Affinage

ICMT

Protein-S-isoprenylcysteine O-methyltransferase · UniProt O60725

Length
284 aa
Mass
31.9 kDa
Annotated
2026-04-28
37 papers in source corpus 17 papers cited in narrative 17 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

ICMT is an endoplasmic reticulum-resident integral membrane methyltransferase that catalyzes the terminal post-translational modification of isoprenylated CAAX and CXC proteins—including all four RAS isoforms, RHO GTPases, and RAB proteins—by transferring a methyl group from S-adenosylmethionine (SAM) to the C-terminal isoprenylcysteine carboxylate, thereby promoting plasma membrane targeting and downstream signaling (PMID:16757321, PMID:31181882). Structurally, ICMT contains a five-transmembrane-helix core with a hydrophobic tunnel linking the cytosolic SAM-binding pocket to the membrane-embedded prenylcysteine-binding site, and catalysis proceeds through an ordered sequential mechanism in which SAM binds first (PMID:22195972, PMID:29342140, PMID:15625008). Loss of ICMT mislocalizes RAS from the plasma membrane, destabilizes RhoA, induces p21(Cip1)-dependent growth arrest, and blocks oncogenic K-Ras-driven transformation, while its expression is transcriptionally repressed by wild-type p53 (PMID:14966563, PMID:30655292). Genetic deletion of Icmt in a Hutchinson-Gilford progeria syndrome mouse model improves survival, implicating ICMT-mediated methylation of farnesylated progerin in disease pathology (PMID:33526168).

Mechanistic history

Synthesis pass · year-by-year structured walk · 11 steps
  1. 2004 High

    Establishing the catalytic mechanism: the question of substrate binding order and cofactor release was resolved, showing ICMT uses an ordered sequential mechanism with SAM binding first and S-adenosyl-L-homocysteine released last, providing a kinetic framework for inhibitor design.

    Evidence In vitro kinetic analysis with recombinant human ICMT using dead-end competitive inhibitors and product inhibition studies

    PMID:15625008

    Open questions at the time
    • No structural basis for the ordered mechanism at this point
    • Kinetics determined with small-molecule substrates, not full-length prenylated proteins
  2. 2004 High

    Defining the biological requirement for ICMT in oncogenic signaling: genetic inactivation of Icmt in K-Ras-transformed fibroblasts blocked transformation, revealing that ICMT loss destabilizes RhoA and induces p21(Cip1)-dependent growth arrest—establishing ICMT as a gatekeeper for RAS-driven oncogenesis.

    Evidence Conditional Cre-mediated Icmt knockout in K-Ras-expressing MEFs, soft agar colony formation, nude mouse xenograft, p21 double-knockout rescue

    PMID:14966563

    Open questions at the time
    • Whether ICMT loss blocks transformation by all RAS isoforms equally was untested
    • Mechanism linking RhoA destabilization to p21 induction was not fully resolved
  3. 2006 High

    Establishing ICMT as the sole mammalian isoprenylcysteine carboxyl methyltransferase: ICMT methylates both CAAX and CXC (Rab) classes of prenylated proteins, and its absence mislocalizes RAS from the plasma membrane.

    Evidence Icmt-knockout cells with subcellular fractionation and electrophoretic mobility shift of unmethylated Ras

    PMID:16757321

    Open questions at the time
    • Relative dependence of individual Ras isoforms on methylation for membrane targeting was not differentiated
  4. 2011 High

    Revealing the structural basis of membrane-embedded catalysis: the first crystal structure of an ICMT ortholog showed a five-transmembrane-helix core with a tunnel connecting the SAM-binding pocket to the membrane interior, explaining how a single enzyme accommodates both hydrophilic and lipophilic substrates.

    Evidence X-ray crystallography of prokaryotic ICMT ortholog

    PMID:22195972

    Open questions at the time
    • Prokaryotic ortholog may not capture all features of mammalian ICMT
    • Substrate-bound structure not obtained
  5. 2014 High

    Mapping the functional residue landscape: systematic mutagenesis identified 62 activity-reducing mutations, confirming that transmembrane residues form the isoprenylcysteine binding site and cytosolic residues engage SAM, and revealed that certain substitutions cause substrate inhibition.

