Arrestins as Regulators of Kinases and Phosphatases - Progress in Molecular Biology and Translational Science - page 116

associated with the receptor-arrestin complex 14,15 ( Fig. 5.2 A). In fact, an
estimated 75-80% of the active ERK1/2 produced in response to short-term
stimulation of the PAR2 receptors is associated with the GPCR-arrestin
signalsome. 14 As a result, nuclear translocation of active ERK1/2 is retarded
and its kinase activity is directed away from nuclear, and toward cytosolic,
targets. 31,34,35
2.2. Mechanism of arrestin-signaling scaffolds
In contrast to the clathrin and AP-2 binding sites in the C-terminus, and
receptor recognition motifs within the N- and C-terminal globular domains
of arrestin (see Chapter 3 ), the binding sites for most arrestin-regulated sig-
naling partners have not been precisely mapped. In fact, all three compo-
nents of the c-Raf1-MEK1-ERK1/2 and ASK1-MKK4-JNK3 cascades
can bind to separately expressed arrestin N- and C-domains, suggesting that
they each make multiple contacts across the exposed cytosolic face of the
receptor-bound arrestin. 36,37
Since arrestins undergo a conformational change when they bind to
GPCRs, activation of arrestin-bound effectors might then proceed by either
of two mechanisms. Receptor binding might change the affinity of arrestin
for its nonreceptor partners, allowing the signalsome complex to form only
on the receptor. Alternatively, the arrestin-effector complex may be pref-
ormed, with activation resulting either from conformational changes in
the arrestin that provide the proper orientation of pathway components
or from the receptor-dependent translocation of components to the plasma
membrane where they gain access to otherwise unavailable upstream path-
way activators. Evidence suggests that both mechanisms play a role in
arrestin signaling.
ERK1/2 and, to a lesser extent, c-Raf1 bind with high affinity only to
the receptor-bound conformation of arrestin. 38 Although MEK1 binds
equivalently to both receptor-bound and free arrestin, assembly of a produc-
tive c-Raf1-MEK1-ERK1/2 scaffolding complex requires an active
GPCR docking site. Membrane translocation may also play a role. Expres-
sion of a G protein-uncoupled neurokinin NK1-arrestin2 chimera leads
to constitutive activation of a pool of ERK1/2 that remains bound,
along with c-Raf1 and MEK1/2 to the endosomal membrane-delimited
receptor-arrestin chimera. 39 Since membrane targeting of c-Raf1 is itself
sufficient to activate ERK1/2, 40 one possibility is that the arrestin
functions simply to move cytosolic c-Raf1 to the membrane for activation.