Extended Data Table 1 |.

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 Ca2+ state 1 (EMDB-20662) (PDB 6U6S)	MthK Ca2+ state2 (EMDB-20664) (PDB 6U6E)	MthK Ca2+ state3 (EMDB-20665) (PDB 6U6H)	MthK Ca2+ closed (EMDB-20653) (PDB 6U5R)	RCK state 1 (EMDB-20652) (PDB 6U5P)	RCK state2 (EMDB-20650) (PDB 6U5N)
Data collection and processing
Magnification	22500	22500	22500	22500	22500	22500
Voltage (kV)	300	300	300	300	300	300
Electron exposure (e-/Å2)	52	52	52	52	52	52
Defocus range (urn)	−1.6 to-2.4	−1.6 to-2.4	−1.6 to-2.4	−1.6 to−2.4	−1.6 to−2.4	−1.6 to−2.4
Pixel size (Å)	1.096	1.096	1.096	1.096	1.096	1.096
Symmetry imposed	Cl	Cl	Cl	C4	C2	C2
Initial particle images	1,426,050	1,426,050	1,426,050	1,426,050	1,426,050	1,426,050
Final particle images	29,448	14,483	15,144	22,677	150,542	113,103
Map resolution (Å)	4.5	6.3	5	3.6	3.3	3.2
FSC threshold	0.143	0.143	0.143	0.143	0.143	0.143
Map resolution range (Å)	4.5‒6.5	6‒7.5	4.5‒6.5	3.5‒5.5	2.8‒3.4	2.6‒3.2
Refinement	*	*	*
Initial model used (PDB code)	3LDC	3LDC	3LDC	2FY8	4L73	4L73
Model resolution (Å)	6.2	6.7	6.4	3.7	3.3	3.2
FSC threshold	0.5	0.5	0.5	0.5	0.5	0.5
Model resolution range (Å)	4.5−6.5	6−7.5	4.5−6.5	3.5−5.5	2.8−3.4	2.6−3.2
Map sharpening B factor (A2)	−96	−186	−146	−124	−102	−97
Model composition
Non-hydrogen	10489	10422	10352	16516	13308	13262
atoms	2133	2118	2104	2156	1776	1776
Protein residues	0	0	0	8	24	24
Ligands B factors (Å2)	40	44	44	61	48	34
Protein
Ligand	-	-	-	120	48	34
R.m.s. deviations	0.006	0.006	0.005	0.011	0.006	0.006
Bond lengths (Å)
Bond angles (°)	0.895	0.932	0.762	0.948	0.733	0.743
Validation
MolProbity score	0.79	0.62	0.57	1.68	1.5	1.39
Clashscore	0.46	0.07	0	5.12	4.07	2.73
Poor rotamers (%)	0	0	0	0	0	0
Ramachandran plot
Favored (%)	97.3	97.6	97.7	93.9	95.6	95.2
Allowed (%)	2.6	2.5	2.3	6.1	4.4	4.8
Disallowed (%)	0	0	0	0	0	0

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