al., 1995). In the
case of PI3Kg, the p110g/p101 holoenzyme (p110g catalytic subunit and the
adaptor protein p101) exhibit an increased lipid kinase activity in response to
binding by the Gbg subunit of
the coupled heterotrimeric G protein,
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