Biomedical Engineering Reference
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different pathway than a heterotrimer by itself (Dupre et al.
2006
) . Virtually all
studies of G protein trafficking rely on overexpressed proteins; in the future it will
be important to try to follow the route of G proteins that are endogenous or expressed
at physiological levels.
In summary, heterotrimeric G proteins follow distinct trafficking pathways to
reach the PM after synthesis (Fig.
11.2
). Some of the key steps in trafficking routes
include (1) localization of Gb g at the cytoplasmic surface of the ER to undergo
C-terminal modifications of Gg; (2) Ga and Gb g heterotrimer formation and Ga
palmitoylation at an endomembrane site, either the cytoplasmic surface of the ER or
Golgi; and (3) trafficking of the heterotrimer to the PM, either in a poorly defined
Golgi-independent pathway from the ER to PM or in a Golgi-dependent pathway
attached to the cytoplasmic surface of PM-destined vesicles. The different trafficking
routes may reflect the intracellular formation of distinct protein complexes.
Importantly, all of these novel trafficking steps provide opportunity for G proteins
to regulate novel signaling pathways as well as targets for therapeutic intervention.
11.4
Activation-Dependent Internalization
and Recycling of G Proteins
Once nascent G proteins reach the PM through specific trafficking pathways, they
are able to carry out many of their classical signaling functions through interactions
with GPCRs and effector proteins. However, G protein localization continues to be
dynamic; they can rapidly and reversibly move from the PM to endomembrane
locations. Over the last decade or more, evidence has emerged to support the
notion that G proteins move throughout the cell in both constitutive and activation-
dependent pathways (Figs.
11.3
and
11.4
).
A variety of cell systems and experimental techniques have been used to estab-
lish that many G protein subunits translocate from the PM to intracellular locations
in response to activation by an appropriate GPCR. One experimental system that
has revealed a wealth of information about activation-induced G protein transloca-
tion is the mammalian visual system. A large number of studies have shown that
activation of the GPCR rhodopsin by light in mammalian retinal photoreceptor cells
results in a massive relocation of both the Ga and Gb g subunits of transducin from
the rod outer segment membranes to other subcellular locations (reviewed in Slepak
and Hurley
2008
; Artemyev
2008
; Calvert et al.
2006
). This G protein signaling
system is somewhat unique due to the high concentration of rhodopsin and transdu-
cin and due to the fact that transducin has a weaker hydrophobic lipid attachment to
membranes compared to other G proteins. Ga
t
is not palmitoylated. It is heteroge-
neously N-acylated by myristate (C14:0) and the less hydrophobic C14:1, C14:2
and C12 fatty acids. The transducin Gb
1
g
1
is attached to the membrane by farnesyla-
tion of Gg
1
; recall that farnesylation, like myristoylation, provides a relatively weak
membrane anchor. The translocation of transducin off of the rod outer segment
membrane is driven by activation-induced dissociation of the subunits. The a
t
b
1
g
1
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