Cryo-EM Reveals How the Taurine Transporter Works and Dimerizes via Cholesterol
First high-resolution structures of human TauT expose substrate recognition and a cholesterol-mediated dimer—key to aging, heart, and brain health.
Summary
Researchers used cryo-EM to solve the first high-resolution structures of the human taurine transporter (TauT) in multiple states: an apo inward-facing open form and substrate-bound occluded forms with taurine or GABA. The structures reveal precisely how TauT recognizes taurine versus GABA, explaining its distinct substrate specificity compared to related GABA transporters. Unexpectedly, when reconstituted in lipid nanodiscs, TauT was also captured as a homodimer stabilized by two cholesterol molecules acting as molecular glue between protomers, alongside direct TM5-TM5 contacts. These findings clarify the molecular basis of taurine transport and open new avenues for treating taurine-deficiency disorders including retinal degeneration, cardiomyopathy, and aging-related conditions.
Detailed Summary
Taurine is one of the most abundant amino acids in the human body, concentrated in excitable tissues like the brain, retina, and heart. It plays critical roles in osmoregulation, antioxidation, neurotransmission, and aging. Because taurine biosynthesis declines with age, the taurine transporter (TauT, a member of the SLC6/neurotransmitter sodium symporter family) becomes the primary mechanism for maintaining adequate intracellular taurine levels. Dysfunction of TauT is linked to retinal degeneration, cardiomyopathy, neurological disorders, and metabolic dysregulation, making structural insight into its operation clinically important.
The research team expressed full-length wild-type human TauT in HEK293S GnTI⁻ cells and purified it in detergent micelles (LMNG) or reconstituted it into lipid nanodiscs of varying compositions. Single-particle cryo-EM was used to solve structures in three functional states: the apo inward-facing open state (3.20 Å), and occluded states bound to taurine (3.05 Å) or GABA (3.26 Å). Nanodisc-reconstituted samples additionally revealed both monomeric (3.23 Å) and homodimeric (3.25 Å) forms of TauT.
All structures share the canonical LeuT fold—12 transmembrane helices with an inverted pseudo-two-fold symmetry—common to SLC6 family members. The substrate-binding pocket accommodates taurine via specific electrostatic and hydrogen-bond interactions that differ subtly but critically from those in GAT1, explaining why TauT has a much lower affinity for GABA than dedicated GABA transporters despite ~62% sequence similarity. The apo structure captures the transporter in an inward-open conformation, while substrate binding drives transition to an occluded state, consistent with the alternating-access transport mechanism.
A major novel finding is the cholesterol-mediated homodimerization of TauT. The dimer—observed only in nanodisc conditions, not in detergent—is assembled via two cholesterol molecules that wedge between TM5 of one protomer and TM7/EL3 of the other, forming hydrophobic contacts with residues including Leu258, Val262, Thr266, Phe338, Phe342, and Leu278. Direct TM5-TM5 intermolecular contacts (Val262, Leu265) further stabilize the interface. Charge-substitution mutations at Val262 and Leu265 abolished transport activity, underscoring the functional importance of these interactions. Removal of cholesterol via methyl-beta-cyclodextrin treatment eliminated the dimeric population, confirming cholesterol's essential role. This TauT dimer architecture differs from the recently described NET homodimer, which involves cholesterol, PI, and PIP2 at a TM3/4/9/12 interface.
These findings provide the first atomic-level framework for understanding TauT substrate selectivity and transport mechanism, and reveal cholesterol-dependent dimerization as a potential regulatory feature of TauT. The structural data lay groundwork for rational design of TauT modulators to treat taurine-deficiency-related diseases and potentially slow aging-associated decline.
Key Findings
- Cryo-EM structures of human TauT solved at 3.0–3.3 Å in apo, taurine-bound, and GABA-bound states.
- Substrate-bound structures reveal an occluded state, elucidating the molecular basis of taurine and GABA recognition and selectivity.
- TauT forms a cholesterol-mediated homodimer in lipid nanodiscs; two cholesterols act as molecular glue at TM5/TM7/EL3 interface.
- Cholesterol removal abolishes dimerization; charge mutations at TM5 interface residues Val262 and Leu265 eliminate transport activity.
- TauT dimerization architecture is distinct from the NET homodimer, suggesting SLC6 family members adopt diverse lipid-mediated oligomeric states.
Methodology
Full-length wild-type human TauT was expressed in HEK293S GnTI⁻ cells and purified in LMNG detergent or reconstituted into MSP1D1/MSPE3D1 lipid nanodiscs of varying compositions. Single-particle cryo-EM was performed to solve structures in apo, taurine-bound, and GABA-bound states at resolutions of 3.05–3.26 Å; functional validation used [³H]-taurine uptake assays and site-directed mutagenesis.
Study Limitations
Dimerization was observed only in lipid nanodisc reconstitution systems and not in detergent micelles, so the physiological prevalence and functional significance of the TauT dimer in native membranes remains to be established. The relatively lower map quality of the homodimeric reconstructions (3.25–3.98 Å) led to missing densities for TM1a, substrate, and ions in some dimer states, limiting atomic-level interpretation of the dimer's functional state. The study does not yet address how TauT conformational dynamics and dimerization are modulated under physiologically relevant conditions such as varying membrane cholesterol levels associated with aging.
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