9ZRK: KCa3.1_I/calmodulin channel

Cryo-EM structure of KCa3.1_I/calmodulin channel in complex with SKA111. Determined by electron microscopy at 2.99 Å resolution. Released 14 Jan 2026.

Method: Electron microscopy
Resolution: 2.99 Å
Organism: Homo sapiens
Chains: 8
Atoms: 15,584
Mol. weight: 230.75 kDa
Ligands: A1C3U, CA
Released: 14 Jan 2026

Explore 9ZRK in 3D Show helices and sheets RCSB PDB PDBe

Secondary structure: helices and β-sheets

9ZRK contains 104 α-helices and 16 β-strands across 8 chains. Residue ranges use author residue numbering, as in the PDB file, from PDBe. To see them in 3D, choose the Cartoon representation with Secondary structure coloring: helices, sheets and coils get different colors.

Chain A: 19 helices, 0 β-strands

Element	Residues	Length
α-helix	11-28	18
α-helix	31-50	20
α-helix	63-91	29
α-helix	102-114	13
α-helix	145-146	2
α-helix	147-154	8
α-helix	155-162	8
α-helix	163-170	8
α-helix	172-174	3
α-helix	177-186	10
α-helix	192-202	11
α-helix	206-227	22
α-helix	237-248	12
α-helix	263-287	25
α-helix	293-331	39
α-helix	335-338	4
α-helix	341-368	28
α-helix	369-372	4
α-helix	373-385	13

Chain B: 14 helices, 0 β-strands

Element	Residues	Length
α-helix	11-48	38
α-helix	56-59	4
α-helix	64-91	28
α-helix	102-114	13
α-helix	154-162	9
α-helix	163-170	8
α-helix	177-185	9
α-helix	192-202	11
α-helix	206-227	22
α-helix	237-248	12
α-helix	263-286	24
α-helix	293-330	38
α-helix	334-368	35
α-helix	371-385	15

Chain C: 17 helices, 0 β-strands

Element	Residues	Length
α-helix	10-45	36
α-helix	46-50	5
α-helix	64-91	28
α-helix	102-114	13
α-helix	149-154	6
α-helix	155-162	8
α-helix	163-170	8
α-helix	177-185	9
α-helix	192-202	11
α-helix	206-227	22
α-helix	230-232	3
α-helix	237-248	12
α-helix	263-287	25
α-helix	293-330	38
α-helix	334-343	10
α-helix	347-366	20
α-helix	372-385	14

Chain D: 18 helices, 0 β-strands

Element	Residues	Length
α-helix	11-45	35
α-helix	46-50	5
α-helix	64-91	28
α-helix	102-114	13
α-helix	145-146	2
α-helix	147-154	8
α-helix	155-162	8
α-helix	163-171	9
α-helix	173-175	3
α-helix	177-185	9
α-helix	192-202	11
α-helix	206-227	22
α-helix	237-248	12
α-helix	262-286	25
α-helix	293-300	8
α-helix	304-330	27
α-helix	334-366	33
α-helix	370-385	16

Chains E, F, G and H: 9 helices, 4 β-strands

Element	Residues	Length	Sheet
α-helix	6-19	14
β-strand	27	1	1
α-helix	29-38	10
α-helix	45-53	9
β-strand	63	1	1
α-helix	65-75	11
α-helix	81-92	12
β-strand	100	1	2
α-helix	102-111	10
α-helix	118-121	4
α-helix	124-128	5
β-strand	136	1	2
α-helix	138-145	8

Molecules and chains

Molecule	Chains	Type	Length	Organism	UniProt
Intermediate conductance calcium-activated potassium channel protein 4	A, B, C, D	protein	360	Homo sapiens	O15554 (AlphaFold model)
Calmodulin-1	E, F, G, H	protein	146	Homo sapiens	P0DP23 (AlphaFold model)

