Arrestins as Regulators of Kinases and Phosphatases - Progress in Molecular Biology and Translational Science - page 126

has the opposite effect, enhancing the arrestin-dependent component of the
ERK1/2 signal, 34,46 suggesting that, with respect to ERK1/2 activation,
arrestin3 is the signaling arrestin isoform, while arrestin2 functions only in
desensitization. Arrestin3 is certainly the key isoform for Class
A receptors like the b 2 adrenergic, which do not bind arrestin2 efficiently, 85
but the dichotomy of isoform-specific arrestin function does not appear to
hold for all Class B receptors. Arrestin-dependent ERK1/2 activation by the
type 1 parathyroid hormone receptor, for example, is inhibited when either
isoform is downregulated, suggesting that both are required to assemble
functional signalsomes. 86
Additional evidence suggests that arrestins adopt different “active” con-
formations depending on which GPCR they bind and which GRK
phosphorylated the receptor. Evidence of the former comes from character-
ization of arrestin ubiquitination. Ubiquitination of lysines 11 and 12 of
arrestin3 is necessary for it to remain stably bound to the AT 1A receptor,
yet an arrestin3 (K11,12R) mutant is still ubiquitinated and fully functional
when recruited to the vasopressin V2 receptor. 45 All 31 lysines must be
mutated before arrestin3 ubiquitination is lost upon b 2-adrenergic receptor
binding. 87 The variability suggests that either the conformation or the acces-
sibility of surface epitopes on arrestin3 differs depending on the GPCR-
binding partner. Additional support for different arrestin conformations
comes from data obtained using isoform-selective silencing of GRKs that
suggest GRK2 and GRK3 phosphorylation of the AT 1A and V2 receptors
promotes arrestin-dependent desensitization, while GRK5 and GRK6
appear to be exclusively responsible for initiating arrestin-dependent
ERK1/2 activation. 88,89 Similar work with the b 2-adrenergic receptor sug-
gests that GRK2 and GRK6 phosphorylate the receptor C-terminus at dif-
ferent sites, and that only the GRK6-induced pattern of phosphorylation
supports arrestin-dependent ERK1/2 activation. 90 This has led to the
hypothesis that different GRKs establish a phosphorylation “barcode” that
imparts distinct arrestin3 conformations to regulate its functional activity.
Signalsomes regulate the spatial, temporal, and functional characteristics
of ERK1/2. When recruited to a Class B receptor, active ERK1/2 accumu-
lates in early endosomes, failing to translocate to the cell nucleus. Wild-type
PAR2 receptors predominantly utilize the arrestin-dependent pathway to
activate ERK1/2. 14 As a result, ERK1/2 is excluded from the nucleus
and does not stimulate proliferation. In contrast, a C-terminal phosphory-
lation site mutant PAR2, that does not bind arrestins or internalize, activates
ERK1/2 via a G protein-dependent pathway that promotes its nuclear