glut4

GLUT4 (SLC2A4)

GLUT4 is the insulin-regulated facilitative glucose transporter responsible for the majority of insulin-stimulated glucose uptake into skeletal muscle and adipose tissue. Under basal conditions ~95% of cellular GLUT4 resides in intracellular GLUT4-storage vesicles (GSVs); insulin signaling through insr → irs-1 → pi3k-akt-pathway → akt → as160 drives GSV exocytosis, inserting GLUT4 into the plasma membrane and acutely amplifying glucose import ~10-fold. Impairment of this translocation cycle is the proximate cellular defect in insulin resistance and type-2-diabetes, and GLUT4 levels and trafficking decline measurably with aging in skeletal muscle.

Identity

Field	Value
UniProt	P14672 (GTR4_HUMAN; Swiss-Prot, manually reviewed)
NCBI Gene	6517
HGNC	11009
Ensembl	ENSG00000181856
Mouse ortholog	Slc2a4 (Mus musculus)
Protein length	509 amino acids (canonical isoform)
Molecular weight	~55 kDa (apparent ~46 kDa on SDS-PAGE due to anomalous migration)
Superfamily	Major Facilitator Superfamily (MFS); SLC2A/GLUT subfamily
Chromosome (human)	17p13.1

Structure and domains

GLUT4 is a 12-transmembrane-helix MFS transporter arranged as two bundles of 6 helices separated by a large intracellular loop ¹. Key structural features:

• N-terminal cytoplasmic tail — dileucine motif (LL) mediates intracellular retention and endocytic recycling; critical for basal intracellular sequestration
• Transmembrane helices TM1–12 — form the glucose-transport channel; alternating-access mechanism with outward-facing and inward-facing conformations
• Large exofacial loop (EC2, TM5–TM6) — target of GLUT4-specific antibodies used to track plasma-membrane insertion in real time
• C-terminal cytoplasmic tail — contains the Phe5-containing FQQI (residues 499–502) motif essential for GSV targeting; also harbors Ser488 phosphorylation site

The transporter operates as a monomer facilitating passive, bidirectional glucose flux down its concentration gradient. Under physiological conditions the gradient is inward (blood glucose > cytoplasm), so net glucose entry occurs.

Tissue distribution and GLUT isoform context

GLUT4 expression is restricted to insulin-sensitive tissues:

Tissue	Expression level	Notes
Skeletal muscle	High (predominant)	~80% of insulin-stimulated glucose disposal in humans
Cardiac muscle	High	Required for normal cardiac metabolic flexibility
Adipose tissue (white + brown)	High	Major depot for postprandial glucose buffering
Brain (selective neurons)	Low–moderate	Insulin-independent role in hippocampal glucose sensing

Contrast with other GLUT isoforms:

• GLUT1 (SLC2A1) — ubiquitous basal transporter; red blood cells, blood-brain barrier
• GLUT2 (SLC2A2) — liver hepatocytes and pancreatic beta cells; high-capacity, low-affinity glucose sensor
• GLUT3 (SLC2A3) — neurons; high affinity (Km ~1.5 mM); expression not regulated by insulin
• GLUT5 (SLC2A5) — fructose transporter; intestinal brush border and kidney

Function: insulin-stimulated translocation cascade

Under basal (fasting) conditions, GLUT4 is actively retained in intracellular GSVs by the GDP-loaded Rab GTPase cycle and tethering complexes. Insulin signaling rapidly reverses this retention ²:

1. Receptor activation — Insulin binds insr ectodomain → conformational change → autophosphorylation of cytoplasmic kinase domain → recruitment and phosphorylation of irs-1 and IRS-2 on multiple Tyr residues
2. PI3K → PIP3 — IRS-1 pTyr motifs recruit the p85 regulatory subunit of pi3k; p110 catalytic subunit generates PIP3 from PIP2 at the inner leaflet of the plasma membrane
3. AKT activation — PIP3 recruits akt via its PH domain → PDK1 phosphorylates Thr308; mTORC2 phosphorylates Ser473 → full AKT activation (see akt page)
4. AS160/TBC1D4 phosphorylation — Activated akt phosphorylates as160 (TBC1D4) at multiple sites including Thr642 ³ no-fulltext-access. AS160 is a Rab-GAP (GTPase-activating protein) that normally maintains Rab proteins in their GDP-bound inactive state
5. Rab GTPase activation — Phospho-AS160 has suppressed GAP activity → Rab8A, Rab10, and Rab14 accumulate in their GTP-bound active form → these Rabs recruit motor proteins and tethering factors enabling GSV mobilization
6. GSV trafficking and fusion — Activated Rab proteins couple GSVs to motor proteins (myosin Va/b) and cortical actin; GSVs dock at the plasma membrane via SNARE complexes (VAMP2/SNAP23/Syntaxin4); membrane fusion inserts GLUT4 into the plasma membrane
7. Glucose uptake — Plasma membrane GLUT4 density rises ~5–10-fold within 5–15 minutes of insulin stimulation → glucose uptake increases correspondingly

There is a parallel pathway: exercise/muscle contraction activates ampk (via AMP:ATP ratio rise) → AMPK independently phosphorylates TBC1D1 (a related Rab-GAP) → Rab8A/Rab10 activation → GLUT4 translocation by a route that is largely insulin-independent ². This exercise-stimulated pathway is clinically important: insulin-resistant muscle retains normal or near-normal contraction-stimulated GLUT4 translocation.

