SLC6A6 (TauT) — review notes

UniProt: P31641 (SC6A6_HUMAN). HGNC:11052. Taxon: Homo sapiens (9606). 620 aa.

Identity and family

• RecName: Sodium- and chloride-dependent taurine transporter; AltName: Solute carrier family 6 member 6; Taurine transporter (TauT) [file:human/SLC6A6/SLC6A6-uniprot.txt "RecName: Full=Sodium- and chloride-dependent taurine transporter"].
• Belongs to the sodium:neurotransmitter symporter (SNF) (TC 2.A.22) family. SLC6A6 subfamily [file:human/SLC6A6/SLC6A6-uniprot.txt "Belongs to the sodium:neurotransmitter symporter (SNF)"].
• 12 transmembrane helices confirmed by cryo-EM (TOPO_DOM/TRANSMEM 1..620, N- and C-termini cytoplasmic). Na+, Cl−, and taurine binding sites mapped near TM1/TM6/TM8 residues (G57, F58, N63, Y138, F300, S301, S402, E406, etc.).

Core molecular function and mechanism

• Mediates sodium- and chloride-dependent transport of taurine [file:human/SLC6A6/SLC6A6-uniprot.txt "Mediates sodium- and chloride-dependent transport of taurine,"]. Stoichiometry Na+/Cl−/taurine = 2:1:1 PMID:8010975.
• Catalytic reaction (RHEA:71223): taurine(out) + chloride(out) + 2 Na(+)(out) = taurine(in) + chloride(in) + 2 Na(+)(in) [file:human/SLC6A6/SLC6A6-uniprot.txt].
• Also transports, with lower affinity, beta-alanine, hypotaurine and GABA (4-aminobutanoate) [file:human/SLC6A6/SLC6A6-uniprot.txt "Can also mediate transport of beta-alanine, hypotaurine and"]. Substrate specificity: taurine and other beta-amino acids incl. beta-alanine; high affinity for taurine (KM ~6 µM) PMID:8010975. RPE clone: taurine, beta-alanine and GABA compete for uptake; alpha-alanine and AIB do not PMID:8654117.
• KM for taurine: 5.9 µM (PMID:8010975), 2 µM (PMID:8654117), 3.8 µM (PMID:31903486), 7.62 µM (PMID:41269860). Uniformly high-affinity, low-µM.
• Intestinal TauT: Na+- and Cl−-dependent, high-affinity, low-capacity transporter of taurine and beta-alanine PMID:19074966.

Note on GO term choice: GOA carries both GO:0005369 taurine:sodium symporter activity (the mechanism-specific term, dominant across IDA/IMP/IBA) and GO:0005368 taurine transmembrane transporter activity (parent). The symport is Na+- AND Cl−-coupled (2:1:1). GO:0005369 is the closest well-supported MF term in GOA and is used as the primary core MF. The GABA/beta-alanine symporter activities (GO:0005332, GO:0001761, GO:0015185) are genuine lower-affinity/secondary activities documented structurally and functionally.

Subcellular location

• Cell membrane (plasma membrane); multi-pass [file:human/SLC6A6/SLC6A6-uniprot.txt "SUBCELLULAR LOCATION: Cell membrane"]. IDA plasma membrane from patient-mutation and proteomic studies (PMID:31345061, PMID:31903486, PMID:28112518).
• Mitochondrion inner membrane — newly reported: SLC6A6 also localizes to mitochondria and imports taurine for mt-tRNA modification PMID:41652173. PKA phosphorylation at Ser-21/Ser-25 promotes plasma-membrane localization while inhibiting mitochondrial localization; N-terminal MTS (1–41) targets mitochondria.
• Epithelial polarity: in placental syncytiotrophoblast TAUT is primarily localized in the syncytiotrophoblast microvillous plasma membrane (MVM) — maternal-facing/apical PMID:15166008. In human fetal RPE, TAUT quantified on both apical and basolateral surfaces (PMID:31791063 QTAP of apical vs basolateral membranes).

Tissue expression

• Expressed abundantly in placenta and skeletal muscle, at intermediate levels in heart, brain, lung, kidney and pancreas and at low levels in liver [file:human/SLC6A6/SLC6A6-uniprot.txt]. HPA: tissue enhanced (retina).

Physiology and disease

• Loss of function causes Hypotaurinemic retinal degeneration and cardiomyopathy (HTRDC, MIM:145350), autosomal recessive: low plasma taurine, childhood-onset progressive retinal degeneration, cardiomyopathy [file:human/SLC6A6/SLC6A6-uniprot.txt "Hypotaurinemic retinal degeneration and cardiomyopathy (HTRDC)"].
• Biallelic SLC6A6 mutation linked to early retinal degeneration; variant A78E severely decreases taurine transport in patient cells [PMID:31345061 title].
• Variant G399V; taurine supplementation treats retinal degeneration and cardiomyopathy in a consanguineous family [PMID:31903486 title].
• Taurine is the most abundant amino acid in human placenta; TauT knockdown compromises trophoblast differentiation and increases apoptosis under inflammatory cytokine PMID:23519128.
• Anti-aging/senescence: taurine acts as an anti-aging agent, levels decline with age; TauT structure work framed around alleviating cellular senescence (PMID:40108449) and aging-associated disorders (PMID:40615403). Negative regulation of cellular senescence annotated by ISS/IEA from mouse ortholog O35316.
• Cancer / drug delivery: SLC6A6 (with SLC6A13) mediates uptake of 5-aminolevulinic acid (ALA), enhancing protoporphyrin IX accumulation / photodamage in cancer cells PMID:24842606. Note this is a substrate-promiscuity / drug-uptake role, not a canonical physiological function.