Sequence of entity 1 (A, B, C, D), FASTA

>9ZRK_1 Intermediate conductance calcium-activated potassium channel protein 4 (chains A, B, C, D)
LGALRRRKRLLEQEKSLAGWALVLAGTGIGLMVLHAEMLWFGGCSWALYLFLVKCTISIS
TFLLLCLIVAFHAKEVQLFMTDNGLRDWRVALTGRQAAQIVLELVVCGLHPAPVRPGFLG
QGEALLSLAMLLRLYLVPRAVLLRSGVLLNASYRSIGALNQVRFRHWFVAKLYMNTHPGR
LLLGLTLGLWLTTAWVLSVAERQAVNATGHLSDTLWLIPITFLTIGYGDVVPGTMWGKIV
CLCTGVMGVCCTALLVAVVARKLEFNKAEKHVHNFMMDIQYTKEMKESAARVLQEAWMFY
KHTRRKESHAARRHQRKLLAAINAFRQVRLKHRKLREQVNSMVDISKMHMILYDLQQNLS

Sequence of entity 2 (E, F, G, H), FASTA

>9ZRK_2 Calmodulin-1 (chains E, F, G, H)
DQLTEEQIAEFKEAFSLFDKDGDGTITTKELGTVMRSLGQNPTEAELQDMINEVDADGNG
TIDFPEFLTMMARKMKDTDSEEEIREAFRVFDKDGNGYISAAELRHVMTNLGEKLTDEEV
DEMIREADIDGDGQVNYEEFVQMMTA

Ligands and cofactors

ID	Name	Formula	Copies
A1C3U	5-methylnaphtho[1,2-d][1,3]thiazol-2-amine	C12 H10 N2 S	4
CA	Calcium ion	Ca	16

Water and common crystallization additives (K) are not listed.

Primary citation

Structural basis for the subtype-selective activation of K Ca 3.1 channels. Ramanishka, A., Nasburg, J.A., Xu, Y. et al. Structure (2026) 34:1040-1049.e3. DOI 10.1016/j.str.2026.04.010 · PubMed

Other PDB entries of the same protein (UniProt O15554 (AlphaFold model), which also has an AlphaFold model), best resolution first:

6D42 1.75 Å, Crystal structure of the KCa3.1 C-terminal four-helix bundle (with copper)
9O48 3.1 Å, Cryo-EM structure of the human SK2-4 chimera/calmodulin channel complex in the Ca2+…
9O5O 3.1 Å, Cryo-EM structure of the human SK2-4 chimera/calmodulin channel complex bound to a small…
9O52 3.18 Å, Cryo-EM structure of the human SK2-4 chimera/calmodulin channel complex bound to the bee…
9O53 3.3 Å, Cryo-EM structure of the human SK2-4 chimera/calmodulin channel complex bound to a small…
9ZRL 3.38 Å, Cryo-EM structure of KCa3.1_II/calmodulin channel in complex with SKA111.
9ZPT 3.39 Å, Cryo-EM structure of KCa3.1_II/calmodulin channel in complex with SKA31.
6CNM 3.4 Å, Cryo-EM structure of the human SK4/calmodulin channel complex
9O51 3.4 Å, Cryo-EM structure of the human SK2-4 chimera/calmodulin channel complex in the Ca2+ free…
9YDZ 3.4 Å, Cryo EM structure of KCa3.1_R355K_II/calmodulin channel in complex with rimtuzalcap
6CNN 3.5 Å, Cryo-EM structure of the human SK4/calmodulin channel complex in the Ca2+ bound state I
9ED1 3.5 Å, Cryo-EM structure of the human KCa3.1/calmodulin channel in complex with Ca2+ and…

Browse structure collections

About this viewer

MolViewer shows 9ZRK directly in your browser with nothing to install. Switch between cartoon, ball-and-stick, spacefill and surface views, color by chain, secondary structure or B-factor, measure distances, angles and dihedrals, and share or embed the view.