    Evidence Scanning mutagenesis of ~50% of Anopheles gambiae ICMT residues with methyltransferase activity assays

    PMID:25059662

    Open questions at the time
    • Insect ortholog mutagenesis; not all findings may transfer quantitatively to human ICMT
    • No crystal structure of mutant enzymes to confirm structural interpretation
  6. 2016 High

    Demonstrating a physiological role for ICMT in sensory neurobiology: retina-specific Icmt knockout caused progressive photoreceptor degeneration by disrupting membrane association and stability of prenylated phototransduction proteins including transducin and cone PDE6.

    Evidence Retina-specific conditional Icmt knockout mice with electroretinography and subcellular fractionation

    PMID:27147662

    Open questions at the time
    • Whether methylation-deficient transducin retains partial signaling activity was not determined
    • Contribution of individual ICMT substrates to the degenerative phenotype was not dissected
  7. 2018 High

    Achieving the eukaryotic structure: the 2.3 Å crystal structure of ICMT in complex with SAM and a monobody inhibitor confirmed distinct cytosolic and membrane-embedded entry routes for cofactor and substrate, providing a template for rational drug design.

    Evidence X-ray crystallography of eukaryotic ICMT with monobody co-crystallization

    PMID:29342140

    Open questions at the time
    • No structure with a bound prenylcysteine substrate or full-length protein substrate
    • Dynamics of substrate entry through the membrane not captured by crystallography
  8. 2019 High

    Pharmacologic validation of ICMT as a druggable target for RAS-driven cancers: ICMT inhibitor UCM-1336 impaired membrane association and GTP loading of all four RAS isoforms and inhibited downstream signaling, with efficacy in an AML model.

    Evidence UCM-1336 treatment with subcellular fractionation, Ras-GTP pull-down, immunoblotting, and in vivo AML xenograft

    PMID:31181882

    Open questions at the time
    • Off-target effects on non-RAS prenylated substrates not fully characterized
    • Long-term resistance mechanisms to ICMT inhibition not explored
  9. 2019 Medium

    Identifying transcriptional control of ICMT: wild-type p53 directly represses ICMT transcription by binding its promoter, while cancer-associated p53 mutants upregulate ICMT, connecting tumor suppressor status to the prenylation-methylation axis.

    Evidence ICMT-luciferase reporter, promoter truncation, and chromatin immunoprecipitation (ChIP) for p53

    PMID:30655292

    Open questions at the time
    • Physiological relevance of p53-dependent ICMT repression in tumor suppression not demonstrated in vivo
    • Mechanism by which mutant p53 activates ICMT expression not resolved
  10. 2021 High

    Resolving isoform-specific dependence on methylation: NRAS uniquely requires ICMT for plasma membrane delivery because its single palmitoylation site makes it dependent on methylation for efficient palmitoylation and PDE6δ chaperone binding.

    Evidence ICMT-knockout cells with photoactivation/FRAP of GFP-NRAS, palmitoylation assays, and PDE6δ co-immunoprecipitation

    PMID:33579760

    Open questions at the time
    • Whether NRAS-specific dependence translates to differential therapeutic vulnerability in NRAS-mutant cancers was not tested
    • Quantitative contribution of each step (palmitoylation, PDE6δ binding, Golgi flux) not resolved
  11. 2021 High

    Establishing ICMT as a therapeutic target in progeria: genetic Icmt deletion and pharmacologic ICMT inhibition (C75) improved survival in HGPS mice and rescued senescence in HGPS cells, with on-target specificity confirmed in double-knockout controls.

