C2CD2L

C2CD2L (TMEM24) is an ER-anchored lipid transport protein that operates at ER–plasma membrane (ER-PM) contact sites to sustain phosphoinositide homeostasis and calcium signaling, most prominently in β-cells and neurons (PMID:28209843, PMID:30819882). Its SMP domain transfers phosphatidylinositol between bilayers, replenishing PM PI(4,5)P2 consumed during signaling, while a polybasic C-terminal motif tethers the ER to the PM (PMID:28209843, PMID:39158698). Its residence at ER-PM junctions is dynamic: nanomolar elevations of cytosolic Ca2+ and diacylglycerol trigger C-terminal phosphorylation that redistributes TMEM24 away from the PM into the broader ER, coupling lipid delivery to the oscillatory state of the cell (PMID:28209843, PMID:30819882, PMID:34821358). Loss of TMEM24 abolishes Ca2+ oscillations and triggered insulin release, causing aberrant ER and mitochondrial Ca2+ accumulation and impaired glucose-stimulated ATP production (PMID:28209843, PMID:34821358). TMEM24 forms a complex with band 4.1 family proteins that enriches and stabilizes ER-PM junctions at cell-cell contacts, exempting these junctions from Ca2+-triggered shedding (PMID:39158698), and interacts with the microprotein alt-RPL36 through phosphoserine-dependent binding to restrain PM PI(4,5)P2 levels and PI3K-AKT-mTOR signaling and cell size (PMID:33479206).

1. 2013 Medium

   Before any mechanistic role was known, TMEM24 was placed in the β-cell insulin secretory machinery, establishing it as a regulator of glucose-stimulated insulin release from a reserve granule pool.

   Evidence Conformation-based pulldown plus mass spectrometry (insulin BIN proteomics) with glucose-stimulated insulin secretion assays

   PMID:24012759

   Open questions at the time

   • Did not define a molecular activity for TMEM24
   • Interaction network from a single lab without orthogonal validation
   • No link to lipid transport or Ca2+ signaling yet
2. 2017 High

   This work defined the core biochemical function — that TMEM24 is an ER-anchored SMP-domain lipid transfer protein at ER-PM contacts whose phosphorylation tracks Ca2+ oscillations, mechanistically explaining how it supports insulin release.

   Evidence In vitro reconstitution of lipid transport, mutagenesis, phosphorylation mapping, live-cell ER-PM contact imaging, and β-cell loss-of-function with Ca2+ and insulin readouts

   PMID:28209843

   Open questions at the time

   • Did not resolve the kinase/phosphatase pair driving the Ca2+-responsive cycle
   • Cargo specificity beyond PI not fully delineated
   • No structural model of the contact-site tether
3. 2019 High

   Extended the Ca2+-regulated localization model to neurons and showed paralog divergence, establishing that the density of conserved C-terminal phosphosites sets Ca2+ sensitivity and distinguishes TMEM24 from C2CD2.

   Evidence Live-cell fluorescence imaging of Ca2+-triggered ER redistribution, phosphosite mapping, and comparative analysis of TMEM24 vs C2CD2

   PMID:30819882

   Open questions at the time

   • Functional consequence of redistribution in neurons not defined
   • Identity of the responsible kinases unresolved
   • Physiological role of C2CD2's lower Ca2+ sensitivity unclear
4. 2021 Medium

   Connected TMEM24 loss to downstream organellar Ca2+ handling, showing it prevents ER Ca2+ overload and excess mitochondrial Ca2+ entry that otherwise impairs ATP production and oscillations.

   Evidence TMEM24 knockout β-cells with ER and mitochondrial Ca2+ measurements, ATP assays, and DAG/Ca2+-triggered dissociation imaging

   PMID:34821358

   Open questions at the time

   • Causal chain from PI(4,5)P2 replenishment to ER Ca2+ buffering not directly demonstrated
   • Single-lab knockout system
   • Whether DAG and Ca2+ act through the same phosphosites unresolved
5. 2021 Medium

   Identified a regulatory partner, the microprotein alt-RPL36, revealing that phosphoserine-dependent binding to TMEM24 tunes PM PI(4,5)P2 levels, PI3K-AKT-mTOR signaling, and cell size.

   Evidence Reciprocal Co-IP, phosphoserine point mutations, PI(4,5)P2 biosensor imaging, and PI3K-AKT-mTOR pathway and cell-size assays in knockout cells

   PMID:33479206

   Open questions at the time

   • How alt-RPL36 binding modulates TMEM24 transport activity mechanistically unknown
   • Single-lab finding
   • Relevance to β-cell physiology not tested
6. 2024 Medium

   Revealed spatial regulation of ER-PM junctions by showing TMEM24/C2CD2 complex with band 4.1 proteins enriches junctions at cell-cell contacts and protects them from Ca2+-triggered shedding.

   Evidence Proximity ligation, Co-IP, and live-cell imaging comparing Ca2+ dynamics at cell-contact vs non-contact junctions

   PMID:39158698

   Open questions at the time

   • Functional output of contact-localized junctions not defined
   • Role of SynCAM 1 link unclear
   • Single-lab evidence
7. 2025 Low

   Proposed a disease-relevant upstream regulator, with TLR4 binding TMEM24 and downregulating it to suppress PI3K/AKT signaling during β-cell lipotoxicity.

   Evidence Lipotoxic β-cell model, TLR4 knockout rats, TLR4-TMEM24 Co-IP, and PI3K/AKT and insulin secretion measurements

   PMID:40170616

   Open questions at the time

   • Single Co-IP without reciprocal validation for the TLR4-TMEM24 interaction
   • Mechanism of TMEM24 downregulation by TLR4 not defined
   • Direct binding versus indirect effect not distinguished

• The kinases and phosphatases that read cytosolic Ca2+ to drive TMEM24's reversible contact-site association, and how lipid transfer is mechanistically coupled to organellar Ca2+ buffering, remain unresolved.
• Identity of responsible kinase/phosphatase not established
• No structural model of the SMP tether at contacts
• Causal link between PI delivery and ER/mitochondrial Ca2+ handling not directly shown