Arrestins as Regulators of Kinases and Phosphatases - Progress in Molecular Biology and Translational Science - page 114

between parallel cascades, and localize signaling to specific subcellular
locations. 26
The prototypic MAP kinase scaffold is the Saccharomyces cerevisiae protein
Ste5p. 27 In the yeast pheromone mating pathway, Ste5p binds to each of the
three components of the yeast MAP kinase pathway, Ste11p, Ste7p, and
either Fus3 or Kss1. Binding of mating factor to the pheromone receptor,
a GPCR, leads to translocation of Ste5 to the plasma membrane and activa-
tion of the Fus3/Kss1 cascade. Deletion of the Ste5 gene disrupts the yeast
mating process, indicating the essential role of this scaffold in MAP kinase
regulation. While there are no structural homologues of Ste5 in mammalian
cells, arrestins perform an analogous function, organizing individual path-
way components to increase efficiency and fidelity of signaling, imposing
spatial constraints, and, importantly, providing for pathway regulation by
extracellular signals detected via GPCRs on the plasma membrane.
This scaffolding property is illustrated by the effect of arrestin3 on the
activation state of c-Jun N-terminal kinase 3 (JNK3). JNK3 is a neuronal
MAPK that is regulated by a phosphorylation cascade composed of the
upstream MAPK kinase kinase, Ask1; the MAPK kinase, MKK4; and
JNK3. Arrestin3 binds the three component kinases, increasing the effi-
ciency of phosphorylation. Ectopic expression of Ask1 and JNK3 in
COS-7 cells is not sufficient to activate JNK3, but simultaneous expression
of arrestin3 with Ask1 and JNK3 results in dramatic activation. 13 Arrestin3
is excluded from the nucleus by the presence of a classical leucine-rich
nuclear export signal in its C-terminus that is absent in arrestin2. 28,29 Con-
sequently, expression of arrestin3 also leads to a dramatic redistribution of
JNK3 from the nucleus into an arrestin-bound cytosolic pool. 13,30
Arrestin-dependent regulation of the ERK1/2 MAPK cascade provided
another early example of arrestin scaffolding. Both arrestin2 and 3 can assem-
ble a complex composed of c-Raf1, MEK1/2, and ERK1/2, and enhance
ERK1/2 activation in response to stimulation of protease-activated receptor
2 (PAR2) and angiotensin II AT 1A receptors. 14,15,25,31 Agonist binding
induces the assembly of a protein complex containing the internalized recep-
tor, arrestin, and activated ERK1/2. This arrestin-mediated signal is inde-
pendent of heterotrimeric G protein activation, as it can be produced by
mutated AT 1A receptors that lack G protein-coupling efficacy and by
“biased” AT 1A receptor agonists that promote arrestin recruitment and
receptor internalization without G protein activation. 32 Because the com-
plex formed between PAR2 and AT 1A receptors and arrestins is stable, these
signalsomes localize activated ERK1/2 to early endosomes, where it remains