Discovery

GLUT4 was independently cloned in 1989:

• James, Strube & Mueckler 1989 — isolated a cDNA from rat adipose tissue and muscle encoding a 509-aa facilitative glucose transporter with insulin-responsive expression; demonstrated this differed from the ubiquitous GLUT1 ¹
• Multiple additional groups cloned the same sequence from adipocytes (Birnbaum 1989) and muscle (Charron 1989) in the same period, establishing it as a distinct insulin-responsive isoform

Knockout and overexpression phenotypes

Model	Phenotype	Reference
Glut4-null (whole-body, mouse)	Viable; severe cardiac hypertrophy (heart:body ratio 2.3–2.5× age-matched controls); growth retardation; severely diminished adipose tissue deposits (females: no dissectable ovarian fat pad); sexually dimorphic glucose phenotype — females clear glucose normally (fasted and fed blood glucose not significantly elevated), males show moderately reduced fasting glycaemia and increased fed glycaemia, but overt diabetes does not develop in either sex; fed insulin levels ~5–6× control (hyperinsulinemia drives compensation); decreased longevity, attributed to cardiac hypertrophy; compensatory GLUT2 upregulation in liver (1.7-fold) and GLUT1 upregulation in heart (1.5-fold) but not in skeletal muscle	4
Muscle-specific Glut4 KO (mouse)	Marked insulin resistance (skeletal muscle); systemic glucose intolerance; hepatic insulin resistance secondary to muscle defect	needs-replication
Adipose-specific Glut4 KO (mouse)	Whole-body insulin resistance disproportionate to adipose-specific loss, suggesting adipokine/signal-relay mechanism	needs-replication
GLUT4 muscle overexpression (transgenic)	Enhanced glucose uptake; protection from diet-induced insulin resistance	needs-human-replication

The whole-body Glut4-null phenotype is notably dominated by cardiac and growth effects, whereas the metabolic defect is less severe than expected — likely because GLUT2 overexpression in the liver (1.7-fold) and marked fed hyperinsulinemia (~5–6× control) drive compensatory glucose disposal by non-GLUT4-dependent routes. GLUT1 is upregulated in the heart (1.5-fold) but neither GLUT1 nor GLUT2 is upregulated in skeletal muscle in Glut4-null animals ⁴.

Dimension	Status	Notes
Pathway conserved in humans?	yes	INSR→IRS→PI3K→AKT→AS160→Rab→GLUT4 axis is identical in humans; AS160 pThr642 confirmed in human tissue
Phenotype conserved in humans?	yes	Insulin resistance in type 2 diabetes involves GLUT4 translocation defect in human muscle; confirmed by muscle biopsy studies
Replicated in humans?	yes (for T2D defect)	Reduced GLUT4 membrane translocation documented in muscle biopsies from T2D patients and insulin-resistant subjects

Aging relevance

Reduced GLUT4 expression and translocation in aged muscle

Skeletal muscle GLUT4 protein content and insulin-stimulated translocation efficiency both decline with aging in humans and rodents ². Contributing mechanisms include:

• Reduced GLUT4 gene expression — aging muscle shows lower SLC2A4 mRNA, partly attributable to reduced PPAR-γ coactivator activity and epigenetic changes at the GLUT4 promoter needs-replication
• Impaired AKT → AS160 phosphorylation — aged muscle shows reduced insulin-stimulated AKT Thr308 and AS160 Thr642 phosphorylation, reflecting upstream IRS-1 and PI3K impairment (see insulin-igf1, akt pages)
• Actin cytoskeleton remodeling defects — GSV trafficking requires dynamic cortical actin reorganization; aged muscle myocytes show impaired insulin-stimulated F-actin remodeling no-mechanism

These changes contribute to the well-characterized age-associated decline in glucose disposal rate measured by glucose clamp techniques.

Type 2 diabetes connection

GLUT4 translocation failure is the proximate cellular defect responsible for impaired insulin-stimulated glucose disposal in type-2-diabetes ². In T2D skeletal muscle:

• Total cellular GLUT4 protein is modestly reduced (~20–40%)
• More importantly, the insulin-stimulated translocation efficiency is severely impaired — GLUT4 fails to translocate to the plasma membrane in proportion to the insulin signal
• Downstream of this failure, glucose disposal is impaired even when insulin secretion is adequate

Exercise as GLUT4 enhancement therapy

Aerobic and resistance exercise acutely and chronically increase GLUT4 in skeletal muscle — the most potent and reliable non-pharmacological intervention:

• Acute exercise stimulates GLUT4 translocation via AMPK (contraction-stimulated, insulin-independent pathway) — effective even in insulin-resistant muscle
• Chronic training increases total cellular GLUT4 protein expression (via transcriptional upregulation, partly through PGC-1α) — this is one mechanism by which exercise reverses insulin resistance

dose-response-unclear — optimal exercise modality (aerobic vs resistance vs combined), frequency, and intensity for maximal GLUT4 restoration in aged muscle remain incompletely defined.

