Structural basis of sodium ion-dependent carnitine transport by OCTN2
James S. Davies, Yi Cheng Zeng, Chelsea Briot, Simon H. J. Brown, Renae Monique Ryan, Alastair G. Stewart · 2025 · 8 citationsRead the paper
Abstract Carnitine is essential for the import of long-chain fatty acids into mitochondria, where they are used for energy production. The carnitine transporter OCTN2 (novel organic cation transporter 2, SLC22A5) mediates carnitine uptake across the plasma membrane and as such facilitates fatty acid metabolism in most tissues. OCTN2 dysfunction causes systemic primary carnitine deficiency (SPCD), a potentially lethal disorder. Despite its importance in metabolism, the mechanism of high-affinity, sodium ion-dependent transport by OCTN2 is unclear. Here we report cryo-EM structures of human OCTN2 in three conformations: inward-facing ligand-free, occluded carnitine- and Na + -bound, and inward-facing ipratropium-bound. These structures define key interactions responsible for carnitine transport and identify an allosterically coupled Na + binding site housed within an aqueous cavity, separate from the carnitine-binding site. Combined with electrophysiology data, we provide a framework for understanding variants associated with SPCD and insight into how OCTN2 functions as the primary human carnitine transporter.
What the full text shows
Shown in the lab- OCTN2 mediates carnitine transport via a 1:1 stoichiometry of carnitine-Na+ symport. — 1:1 stoichiometry (Human OCTN2 expressed in Xenopus laevis oocytes)
- Wild-type hOCTN2 exhibits high affinity for carnitine. — KM = 12.0 ± 3.0 μM (100 mM external [Na+])
- Wild-type hOCTN2 exhibits high affinity for sodium. — KM = 8.62 ± 1.8 mM (100 μM carnitine, Cs+ for osmolarity)
- Mutation of Na+-coordinating residues significantly reduces carnitine affinity. — ~18-fold increase in KM (N32A); ~11-fold increase in KM (N210A) (Human OCTN2 mutants)
- Ipratropium binds to OCTN2 but inhibits rather than is transported by it. — IC50 = 615 ± 174.5 μM (Human OCTN2 expressed in Xenopus laevis oocytes)
- TEA is transported by OCTN2 in a Na+-independent manner but with very low efficiency. — 0.2-3% transport rate relative to carnitine (Human OCTN2)
- Setup
- Full-length human OCTN2 expressed in HEK293F cells for structural studies; expressed in Xenopus laevis oocytes for functional assays.
- How
- Cryo-EM was used to solve structures of human OCTN2 in three conformations (inward-facing ligand-free, occluded carnitine/Na+-bound, and inward-facing ipratropium-bound). Two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes was used to verify Na+-dependence and substrate affinity.
- Limitations
- Lack of outward-facing conformation structures. · Some variants (e.g., N210) show mixed membrane localization, complicating functional interpretation. · Limited resolution of glycan chains (~20 Å).
Summarized by Gemma 4 from the open-access full text · 8 Oct 2026. Read it