LLGL2

LLGL2 is a scaffolding polarity protein that links cell-surface nutrient transport, cytoskeletal organization, and epithelial polarity to the control of proliferation and invasion in epithelial tissues (PMID:30996345, PMID:21606200). As a polarity determinant, murine Llgl2 is required for trophoblast cell polarization and polarized invasion during placental branching morphogenesis (PMID:21606200). In ER+ breast cancer it assembles into a trimeric complex with the leucine transporter SLC7A5 and the membrane-fusion regulator YKT6 to raise SLC7A5 cell-surface levels and leucine uptake, sustaining proliferation under nutrient stress, and its expression is driven by oestrogen receptor signaling (PMID:30996345). LLGL2 expression is itself controlled at the promoter level by Sp-1-dependent basal transcription and by Snail, which binds E-boxes to repress LLGL2; reciprocally, restored LLGL2 reduces Snail nuclear localization and reverses EMT (PMID:22580609, PMID:18155665). LLGL2 protein is targeted for proteasomal degradation by the E3 ligase MDM2, and when present it engages the CCR4-NOT subunit CNOT1 to stabilize THBS3 mRNA and modulate PI3K-Akt signaling (PMID:40619612). Through interaction with ACTN1 it redirects alpha-actinin localization to impair actin filament bundling and cytoskeletal-remodeling-dependent invasion (PMID:38136424), and it restrains autophagosome formation via Vps34/ATG14L, an axis that in turn shapes EMT and proliferation (PMID:36009528, PMID:38720397).

1. 2011 High

   Establishing whether mammalian Llgl2 has a non-redundant role in vivo, the knockout defined it as a polarity factor required for directed cell behavior during development.

   Evidence Llgl2-knockout mice with histological and cell-biological analysis of placental trophoblasts

   PMID:21606200

   Open questions at the time

   • Molecular partners mediating polarized trophoblast invasion not defined
   • Did not address LLGL2 role outside placenta
2. 2007 Medium

   Addressing how LLGL2 transcription is set, promoter dissection identified Sp-1 as a driver of basal expression and EGF signaling as a repressive input.

   Evidence Luciferase reporter truncations, mithramycin A (Sp-1 inhibition), and EGF treatment of primary hepatocytes

   PMID:18155665

   Open questions at the time

   • EGF-to-promoter signaling intermediates unmapped
   • No protein-level consequence demonstrated
3. 2012 High

   To connect LLGL2 to EMT control, work showed a bidirectional antagonism with Snail at the transcriptional level.

   Evidence Promoter reporter assays, E-box deletion mutants, ChIP, nuclear localization analysis, and gain-of-function rescue

   PMID:22580609

   Open questions at the time

   • Mechanism by which LLGL2 reduces Snail nuclear localization unresolved
   • No direct LLGL2-Snail physical interaction shown
4. 2019 High

   Defining a concrete molecular function, LLGL2 was shown to scaffold a transporter complex coupling nutrient uptake to proliferation, downstream of ER.

   Evidence Reciprocal Co-IP identifying an SLC7A5-YKT6-LLGL2 trimer, surface and uptake assays, and ER-modulation expression analysis in ER+ breast cancer cells

   PMID:30996345

   Open questions at the time

   • Structural basis of trimer assembly unknown
   • Whether YKT6/SLC7A5 partnership operates outside breast cancer untested
5. 2021 Low

   Testing a downstream signaling role, LLGL2 was linked to calcium influx and PI3K/AKT activation in hepatocellular carcinoma.

   Evidence In vitro functional assays, calcium influx measurements, and in vivo tumor models

   PMID:34178676

   Open questions at the time

   • Mechanistic link between LLGL2 and calcium influx not defined
   • Single lab, limited mechanistic detail
6. 2022 Medium

   Probing how LLGL2 promotes proliferation, gain/loss-of-function placed it as a suppressor of autophagosome formation in prostate cells.

   Evidence siRNA knockdown, overexpression, and Western blot for autophagy markers with proliferation assays

   PMID:36009528

   Open questions at the time

   • Direct molecular target on the autophagy machinery not identified
   • Single context
7. 2023 Medium

   Identifying a cytoskeletal effector, IP-MS revealed ACTN1 as a partner whose localization LLGL2 redirects to suppress invasion.

   Evidence IP-MS, localization analysis, in vitro invasion/migration assays, and an in vivo metastasis model in ovarian cancer cells

   PMID:38136424

   Open questions at the time

   • Binding interface and stoichiometry undefined
   • How LLGL2 alters ACTN1 localization mechanistically unknown
8. 2023 Medium

   Assessing tumor-suppressive polarity function in vivo, combined Llgl1/2 loss was shown to activate aPKC and NF-κB and cooperate with p53 loss in driving squamous cell carcinoma.

   Evidence Conditional double-knockout mice with Trp53 epistasis and signaling analysis (preprint)

   PMID:36945368

   Open questions at the time

   • Cannot resolve LLGL2-specific contribution from LLGL1
   • Preprint, not peer-reviewed
9. 2024 Medium

   Extending the autophagy link, LLGL2 knockdown was shown to induce Vps34/ATG14L-dependent autophagy flux that suppresses EMT.

   Evidence siRNA knockdown, rapamycin/3-MA pharmacological epistasis, and shLLGL2 xenografts in prostate cancer cells

   PMID:38720397

   Open questions at the time

   • Whether LLGL2 acts directly on Vps34/ATG14L untested
   • Connection to the breast-cancer transporter function unexplored
10. 2024 Low

    Two studies placed LLGL2 upstream of additional invasion/signaling outputs (occludin regulation; Hedgehog pathway activation).

    Evidence siRNA knockdown with invasion assays (OSCC) and overexpression/knockdown with JK184 pharmacological epistasis and xenografts (endometrial cancer)

    PMID:39394821 PMID:39647764

    Open questions at the time

    • No direct molecular interactions established for either axis
    • Pathway placements rest on expression changes alone
11. 2025 Medium

    Defining upstream and downstream regulation of LLGL2 levels, MDM2 was identified as its E3 ligase and CNOT1-mediated THBS3 mRNA stabilization as a downstream effector tied to PI3K-Akt.

    Evidence RIP-seq, shotgun MS (CNOT1), RNA-seq, and proteasome inhibition in colorectal cancer

    PMID:40619612

    Open questions at the time

    • MDM2 ubiquitination site on LLGL2 not mapped
    • Whether LLGL2 directly binds THBS3 mRNA versus via CNOT1 unresolved

• How LLGL2's distinct activities — transporter scaffolding, ACTN1-cytoskeletal control, autophagy suppression, and mRNA stabilization — are coordinated within a single polarity-dependent program remains unresolved.
• No unifying biochemical mechanism integrating the multiple reported partners
• Context-dependence of tumor-suppressive versus tumor-promoting roles unexplained
• No structural model of LLGL2 in any complex