Pathway membership

• insulin-igf1 — GLUT4 is the primary effector for insulin-stimulated glucose uptake
• pi3k-akt-pathway — AKT-AS160 axis is the proximal driver of translocation
• ampk — contraction-stimulated, insulin-independent GLUT4 translocation route

Key interactors

Interactor	Role	Evidence type
akt	Phosphorylates AS160 → enables GSV release	In vitro + in vivo; Thr642 phosphorylation confirmed
as160	Rab-GAP; substrate of AKT; phospho-AS160 releases Rab10/Rab14 from GDP-locked state	3 no-fulltext-access
insr	Upstream receptor; initiates insulin signaling cascade	Established
irs-1	Adaptor; IRS-1 pTyr recruits PI3K p85	Established
VAMP2	v-SNARE on GSVs; drives GSV-plasma membrane fusion	Established (not yet a wiki page)

Pharmacological modulation

Agent	Mechanism	GLUT4 Effect	Evidence level
Insulin	Direct (activates INSR→AKT→AS160)	Acute translocation	Established
Metformin	AMPK activation (complex I inhibition → AMP:ATP rise)	Increased GLUT4 translocation (AMPK route)	Strong (T2D clinical use)
Thiazolidinediones (PPAR-γ agonists)	PPAR-γ transcriptional upregulation of SLC2A4	Increased GLUT4 protein expression	Strong (clinical); long-term-unknown for aging
GLP-1 receptor agonists (e.g., semaglutide)	Indirectly — weight loss + improved insulin sensitivity upstream	Improved translocation (secondary)	Strong (clinical T2D)
Exercise	AMPK + PGC-1α (translocation + transcription)	Acute translocation + chronic protein upregulation	Strong (human biopsy data)
Rapamycin / mTOR inhibition	Complex (mTORC2 contributes to AKT Ser473; mTORC1 affects IRS-1 feedback)	Ambiguous; can impair insulin signaling acutely	Contested contradictory-evidence

Limitations and gaps

• GLUT4 expression decline with aging: magnitude unclear — published fold-changes vary across studies; sex, training history, and biopsy technique all confound; systematic meta-analysis needed. needs-replication
• Tissue-specific KO phenotypes in mouse — muscle- and adipose-specific GLUT4 KO phenotypes are established in mouse but direct human genetic equivalents (rare loss-of-function SLC2A4 variants) are not well characterized. needs-human-replication
• GSV subpopulation heterogeneity — evidence suggests multiple biochemically distinct GLUT4-containing vesicle pools with different insulin sensitivities; functional significance in aging not clear. no-mechanism
• Brain GLUT4 role — neuronal GLUT4 in hippocampus may contribute to cognitive insulin sensitivity; aging-related changes unstudied. needs-replication
• Non-glucose substrates — GLUT4 also transports galactose and dehydroascorbic acid; physiological relevance in aging contexts unexplored. unsourced

Cross-references

• insr — upstream receptor; INSR-AKT-AS160 axis described there (verified-partial)
• insulin — ligand; verified-partial
• insulin-igf1 — pathway page covering the full IIS axis; verified-partial
• akt — kinase substrate (phosphorylates AS160); detailed AKT biochemistry there (verified)
• as160 — direct substrate of AKT in this pathway; key Rab-GAP controlling GSV release
• type-2-diabetes — GLUT4 translocation defect is the proximate mechanism; verified-partial
• skeletal-muscle — primary tissue for GLUT4-dependent glucose disposal
• adipose-tissue — second major GLUT4 tissue (page planned)
• ampk — contraction-stimulated parallel route to GLUT4 translocation
• deregulated-nutrient-sensing — hallmark page; GLUT4 insufficiency is a downstream manifestation

Footnotes

Footnotes

1. doi:10.1038/338083a0 · James DE, Strube M, Mueckler M · n=N/A · in-vitro + expression cloning · model: rat adipose/muscle cDNA library · 907 citations; 100th percentile impact; archive: not_oa (closed access) no-fulltext-access ↩ ↩²

2. doi:10.1152/ajpcell.00069.2014 · Klip A, Sun Y, Chiu TT, Foley KP · review · model: review of mammalian GLUT4 trafficking literature · 162 citations; 100th percentile impact; archive: not_oa (closed access) no-fulltext-access ↩ ↩² ↩³ ↩⁴

3. doi:10.1074/jbc.C300063200 · Sano H et al. · n=N/A · in-vitro (biochemistry + cell biology) · model: 3T3-L1 adipocytes + in vitro kinase assay · 952 citations; 100th percentile impact; archive: failed (hybrid OA — no candidate URLs resolved; no-fulltext-access) ↩ ↩²

4. doi:10.1038/377151a0 · Katz EB et al. · n=N/A · in-vivo (mouse transgenic, whole-body KO) · model: Glut4-null Mus musculus · 472 citations; 100th percentile impact; local PDF available at ↩ ↩²