Science - USA (2020-03-20)

larger effects on Gsactivation [factor of 6 to

8 reduction of potency and 40 to 80% reduc-

tion in maximal responses (Emax)] than their

alanine replacements (factor of 2 to 3 reduc-

tion of potency and no reduction inEmax) (Fig.

4, I and K; fig. S8C; and table S2), but again

were better tolerated in the assay of Gi1acti-

vation (Fig. 4, J and L; fig. S8F; and table S2).

Consistent with these results, the mutation

L3546.45bW substantially reduced both gluca-

gon potency andEmaxin Gsactivation, whereas

it showed much less influence on Gi1activa-

tion (factor of 10 reduction of potency and no

reduction inEmax) (Fig. 4, I to L; fig. S8, C and

F; and table S2). Increasing the size of the hy-

drophobic residues reduces the size of the

intracellular pocket, and this is more detri-

mental to binding of the bulkier and more

polar Gsa5Cterminus.

Most of the alanine replacements—Y2483.53bA,

L2493.54bA, L2533.58bA, L3285.60bA, L3295.61bA,

and L3546.45bA—reduced the maximum level

of Gqi9-mediated IP production by 50 to 90%

(Fig. 4H, fig. S8K, and table S4). These alanine

mutants retainedEmaxvalues in cAMP accu-

mulation that were similar to that of the wild-

type receptor, but decreased potency of glucagon

was observed (by a factor of 2 to 9) (Fig. 4, E

andG;fig.S8H;andtableS3).Althoughsome

of these alanine mutants had a similar over-

all effect on Gsand Gisignaling, Y2483.53bA,

L2493.54bA, L2533.58bA, and L3546.45bA were

more detrimental for Gisignaling (Fig. 4, E

to H, and tables S3 and S4). Similarly, in the

NanoBiT assay, the mutations L3285.60bA and

L3295.61bA reduced glucagon potency by a fac-

tor of >30 in Gi1activation but decreased the

potency of Gsactivation by only a factor of 2 to

3 (Fig. 4, I and J; fig. S8, C and F; and table S2).

The different behaviors of these mutants

indicate that disturbing the hydrophobic

contact between the receptor and the Gaa 5

C terminus has a larger effect on Gisignaling

than on Gssignaling. This aligns well with

the fact that the interaction interface be-

tween GCGR and the GaC terminus (residues

G.H5.16 to G.H5.26), which is mainly com-

posed of hydrophobic residues, accounts for

80% of the GCGR-Gaiinterface but only about

60% of the total interface between GCGR and

Gas. Thus, the hydrophobic cavity within the

receptor intracellular face may play a more

critical role for Girecognition than that of Gs.

Collectively, this work provides a model for

the diverse G protein signaling observed with

class B GPCRs. The G protein–bound GCGR

structures reveal that the less bulky Giprotein

is accommodated in the large intracellular

cavity but forms less extensive, predominantly

hydrophobic, interactions, which account for

G protein coupling specificity. Furthermore,

there are specific conformational differences

in the receptor, which also govern the nature of

GCGR–G protein interactions and may mediate

biased agonism, including in the intracellu-

lar loops and individual residue side chains.

Although there are studies that implicate

Gicoupling of GCGR, physiological relevance

remains unclear. Nonetheless, our structures

of Gsand Gibound to the same GPCR give an

opportunity to study the basis of G protein

specificity and offer new insights into the mo-

lecular details that govern pleiotropic GPCR–G

protein coupling.

