Extended Data Table 2 |.

Cryo-EM data collection, refinement and validation statistics of MthK WT Ca²⁺ bound states 1–3, MthK WT Ca²⁺ bound closed state, and RCK states 1–2.

	MthK EDTA (EMDB-20663 (PDB 6U6D)	MthK Ca2+ Δ2–17 state 1 (EMDB-20930) (PDB 6UX7)	MthK Ca2+ Δ2–17 state2 (EMDB-20931) (PDB 6UXA)	MthK Ca2+ Δ2–17 state3 (EMDB-20932) (PDB 6UXB)	Δ2–17 Ca2+ RCK state 1 (EMDB-20925) (PDB 6UWN)	Δ2–17 Ca2+ RCK state 1 (EMDB-20929) (PDB 6UX4)
Data collection and processing
Magnification	130000	22500	22500	22500	22500	22500
Voltage (kV)	300	300	300	300	300	300
Electron exposure (e-/Å2)	49.32	57	57	57	57	57
Defocus range (urn)	−0.5 to −3	−1.4 to−2.4	−1.4 to−2.4	−1.4 to−2.4	−1.4 to−2.4	−1.4 to−2.4
Pixel size (Å)	1.05878	1.07	1.07	1.07	1.07	1.07
Symmetry imposed	C4	Cl	Cl	Cl	C2	C2
Initial particle images	360,273	1,356,600	1,356,600	1,356,600	1,356,600	1,356,600
Final particle images	80,904	40,536	96,834	68,286	175,149	273,492
Map resolution (Å)	3.6	6.7	4.5	4.9	3.5	3.5
FSC threshold	0.143	0.143	0.143	0.143	0.143	0.143
Map resolution range (Å)	2.8–3.8	5–7	5–7	5–7	3.2–3.6	3.2–3.6
Refinement		*	*	*
Initial model used (PDB code)	2FY8	3LDC	3LDC	3LDC	4L73	4L73
Model resolution (Å)	3.7	7	6.5	6.3	3.5	3.4
FSC threshold	0.5	0.5	0.5	0.5	0.5	0.5
Model resolution range (Å)	2.8–3.8	5–7	5–7	5–7	3.2–3.6	3.2–3.6
Map sharpening B	−110	−100	−100	−100	−159	−174
factor (Å2)
Model composition
Non-hydrogen	16770	10344	10344	10344	13258	13258
atoms	2156	2104	2104	2104	1776	1776
Protein residues	2	0	0	0	24	24
Ligands	50	45	45	45	72	58
B factors (Å2)
Protein
Ligand	50	-	-	-	47	33
R.m.s. deviations
Bond lengths (Å)	0.002	0.005	0.005	0.005	0.007	0.011
Bond angles (°)	0.487	0.774	0.775	0.775	0.711	0.816
Validation
MolProbity score	1.58	0.75	0.75	0.75	1.35	1.4
Clashscore	6.53	0	0	0	3.49	3.37
Poor rotamers	0.0	0	0	0	0.15	0.29
(%)
Ramachandran plot
Favored (%)	96.6	96.2	96.2	96.2	96.7	96
Allowed (%)	3.4	3.8	3.8	3.8	3.3	4
Disallowed (%)	0.0	0	0	0	0	0

^*indicates a poly-alanine model with side chains truncated and ligands removed.