9O51: PDB entry 9O51

Cryo-EM structure of the human SK2-4 chimera/calmodulin channel complex in the Ca2+ free state. Determined by electron microscopy at 3.4 Å resolution. Released 9 Jul 2025.

Method: Electron microscopy
Resolution: 3.4 Å
Organism: Homo sapiens
Chains: 8
Atoms: 12,302
Mol. weight: 265.18 kDa
Ligands: CA
Released: 9 Jul 2025

Explore 9O51 in 3D Show helices and sheets RCSB PDB PDBe

Secondary structure: helices and β-sheets

9O51 contains 72 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.

Chains A, B, C and D: 14 helices, 2 β-strands

Element	Residues	Length	Sheet
α-helix	121-158	38
α-helix	165-200	36
α-helix	205-207	3
α-helix	211-224	14
β-strand	234-240	7	1
α-helix	241	1
β-strand	247-253	7	1
α-helix	256-259	4
α-helix	260-266	7
α-helix	267-276	10
α-helix	298-308	11
α-helix	310-332	23
α-helix	345-356	12
α-helix	369-388	20
α-helix	401-437	37
α-helix	443-474	32

Chains E, F, G and H: 4 helices, 2 β-strands

Element	Residues	Length	Sheet
α-helix	83-93	11
β-strand	101	1	5
α-helix	103-108	6
α-helix	120-129	10
β-strand	137	1	5
α-helix	139-146	8

Molecules and chains

Molecule	Chains	Type	Length	Organism	UniProt
Intermediate conductance calcium-activated potassium channel protein 4,Small conductance…	A, B, C, D	protein	435	Homo sapiens	O15554 (AlphaFold model), Q9H2S1 (AlphaFold model)
Calmodulin-1	E, F, G, H	protein	149	Homo sapiens	P0DP23 (AlphaFold model)

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

>9O51_1 Intermediate conductance calcium-activated potassium channel protein 4,Small conductance calcium-activated potassium channel protein 2 chimera (chains A, B, C, D)
MGGDLVLGLGALRRRRALFEKRKRLSDYALIFGMFGIVVMVIETELSWGAYDKASLYSLA
LKCLISLSTIILLGLIIVYHAREIQLFMVDNGADDWRIAMTYERIFFICLEILVCAIHPI
PGNYTFTWTARLAFSYAPSTTTADVDIILSIPMFLRLYLIARVMLLHSKLFTDASSRSIG
ALNKINFNTRFVMKTLMTICPGTVLLVFSISLWIIAAWTVRACERYHDQQDVTSNFLGAM
WLISITFLSIGYGDMVPNTYCGKGVCLLTGIMGAGCTALVVAVVARKLELTKAEKHVHNF
MMDIQYTKEMKESAARVLQEAWMFYKHTRRKESHAARRHQRKLLAAINAFRQVRLKHRKL
REQVNSMVDISKMHMILYDLQQNLSSSHRALEKQIDTLAGKLDALTELLSTALGPRQLPE
PSQQSKSSSLEVLFQ

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

>9O51_2 Calmodulin-1 (chains E, F, G, H)
MADQLTEEQIAEFKEAFSLFDKDGDGTITTKELGTVMRSLGQNPTEAELQDMINEVDADG
NGTIDFPEFLTMMARKMKDTDSEEEIREAFRVFDKDGNGYISAAELRHVMTNLGEKLTDE
EVDEMIREADIDGDGQVNYEEFVQMMTAK

Ligands and cofactors

ID	Name	Formula	Copies
CA	Calcium ion	Ca	4

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

Primary citation

Mechanism of SK2 channel gating and its modulation by the bee toxin apamin and small molecules. Cassell, S.J., Li, W., Krautwald, S. et al. Elife (2025) 14. DOI 10.7554/eLife.107733 · 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)
9ZRK 2.99 Å, Cryo-EM structure of KCa3.1_I/calmodulin channel in complex with SKA111.
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
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

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