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ACKNOWLEDGMENTS

We thank Y.-L. Liang for expert advice on class B GPCR–G protein

complex biochemistry, and T. T. Truong and S. Darbalaei for

assistance with the NanoBiT studies. The cryo-EM studies were

performed at the Center for Biological Imaging (CBI, http://cbi.ibp.

ac.cn), Institute of Biophysics, Chinese Academy of Sciences; we

thank B.-L. Zhu and X. Huang from CBI for their help on cryo-EM

data collection.Funding:Supported by the National Key R&D

Program of China 2018YFA0507000 (B.W., Q.Z., and M.-W.W.) and

2017YFA0504703 (F.S.); National Science Foundation of China

grants 31825010 (B.W.), 81525024 (Q.Z.), 31830020 (F.S.),

81872915 (M.-W.W.), and 81773792 (D.Y.); CAS Strategic Priority

Research Program XDB37000000 (B.W.); Shanghai Outstanding

Academic Leaders Plan of Shanghai Municipal Science and

Technology Committee 18XD1404800 (Q.Z. and S.H.); the National

Science & Technology Major Project–Key New Drug Creation and

Manufacturing Program, China grant 2018ZX09711002 (L.M., D.Y.,

and M.-W.W.); the National Mega R&D Program for Drug Discovery

and D, and fig. S1E). Additionally, the NanoBiT
assay revealed that these alanine replacements
heavily impaired both Gs and Gi1 activation
(Fig. 4, I to L; fig. S8, B and E; and table S2).
The effect of these mutations on Gs and Gi ac-
tivation could be explained by the disruption
of the interaction networks within the receptor
and impairment of the global conformational
rearrangement that is required for G protein
recognition and signaling. Nonetheless, in the
NanoBiTGproteinactivationassay,therewas
a greater impact of the H1772.50bA mutant
on Gs than on Gi1 (Fig.4,Ito L, andfig.S8,Band
E), which may reflect the additional loss of direct

interaction that occurs with Gas Y391G.H5.23.
The interaction patterns of GCGR with Gs
and Gi also differ for the residue at position

G.H5.24 of the Gaa 5 helix. The side chain of
the Gas residue E392G.H5.24^ is within interac-
tion distance of N4047.61b^ and K4057.62b^ at the
hinge region between helices VII and VIII in
the glucagon-GCGR-Gs structure (Fig. 4A and
fig. S7B), whereas these interactions are not
possible in the Gi complex because of glycine

(G352G.H5.24) substitution in Gai (Fig. 4B and
fig. S7H). In the NanoBiT assay, a factor of 31
reduction of the EC 50 in glucagon-induced Gs
activation occurred for the mutant N4047.61bA,
which in contrast had little effect on Gi ac-
tivation (factor of 4 reduction) (Fig. 4, I and J;
fig. S8, B and E; and table S2). However, the
mutant K4057.62bA was not different from the
wild-type GCGR for either G protein, which sug-
gests that interaction with the side chain of this
residue is less critical for engagement with Gs.
In both Gs- and Gi1-bound GCGR structures,

aGprotein–binding cavity formed by a cluster
of hydrophobic residues from helices III, V,
and VI is observed at the intracellular face
of the receptor transmembrane domain. It
recognizes different hydrophobic patches

at the a5Cterminus in Gas and Gai (Gas:
L388G.H5.20, Y391G.H5.23, L393G.H5.25, and

L394G.H5.26;Gai: I344G.H5.16, L348G.H5.20,
L353G.H5.25, and F354G.H5.26) (Fig. 4, C and D).
The importance of this hydrophobic cavity in
receptor signaling was reflected in our muta-
genesis studies, where introduction of alanine
or tryptophan mutation within the cavity not
only decreased glucagon potency on Gs sig-
naling (Fig. 4, E and G; fig. S8, H and I; and
table S3) but also reduced Gqi9-mediated IP
production (Fig. 4, F and H; fig. S8, K and L;
and table S4). Of note, tryptophan mutations
within this pocket were more detrimental for
Gs signaling than were alanine mutations (Fig.
4E and fig. S8, H and I); this was not the case
for Gi, where some tryptophan mutations such
as Y2483.53bW and L3285.60bW were better
tolerated than alanine (Fig. 4, F and H, and
fig. S8, K and L). The distinct effects of the
tryptophan mutants on Gs and Gi activation
were also observed in the NanoBiT assay,
where L3285.60bW and L3295.61bW exhibited

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