A Snapshot
of
Molecules at Work
|
The G proteins are heterotrimers consisting of three subunits (α,
β, and γ). GPCRs activated by the extracellular signal catalyze
nucleotide exchange on G proteins, thereby converting the inactive
(GDP-bound) Gαβγ heterotrimer into activated Gα·GTP
and Gβγ proteins. These subunits
then interact with downstream proteins called effectors to elicit
an appropriate intracellular response. For example, activation of
receptors coupled to Gq leads to formation of Gαq·GTP
and Gβγ, which in turn bind to
and stimulate phospholipase Cβ (PLCβ). Gq represents
one of the four subfamilies of the Gα
subunit (Gαs, Gαt,
Gαq, and Gα12/13).
Heterotrimeric G proteins (ribbon structures above)
undergo a dramatic change in configuration (below) as they become
activated and bind GRK2 (solid spheres). The view is from the perspective
of the membrane surface. The Gαq and Gβγ
subunits separate by about 80 Å and the Gα subunit
rotates more than 100° with respect to Gβγ after GRK2
binding. The so-called “switch” regions of Gαq
that undergo conformational change upon binding GTP are shown in
red.
In order to adapt to new extracellular conditions, Gq-coupled
receptors are desensitized by enzymes such as GRK2, which not only
add phosphate groups to (phosphorylate) activated GPCRs, initiating
their down-regulation, but also sequester activated Gαq
and Gβγ subunits from PLCβ and presumably other effectors.
In order to better understand the arrangement of heterotrimeric
G proteins in complex with GRK2, the Michigan–Illinois team
crystallized the Gαq-GRK2-Gβγ complex.
After visiting three different beamlines, the team obtained the
best diffraction data at ALS Beamline 8.3.1,
which allowed them to solve the structure at a resolution of 3.1
Å by the molecular-replacement method.
The Gαq-GRK2-Gβγ structure is the first structure obtained to
date of Gαq; it also reveals for the first time the configuration
of activated Gα and Gγβ subunits at the cell membrane. In the Gαq-GRK2-Gβγ
complex, activated Gαq is completely dissociated from Gβγ and undergoes
a dramatic, approximately 105° rotation with respect to its
position in the Gαβγ heterotrimer. Because GPCRs also interact
with GRK2, the Gαq-GRK2-Gβγ structure supports the hypothesis that
high-order complexes consisting of receptors, G proteins, and effectors
assemble at the cell membrane in response to extracellular stimuli,
in this case with GRK2 serving as the scaffold.
GRK2 interacts with Gαq through its regulator of G-protein-signaling
(RGS) homology (RH) domain. Many other proteins with RH domains
(e.g., RGS4) function as GTPase-activating proteins (GAPs) for
Gα subunits, thereby terminating signal transduction by converting
the GTP back to GDP. However, the RH domain of GRK2 interacts with
Gαq in a manner that is inconsistent with that of a GAP. Instead,
the GRK2 RH domain binds to an analogous site on Gαq used by the
enzymes adenylyl cyclase or cGMP phosphodiesterase to bind Gαs
or Gαt, respectively. This binding site could mean that GRK2, like
PLCβ, is in fact a downstream effector target of Gαq. In support
of this hypothesis, GRK2 does not appear to impede the binding
of RGS4 to Gαq. Therefore, RGS proteins such as RGS4 potentially
also participate in the high-order membrane complexes assembled
by GRK2.
With the Gαq-GRK2-Gβγ
structure, each of the four Gα subunit families has been structurally
characterized. Moreover, structures of effector complexes involving
each of these subfamilies have been determined. Comparison of these
complexes reveals remarkable similarities in how structurally unrelated
effectors bind to Gα subunits. Remarkably,
each of these effector complexes leaves room for the simultaneous
binding of a GAP (such as RGS4). This feature explains how Gα
subunits can propagate signals, while remaining responsive to regulatory
proteins that control the duration of the signal.
Research conducted by V.M. Tesmer, A. Shankaranarayanan, and J.J.G.
Tesmer (University of Michigan, Ann Arbor) and T. Kawano and T.
Kozasa (University of Illinois, Chicago).
Research funding: American Heart Association, U.S. National Institutes
of Health, and the American Cancer Society. Operation of the ALS
is supported by the U.S. Department of Energy, Office of Basic
Energy Sciences.
Publication about this research: V.M. Tesmer, T.
Kawano, A. Shankaranarayanan, T. Kozasa, and J.J.G. Tesmer, “Snapshot
of activated G proteins at the membrane: The Gαq-GRK2-Gβγ complex,” Science 310,
1686 (2005). ALSNews
Vol. 266, June 28, 2006 |