    Evidence Icmt knockout in HGPS mouse model, C75 inhibitor treatment, proliferation/senescence assays, Icmt/Zmpste24 double-KO specificity control

    PMID:33526168

    Open questions at the time
    • Whether ICMT inhibition reverses established disease or only prevents progression was not determined
    • Potential toxicity from global ICMT inhibition in vivo not thoroughly assessed

Open questions

Synthesis pass · forward-looking unresolved questions
  • A substrate-bound structure of eukaryotic ICMT with a prenylated peptide or full-length protein substrate has not been determined, leaving the precise geometry of methyl transfer and the structural basis for substrate selectivity among diverse prenylated proteins unresolved.
  • No substrate-bound eukaryotic ICMT structure
  • Structural basis for selectivity among CAAX vs CXC substrates unknown
  • Allosteric regulation and role of interactors such as PFKFB4 structurally uncharacterized

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0016740 transferase activity 5
Localization
GO:0005783 endoplasmic reticulum 3
Partners
KRASNRASPDE6DPFKFB4RAC1RHOA

Evidence

Reading pass · 17 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2011 Crystal structure of a prokaryotic ICMT ortholog revealed the enzyme comprises a core of five transmembrane α-helices with a cofactor (SAM)-binding pocket in a highly conserved C-terminal catalytic subdomain, and a tunnel linking the reactive methyl group of SAM to the inner membrane provides access for the prenyl lipid substrate, explaining how the integral membrane methyltransferase accommodates both a hydrophilic cofactor and a lipophilic prenyl substrate. X-ray crystallography of prokaryotic ICMT ortholog Molecular Cell High 22195972
2018 X-ray crystal structure of eukaryotic ICMT at 2.3 Å resolution (in complex with SAM cofactor, an ordered lipid, and a monobody inhibitor) demonstrated that the active site spans cytosolic and membrane-exposed regions, indicating distinct entry routes for the cytosolic methyl donor SAM and for prenylcysteine substrates associated with the ER membrane. X-ray crystallography with monobody inhibitor co-crystallization Nature High 29342140
2004 Human ICMT catalysis proceeds through an ordered sequential kinetic mechanism in which SAM binds first; dead-end competitive inhibitor analysis with S-farnesylthioacetic acid and product inhibition studies with S-adenosyl-L-homocysteine (AdoHcy) and N-acetyl-S-farnesyl-L-cysteine methylester established that AdoHcy is the final product released. In vitro kinetic analysis with recombinant human ICMT, dead-end inhibitor and product inhibition studies BMC Biochemistry High 15625008
2014 Scanning mutagenesis of nearly half of the residues of the Anopheles gambiae ICMT ortholog, combined with methyltransferase assays, identified 62 mutations causing reduced or undetectable activity; residues within transmembrane regions contribute to the isoprenylcysteine binding site, and certain substitutions caused substrate inhibition, while cytosolic residues contribute to AdoMet binding in a manner similar to a related prokaryotic methyltransferase (Ma MTase). Scanning mutagenesis combined with in vitro methyltransferase activity assays Journal of Biological Chemistry High 25059662
2004 Genetic inactivation of Icmt in mouse embryonic fibroblasts expressing oncogenic K-Ras inhibited K-Ras-induced oncogenic transformation (soft agar and nude mice assays); mechanistically, Icmt loss caused accelerated RhoA protein turnover/reduced RhoA levels, a Ras/Erk1/2-dependent increase in p21(Cip1), and deletion of p21(Cip1) rescued soft-agar growth, placing Icmt upstream of RhoA stability and p21(Cip1) induction. Conditional Cre-mediated Icmt knockout in K-Ras-expressing fibroblasts, soft agar assay, nude mouse xenograft, p21(Cip1) double knockout rescue Journal of Clinical Investigation High 14966563
2006 ICMT is the only enzyme in mammalian cells capable of methylating isoprenylated CAAX proteins; in addition to CAAX proteins (including Ras), ICMT also methylates the CXC class of isoprenylated Rab proteins. In the absence of carboxyl methylation, Ras proteins are mislocalized away from the plasma membrane and exhibit a shift in electrophoretic mobility. Genetic knockout studies and subcellular fractionation/localization in Icmt-deficient cells Methods in Enzymology High 16757321
2021 Among the four RAS proteins, NRAS uniquely requires ICMT for delivery to the plasma membrane because it has only a single palmitoylation site; ICMT-mediated carboxyl methylation is required for efficient NRAS palmitoylation, for binding to the prenyl-protein chaperone PDE6δ, and for homeostatic Golgi flux, all of which direct NRAS to the plasma membrane. ICMT knockout cells, photoactivation and FRAP of GFP-NRAS, palmitoylation assays, PDE6δ binding assays, subcellular localization imaging Life Science Alliance High 33579760
