Fig. 2: Tethering of the S3-S4 β-hairpins to the selectivity filter and 180o flip of the critical tyrosine in the signature sequence in apo_KCa2.2.
From: Cryo-EM structures of the small-conductance Ca2+-activated KCa2.2 channel
a Cryo-EM density map with fitted model of two subunits of apo_KCa2.2 viewed from the plane of the membrane. K+ is seen at positions S0 (at the aromatic box), S2, S3, and S4 within the filter; S1 is missing. Residues in the filter (blue) are highlighted. Extracellular S3-S4 β-hairpins (magenta) from two neighboring subunits are positioned on either side of K+ at S0 in (a, b). Note, the outer filter is widened at the level of Y362-G363. b Each S3–S4 loop is tethered to the filter in the neighboring subunit. Asp364 in the filter of subunit A (blue) forms inter-subunit hydrogen bonds and salt bridge with Arg241, Phe244, and Tyr246 in the S3-S4 loop of neighboring subunit B (green). In addition, an inter-subunit interaction between Trp351 (blue) and Tyr362 (green) in the filter stabilizes the pore. c In apo_KCa3.1_1, Asp255, Val256 and Trp242 (yellow) in the selectivity filter, corresponding to Asp364, Met365 and Trp351 in apo_KCa2.2, form intra-subunit hydrogen bonds. In addition, Tyr253 in apo_KCa3.1_1 (yellow), corresponding to Tyr362 in apo_KCa2.2, forms an inter-subunit hydrogen bond with Thr247 (green). d Extracellular view of the critical tyrosine (Tyr362) in the G[Y/F]G motif of the selectivity filter in apo_KCa2.2 (blue) overlaid on the corresponding aromatic residues in the selectivity filters of the open-conducting conformations of apo_KCa3.1_1 (Tyr253, yellow; PDB 6cnn), apo_KCa3.1_2 (Tyr253, green; PDB 6cno), KVChim (Tyr373, brown, PDB 2r9r), KCa1.1 (Phe279, purple, PDB 5tj6), and AlphaFold_KCa2.2 (Tyr362, gray, https://alphafold.ebi.ac.uk/entry/E9PSQ3). Note, Tyr362 in apo_KCa2.2 is flipped 180° compared to the critical aromatic residues in the other channels.
