Extended Data Table 1.

Cryo-EM data collection, refinement and validation statistics

	GABAB apo	GABAB/SKF97541 Int-1	GABAb/SKF97541 Int-2	GABAB/SKF97541/GS39783
	(EMDB-21219)	(EMDB-20824)	(EMDB-20823)	(EMDB-20822)
	(PDB 6VJM)	(PDB 6UOA)	(PDB 6U09)	(PDB 6UO8)
Data collection and processing
Magnification	135,000	29,000	29,000	29,000
Voltage (kV)	300	300	300	300
Electron exposure (e−/Å2)	8	15	15	15
Defocus range (μm)	−1.5 - −2.5	−1.5 - −3.0	−1.5 - −3.0	−1.5 - −3.0
Pixel size (Å)	1.08	0.8521	0.8521	0.8521
Symmetry imposed	C1	C1	C1	C1
Initial particle images (no.)	2,010,390	5,641,714	5,641,714	4,173,286
Final particle images (no.)	113,093	48,600	114,552	89,001
Map resolution (Å)	4.0	6.3	4.8	3.6
FSC threshold	0.143	0.143	0.143	0.143
Map resolution range (A)	3.7 - 4.6	5.7 - 12.2	4.5 - 7.3	3.4 - 7.0
Refinement
Initial model used (PDB code)	4MQE, 6UO8	4MQE, 6VJM	4MS3, 6UO8	4MS3, 40R2
Model resolution (A)	4.2	7.6	5.2	3.8
FSC threshold	0.5	0.5	0.5	0.5
Model resolution range (Å)	29.1 - 3.4	29.3 - 1.8	34.3 - 3.5	33.7 - 3.0
Map sharpening B factor (Å2)	−143.0	−378.1	−230.7	−125.8
Model composition
Non-hydrogen atoms	10,629	9,153	9,915	10,958
Protein residues	1,369	1,323	1,365	1,385
Ligands	0	0	1	2
N-glycans	11	11	13	13
B factors (Å2)
Protein	147.4	361.4	209.8	116.0
Ligands	N/A	N/A	141.0	120.8
N-glycans	191.8	421.3	260.1	145.3
R.m.s. deviations
Bond lengths (Å)	0.005	0.006	0.006	0.004
Bond angles (°)	0.912	1.300	1.360	0.713
Validation
MolProbity score	1.86	1.87	1.86	1.68
Clashscore	6.77	6.80	6.36	4.16
Poor rotamers (%)	0.19	1.14	1.07	0.00
Ramachandran plot
Favored (%)	91.75	92.94	92.03	92.23
Allowed (%)	8.25	7.06	7.97	7.77
Disallowed (%)	0.0	0.0	0.0	0.0