Regulation

• Stimulated by thyrotropin (TSH) via cAMP in thyroid (FRTL-5) cells PMID:8382624.
• Stimulated by hypertonic stress (osmoregulation; by similarity); inhibited by GABA, hypotaurine, beta-alanine and various sulfonate analogs; down-regulated by Ser-322 phosphorylation (by similarity) and by PKC activation / NO in placenta.
• Induction regulated by NFAT5 (tonicity-responsive), which regulates mitochondrial function of SLC6A6 [file:human/SLC6A6/SLC6A6-uniprot.txt "Expression is regulated by NFAT5"].

Annotation-review reasoning highlights

• Core MF: GO:0005369 taurine:sodium symporter activity (huge IDA/IMP/IBA support, incl. 6+ structural/functional papers) — ACCEPT.
• Core CC: GO:0005886 plasma membrane (IDA/ISS/IBA/TAS) — ACCEPT. Apical (GO:0016324) + basolateral (GO:0016323) IDA — accept as epithelial context refinements (core plasma-membrane sub-locations).
• Core BP: GO:0015734 taurine transmembrane transport (IDA/IMP/ISS/IBA/TAS) — ACCEPT.
• Mitochondrial inner membrane (GO:0005743) IDA (PMID:41652173) + positive regulation of mitochondrial translation (GO:0070131) IDA — accept as genuine newly-discovered non-canonical/moonlighting localization+role; the IEA SubCell version is redundant but reflects the same finding.
• beta-alanine (GO:0001761/GO:0001762) and GABA (GO:0015185/GO:0005332/GO:0051939) activities: genuine but secondary/lower-affinity substrates — KEEP_AS_NON_CORE (functionally real, not the defining physiological role).
• GO:0022858 alanine transmembrane transporter activity + GO:0032328 alanine transport (IDA PMID:19074966): the paper studies taurine/beta-alanine, and TauT does NOT accept alpha-alanine (PMID:8654117). "alanine" here almost certainly refers to beta-alanine handling in an SLC6A6 context; but the GO term GO:0022858 is for alanine (alpha) transport. Cannot verify alpha-alanine transport; ARUK-UCL experimental IDA — do not REMOVE per policy; MARK_AS_OVER_ANNOTATED (likely a beta-alanine finding over-mapped to the generic alanine term).
• GO:0006836 neurotransmitter transport (IEA InterPro) and GABA-related terms reflect family-level (SNF) inheritance; taurine is not a classical neurotransmitter. KEEP_AS_NON_CORE / non-core, family-derived.
• GO:0150104 transport across blood-brain barrier (NAS, review PMIDs 30280653 & 26590417): both are general BBB reviews that do not specifically establish SLC6A6 BBB transport in their abstracts/text. Author statement (NAS) — KEEP_AS_NON_CORE but flag low relevance; not core.
• GO:0045597 positive regulation of cell differentiation (IMP PMID:23519128): trophoblast differentiation compromised in TauT-deficient cells — genuine but tissue-specific downstream role. KEEP_AS_NON_CORE.
• GO:0089718 amino acid import across plasma membrane / GO:0098739 import across plasma membrane / GO:0006865 amino acid transport / GO:0015171 amino acid transmembrane transporter activity / GO:0005283 amino acid:sodium symporter activity: correct but general parents of the taurine-specific function; ACCEPT the redundant-but-correct ones where experimentally grounded, otherwise KEEP as generalizations. Prefer taurine-specific terms as core.
• GO:0035725 sodium ion transmembrane transport (IBA): mechanistically correct (Na+ is co-transported) but the co-ion role, not the substrate; KEEP_AS_NON_CORE.
• GO:2000773 negative regulation of cellular senescence (IEA/ISS from mouse O35316): plausible (taurine anti-senescence) but by orthology only; KEEP_AS_NON_CORE.
• CC dendrite / neuronal cell body (ISS from rat GAT P31643): transferred from a GABA transporter paralog; SLC6A6 neuronal-compartment localization not established in human — MARK_AS_OVER_ANNOTATED (ISS from a paralog, weak).
• GO:0031528 microvillus membrane (IDA PMID:15166008): placental MVM localization directly shown — ACCEPT (epithelial-context CC).
• GO:0016020 membrane (TAS/IEA): uninformative generic parent — MARK_AS_OVER_ANNOTATED.

Full-text availability of cited pubs

• full_text_available: true for PMID:19074966, PMID:23519128, PMID:26590417 (and 31903486 pdf_partial).
• Abstract-only (full_text_available: false): 8382624, 8010975, 8654117, 24842606, 41652173, 31345061, 15166008, 28112518, 31791063, 40108449, 40615403, 40789850, 41269860. Quotes taken only from cached text (abstracts) for these.