Extended Data Fig. 6: Sequence and structural comparison with VSD in SLC9C1 versus VSD in voltage-gated ion channels.

A. Clustal omega⁶² protein sequence alignment of the VSD domain of sea urchin SLC9C1, human SLC9C1 (uniport: Q4G0N8), mouse SLC9C1 (Q6UJY2), drosophila K⁺-shaker channel (P08510), and VSD_IV of human sodium channel Na_V1.7 (Q15858). Conserved residues are highlighted in red and gating-charge residues in S4 are further highlighted with a + symbol. b. Superimposition of the SLC9C1 VSD (cyan, labelled S1 to S4) with VSD from the Shaker Kv K⁺-channel (PDB:7SIP, grey) with an r.m.s.d 2.4 Å for all C_α pairs. c. Cartoon representation of the VSD in the sea urchin SLC9C1 structure in detergent (cyan) highlighting the conserved salt-bridge and hydrogen bond interactions between R2 and R4 residues in S4 and E698 and N691 in S1 (yellow-sticks), respectively. The equivalent conserved residues in VSD_IV of human Na_V1.7 (PDB: 7XMG) are further shown (pink sticks). The aromatic residue Y743 (F1558 in Na_V1.7) in the charge transfer centre is also highlighted. d. Cartoon representation of the gating-charge residues in SLC9C1 (yellow sticks) and Na_V1.7 (pink sticks) as well as the charge transfer centre. Inset above: The gating-charge residue R806 in SLC9C1 forms a hydrogen-bond to N736 in S2 and salt bridged to E698 in S1 as in human Na_V1.7 (pink sticks). The gating-charge residue R809 in SLC9C1 also forms a hydrogen bond to N736 in S2. Inset below: Apart from the gating aromatic residue, all the residues in the charge transfer centre are conserved and similarly positioned between the VSD in sea urchin SLC9C1 (yellow sticks) and VSD_IV of human Na_V1.7 (pink sticks).