Fig. 1: Functional characterization and structure determination of sea urchin SLC9C1.

a, SLC9C1 is essential for the pH-mediated regulation of sAC, which leads to CatSper Ca²⁺ channel activation and thereby drives sperm motility. In sea urchin sperm, the binding of the chemoattractant speract to its extracellular receptor enhances intracellular cGMP production, CNGK channel opening and the hyperpolarization-induced activation of SLC9C1^2,3,5,8. The diagram was created using BioRender.com. b, The NHE transporter module has 13 TM segments, forming a core ion-transport domain and dimer domain (grey). Downstream of the NHE transporter module is an S4 VSD (cyan) and a CNBD (brown). c, SSM-based electrophysiology analysis. Transient currents after the addition of 150 mM NaCl were recorded from SLC9C1 proteoliposomes and empty liposomes at a symmetrical pH of 6.5, 7.5 and 8.5 as labelled. Inset: normalized peak currents in response to 80 mM NaCl at pH 6.5 for varied LPR. d, Representative transient currents at pH 6.5 with varying concentrations of NaCl. The key SLC9C1 ion-binding aspartate D238A mutant response to 80 mM NaCl is also shown. e, Fit of the transient currents as a function of Na⁺ concentrations for SLC9C1(WT), SLC9C1(D238A), rat fructose transporter GLUT5, horse NHE9(D244A/N243A) non-functional variant and empty liposomes at a symmetrical pH of 6.5 as labelled. Data are mean ± s.d. of n = 3 titrations (sensors) for SLC9C1(WT), SLC9C1(D238A), empty liposomes and a range of n = 2 titrations for rat GLUT5 and horse NHE9. The K_d for Na⁺ is shown for WT SLC9C1. f, Cryo-EM maps of the sea urchin SLC9C1 homodimer in GDN detergent (C₂ symmetry). The NHE transporter module (grey), the VSD domain (cyan) and CNBD domain (brown) are shown. Connecting the NHE transporter to the VSD are intracellular helices ICH1 to ICH4 (orange), and connecting the VSD domain to the CNBD are intracellular helices ICH5 to ICH7 (purple). a, Adapted from ref. ², CC BY 4.0.

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