SLC9C1

Model summary

A multi-pass membrane Na+/H+ exchanger that resides at the cell surface and uses a conserved ion-exchange core to couple sodium influx to proton efflux, thereby controlling intracellular pH and ion balance. Its cytosolic regulatory tail and auxiliary ion-transport fold enable assembly with partner proteins that tune activity. By stabilizing optimal pH and membrane potential, it supports the energetic and regulatory demands of the flagellum and thereby sustains sperm motility.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture and its order along the sequence.

   The N-terminus (residues 27–418) carries IPR006153 (Cation/H+ exchanger, transmembrane domain), embedded within the broader IPR018422 (Cation/H+ exchanger, CPA1 family, residues 22–492). This CPA1 exchanger core is the hallmark of Na+/H+ antiporters and dictates an alternating-access mechanism that couples sodium influx to proton efflux across membranes. Mid-protein, a second transmembrane/ion-transport module appears: IPR027359 (Voltage-dependent channel domain superfamily, residues 602–731) overlaps with IPR005821 (Ion transport domain, residues 613–726). Although annotated as a voltage-dependent channel-like fold, in this context it functions as an auxiliary ion-transport/scaffold module that stabilizes the antiporter core and tunes gating rather than forming a classical pore. The C-terminus is cytosolic and bipartite: IPR014710 (PpnP-like, jelly roll fold, residues 862–1012) overlaps with IPR018490 (Cyclic nucleotide-binding domain superfamily, residues 864–997) and the specific IPR000595 (Cyclic nucleotide-binding domain, residues 882–998). This CNB-like jelly-roll platform is known in membrane proteins to mediate allosteric regulation and protein–protein interactions rather than canonical cyclic nucleotide binding. Together, the ordered layout—CPA1 antiporter core (N- to mid-region), followed by an ion-transport/scaffold module and a C-terminal CNB-like regulatory platform—creates a multi-pass membrane transporter whose activity and assembly are tuned by cytosolic binding events.

2. 02

   This architecture causes sodium:proton exchange as the primary molecular function.

   The CPA1 exchanger domains (IPR006153 and IPR018422) specify sodium/proton antiporter chemistry, aligning with GO:0015385 sodium:proton antiporter activity. The presence of a sizable cytosolic CNB-like tail and auxiliary ion-transport fold predicts extensive regulatory and assembly interfaces, supporting GO:0005515 protein binding as an intrinsic property necessary for oligomerization and modulation.

3. 03

   Antiport of H+ and Na+ directly sets intracellular pH and ion balance.

   In sperm, this ion-coupled pH control is a prerequisite for axonemal dynein function and energy coupling, which drives vigorous beating. Thus, the transporter’s activity feeds into the broader physiological role formalized as GO:0030317 flagellated sperm motility: by sustaining optimal intracellular pH and membrane potential, it sustains the motile apparatus of the flagellum.

4. 04

   The multi-pass arrangement and transporter identity place the protein in the plasma membrane system.

   The CPA1 exchanger core and the auxiliary ion-transport fold together enforce a membrane-embedded topology, consistent with a multi-pass architecture at the cell surface. This supports GO:0005886 plasma membrane and the experimentally observed cell membrane localization.

5. 05

   Mechanistically, I hypothesize that the N-terminal CPA1 exchanger core executes alternating-access exchange, while the mid-region ion-transport fold stabilizes conformational equilibria and the C-terminal CNB-like jelly-roll recruits regulatory partners.

   This arrangement predicts assembly into higher-order complexes that couple ion exchange to cytoskeletal and signaling modules. Likely interaction partners include membrane scaffolds and cytoskeletal adaptors that localize and tune antiporter function in motile cells. In sperm, plausible partners include AKAP-anchored complexes and flagellar scaffolds that coordinate pH regulation with axonemal motility.