2016 Retina-specific Icmt knockout in mice caused progressive loss of rod and cone photoreceptor function; ICMT-mediated methylation is required for proper membrane association of isoprenylated transducin and cone phosphodiesterase 6 (PDE6α'), and for maintaining protein levels of transducin, PDE6α', and cone GRK1, demonstrating that carboxyl methylation is essential for phototransduction protein membrane targeting. Retina-specific conditional Icmt knockout mice, electroretinography, subcellular fractionation, immunoblotting Journal of Neuroscience High 27147662
2021 Genetic knockout of Icmt in HGPS mice improved survival and restored vascular smooth muscle cell numbers in the aorta; pharmacologic ICMT inhibition with compound C75 delayed senescence and stimulated proliferation of late-passage HGPS cells and Zmpste24-deficient fibroblasts, acting specifically through ICMT (C75 had no effect on Zmpste24-deficient cells also lacking Icmt), implicating ICMT-mediated methylation of farnesylated progerin in HGPS pathology. Icmt knockout HGPS mouse model, ICMT inhibitor C75, proliferation and senescence assays, specificity confirmed in double-KO cells eLife High 33526168
2019 Pharmacologic ICMT inhibition with compound UCM-1336 (IC50 = 2 μM) significantly impaired membrane association of all four Ras isoforms, decreased Ras-GTP levels, and inhibited downstream Ras signaling pathways, demonstrating that ICMT-mediated methylation is required for Ras membrane localization and activity regardless of activating mutation. ICMT inhibitor treatment, subcellular fractionation, Ras activation (pull-down) assay, downstream signaling by immunoblot, in vivo AML model Journal of Medicinal Chemistry High 31181882
2007 Chemical inhibition of ICMT with AGGC caused redistribution and aggregation of the ER chaperone GRP94, followed by GRP94 protein degradation; these effects were dependent on RhoA activity and caspase activation, and GRP94 depletion augmented ICMT-inhibition-induced endothelial cell apoptosis, linking ICMT/RhoA signaling to the unfolded protein response. ICMT inhibitor (AGGC) treatment, 2D-PAGE proteomics, immunofluorescence, overexpression of constitutively active RhoA, caspase inhibition American Journal of Respiratory Cell and Molecular Biology Medium 17347446
2014 miR-100 directly inhibits ICMT expression by binding to the ICMT 3'-UTR; ICMT knockdown phenocopied miR-100 overexpression in suppressing lamellipodia formation and MMP2 activation, and constitutively active Rac1(Q61L) rescued the anti-metastatic effect of miR-100, placing ICMT upstream of Rac1 in a signaling axis controlling HCC cell migration and invasion. Luciferase 3'-UTR reporter assay, gain- and loss-of-function studies, Rac1(Q61L) rescue, mouse orthotopic xenograft Oncotarget Medium 25361001
2019 Wild-type p53 represses ICMT transcription by binding to the ICMT promoter region between -209 and -14, as established by ICMT-luciferase reporters, promoter truncation analysis, and ChIP assays; in contrast, cancer-associated p53 mutants positively regulate ICMT expression through a distinct promoter region. ICMT-luciferase reporter assay, promoter truncation analysis, chromatin immunoprecipitation (ChIP) Journal of Biological Chemistry Medium 30655292
2022 PFKFB4 physically interacts with ICMT (shown by co-immunoprecipitation), promotes the ICMT/RAS interaction, controls RAS localization at the plasma membrane, and activates AKT signaling to enhance melanoma cell migration, revealing a glycolysis-independent function of PFKFB4 acting through ICMT. Co-immunoprecipitation, RAS localization imaging, AKT signaling assays, migration assays Life Science Alliance Medium 35914811
2024 ICMT overexpression enhanced invadopodia formation and promoted lung metastasis in vivo; ICMT overexpression promoted migration and invasion in vitro, acting through its substrates that regulate actin cytoskeleton dynamics. ICMT overexpression, invadopodia formation assay, in vivo lung metastasis model, migration and invasion assays Biochimie Medium 38301884
2011 Prenylcysteine-based ICMT inhibitors (analogs 1a and 1b) exhibit mixed-mode inhibition of human ICMT with the competitive component predominating; cellular treatment with inhibitor 1b alters subcellular localization of GFP-KRas and inhibits Ras activation and Erk phosphorylation in Jurkat cells. Kinetic inhibitor analysis (Ki determination), GFP-KRas localization microscopy, Ras activation assay, immunoblotting Bioorganic & Medicinal Chemistry Medium 22142613
2025 RAC1P29S requires C-terminal prenylation (and thus ICMT-mediated methylation) to drive MAPKi resistance in BRAF(V600E) melanoma; mechanistically, combined ICMT inhibition (cysmethynil) and MAPK pathway inhibition impaired nuclear translocation of TAZ, whose transcriptional activity accounts for MAPKi resistance in RAC1P29S cells, and constitutively active TAZ phenocopied RAC1P29S-driven resistance. RAC1P29S/C189S prenylation-site mutant, cysmethynil ICMT inhibitor, TAZ nuclear translocation assay, constitutively active TAZ rescue, in vitro and in vivo tumor models bioRxivpreprint Medium bio_10.1101_2025.08.10.669587

