Structure of the
Beta-2 Adrenergic Receptor-Gs Protien Complex
Andrew Maurer '14 and Sam McQuiston '14
complex has been used as a model system for
G-protien coupled receptors(GPCRs) since its discovery over 40 years
ago. As a trans-membrane receptor, the primary function of the
Beta-adrenergic receptor is to bind to extracellular adreniline and in
turn activate the Gs subunit, which then dissociates and continues the
messaging cascade. The exact structure of the receptor complex remained
for many years due to the instability of the complex while in the
detergent solution required for x-ray crystallography. This problem was
worked around by binding a nanobody, Nb35,
to the Gs subunit
and a lysosome,
to B2AR which both increased the stability of the
complex in the x-ray chromotography solution.
a single poly-peptide chain which
weaves inside and outside of the cellular membrane. Its secondary
structure is constituted by 8 α helices, 7 of which are
trans-membrane and one
which runs parallel to the intracellular face of
the cell membrane.
also contains three
extra cellular loops, three intracellular loops, an extracellular
N-terminus and intracellular C-terminus. The agonist binding site lies
within the trans-membrane section of the protein.
The Gs subunit
G-protein, with the subunits, Gαs,
Gγ. The Gαs,
binds to GDP and GTP, is the primary functional subunit of the Gs
protein which binds to and activates the adenyl-cyclase channel,
continuing signal transduction. It is further divided into two
which clamp onto the guanine-nuceloside when it is bound.
Agonist Binding and B2AR Conformation Shift
agonist binding site
the B2AR protein consists of the residues D130,
V131, F288, F289, and N312.
The primary interactions of this binding
site are Hydrogen bonding between the D130 and N312 hydrogen acceptors
and the agonist's proton donors, primarily in the form of hydroxyl
groups. Additionally, the F288 and F289 groups help to stabilize the
agonist in the binding site by providing non-polar-phenyl stacking
interactions which hold the agonist in place.
binding of the agonist to this site results in a conformational shift
in the trans-membrane
helix(TM) 3 and TM 6 of B2AR
activate the Gs subunit. However, this conformational shift also
results in the weakening of the association of between these helices,
which makes them vulnerable to outside attack. This is thought to be
one of the primary reasons for the instability of B2AR in detergent
solution. The reason for Gs activation is not certain because the
B2AR-Gs complex does not include a nucleotide and thus activation can
not be observed.
structural changes in B2AR include an eight residue shift in the TM5
helix and a change in in
secondary structure of the second
intracellular loop(2ICL) into
formation of the nucleotide-free B2AR-Gs protein complex first
requires the carboxy terminus of the α5-helix in to shift
away from the β6-strand in Gs (shown in next section). This
shift allows for proper Gs
interactions with B2AR. Gαs is composed of two GαsRas
which activity bind to
B2AR-GαsRas interface is the primary binding site for the
B2AR-Gs protein complex with a total buried surface of 2,576 A2 .
of GαsRas causes a conformational change in both
B2AR and GαsRas allowing for the stabilization and bond
formation between them. The interaction is formed by
top of β1-strand
There are many specific interactions which hold B2AR and Gs together.
1.) ARG 131, ALA 134,
135, and THR 136
interact with TYR 391, HIS 387,
384, and ARG 380
a D/ERY motif.
2.) SER 143
and ALA 39
3.) ARG 239
and THR 350
4.) ALA 271,
274, and LEU 275
and LEU 393
and GLU 392
Combined these interactions hold the B2AR-Gs complex together
when GDP is released during the building of the B2AR-Gs
protein complex. Surprisingly there is no interaction
between B2AR and Gβϒ
the second subunit of Gs.
Allows Binding of Gs-Nucleotide
conformational shift in the Gs sub-unit allows
GαsRas and GαsAH
bind to the Guanine nucleotide
degrees, from a non-nucleotide bound state to a nucleotide
bound state. The lack of the
stability in the bond with guanine in between
subunits is responsible for the flexibility of the GαsAH
subunit when in the non-nucleotide-bound state.
subunit is less flexible than its counterpart, it still undergoes
significant conformational shift. The
6Å towards the receptor and is rotated so the carboxyl
pointed into the B2AR core in non-nucleotide-bound
state. In addition, the beta6 alpha5 loop, which interacts with the
guanine ring in the active site when bound, is displaced away
the binding pocket.
not totally understood, the β1-α1 loop, a
P-loop motif, is directly involved in the nucleotide binding
interactions between GαsRas
be unaffected by the binding of guanine nucleotides. However, because a
crystal structure of GDP-bound Gs heterotrimer has not yet been
reported and the Nb35 binds to the junction of GαsRas
and could be responsible for the relative stability of the interface,
the veracity of this phenomenon in biological systems can't be certain.
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