Source papers

Stage 0 corpus · 37 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2004 Inactivation of Icmt inhibits transformation by oncogenic K-Ras and B-Raf. The Journal of clinical investigation 133 14966563
2013 18F-ICMT-11, a caspase-3-specific PET tracer for apoptosis: biodistribution and radiation dosimetry. Journal of nuclear medicine : official publication, Society of Nuclear Medicine 74 23949910
2002 icmT is essential for pore formation-mediated egress of Legionella pneumophila from mammalian and protozoan cells. Infection and immunity 70 11748165
2011 Mechanism of isoprenylcysteine carboxyl methylation from the crystal structure of the integral membrane methyltransferase ICMT. Molecular cell 45 22195972
2014 Downregulation of microRNA-100 enhances the ICMT-Rac1 signaling and promotes metastasis of hepatocellular carcinoma cells. Oncotarget 39 25361001
2011 Discovery and SAR of methylated tetrahydropyranyl derivatives as inhibitors of isoprenylcysteine carboxyl methyltransferase (ICMT). Journal of medicinal chemistry 37 21661760
2019 A Potent Isoprenylcysteine Carboxylmethyltransferase (ICMT) Inhibitor Improves Survival in Ras-Driven Acute Myeloid Leukemia. Journal of medicinal chemistry 34 31181882
2018 Atomic structure of the eukaryotic intramembrane RAS methyltransferase ICMT. Nature 34 29342140
2018 Clinical translation of [18F]ICMT-11 for measuring chemotherapy-induced caspase 3/7 activation in breast and lung cancer. European journal of nuclear medicine and molecular imaging 32 30259091
2002 The C-terminus of IcmT is essential for pore formation and for intracellular trafficking of Legionella pneumophila within Acanthamoeba polyphaga. Molecular microbiology 31 11918802
2014 Preclinical assessment of carboplatin treatment efficacy in lung cancer by 18F-ICMT-11-positron emission tomography. PloS one 28 24618809
2004 Analysis of the kinetic mechanism of recombinant human isoprenylcysteine carboxylmethyltransferase (Icmt). BMC biochemistry 27 15625008
2013 Functionalized indoleamines as potent, drug-like inhibitors of isoprenylcysteine carboxyl methyltransferase (Icmt). European journal of medicinal chemistry 23 23514631
2021 A small-molecule ICMT inhibitor delays senescence of Hutchinson-Gilford progeria syndrome cells. eLife 21 33526168
2019 Isoprenylcysteine carboxy methyltransferase (ICMT) is associated with tumor aggressiveness and its expression is controlled by the p53 tumor suppressor. The Journal of biological chemistry 16 30655292
2019 ICMT contributes to hepatocellular carcinoma growth, survival, migration and chemoresistance via multiple oncogenic pathways. Biochemical and biophysical research communications 16 31451223
2007 Inhibition of ICMT induces endothelial cell apoptosis through GRP94. American journal of respiratory cell and molecular biology 15 17347446
2022 Licoricidin combats gastric cancer by targeting the ICMT/Ras pathway in vitro and in vivo. Frontiers in pharmacology 14 36339587
2011 Amide-modified prenylcysteine based Icmt inhibitors: Structure-activity relationships, kinetic analysis and cellular characterization. Bioorganic & medicinal chemistry 14 22142613
2005 Structure-function analysis of the C-terminus of IcmT of Legionella pneumophila in pore formation-mediated egress from macrophages. FEMS microbiology letters 12 15621435
2016 Icmt inhibition exerts anti-angiogenic and anti-hyperpermeability activities impeding malignant pleural effusion. Oncotarget 11 26959120
2022 CircRNA hsa_circ_0018289 exerts an oncogenic role in cervical cancer progression through miR-1294/ICMT axis. Journal of clinical laboratory analysis 10 35312113
2022 PFKFB4 interacts with ICMT and activates RAS/AKT signaling-dependent cell migration in melanoma. Life science alliance 10 35914811
2006 Genetic and pharmacologic analyses of the role of Icmt in Ras membrane association and function. Methods in enzymology 10 16757321
2014 Mutational analysis of the integral membrane methyltransferase isoprenylcysteine carboxyl methyltransferase (ICMT) reveals potential substrate binding sites. The Journal of biological chemistry 9 25059662
2021 NRAS is unique among RAS proteins in requiring ICMT for trafficking to the plasma membrane. Life science alliance 8 33579760
2021 Lidocaine exerts anticancer activity in bladder cancer by targeting isoprenylcysteine carboxylmethyltransferase (ICMT). Translational andrology and urology 8 34984187
2016 Deficiency of Isoprenylcysteine Carboxyl Methyltransferase (ICMT) Leads to Progressive Loss of Photoreceptor Function. The Journal of neuroscience : the official journal of the Society for Neuroscience 8 27147662
2020 Development of [18F]ICMT-11 for Imaging Caspase-3/7 Activity during Therapy-Induced Apoptosis. Cancers 7 32781531
2018 Depicting Changes in Tumor Biology in Response to Cetuximab Monotherapy or Combination Therapy by Apoptosis and Proliferation Imaging Using 18F-ICMT-11 and 18F-FLT PET. Journal of nuclear medicine : official publication, Society of Nuclear Medicine 6 29794225
2007 9 Structure and function of isoprenylcysteine carboxylmethyltransferase (Icmt): A key enzyme in CaaX processing. The Enzymes 6 26718043
2013 Mapping peptide thiol accessibility in membranes using a quaternary ammonium isotope-coded mass tag (ICMT). Bioconjugate chemistry 5 23725486
2018 Use of an Isotope-Coded Mass Tag (ICMT) Method To Determine the Orientation of Cholesterol Oxidase on Model Membranes. Biochemistry 3 30125103
2024 Isoprenylcysteine carboxyl methyltransferase (ICMT) promotes invadopodia formation and metastasis in cancer cells. Biochimie 2 38301884
2024 In silico Exploration of a Novel ICMT Inhibitor with More Solubility than Cysmethynil against Membrane Localization of KRAS Mutant in Colorectal Cancer. Current computer-aided drug design 1 38835128
2026 ICMT deficiency ameliorates weight loss and mortality, but not tumor formation in a mouse model of liver cancer. Cell communication and signaling : CCS 0 41808116
2026 Targeting ICMT: A promising strategy in cancer treatment (Review). Oncology letters 0 41877819