GIMAP2

GIMAP2 is a TRAFAC-class guanine nucleotide-binding protein that functions as a GTP-dependent, self-assembling scaffold on intracellular membranes (PMID:21059949). Crystal structures across nucleotide states establish that nucleotide-free and GDP-bound GIMAP2 is monomeric, with a distinctive amphipathic helix α7 packed against switch II, and that GTP binding drives assembly into linear oligomers through two distinct G-domain interfaces: a switch I-stabilized dimerization interface spanning the nucleotide-binding site and a second interface exposed when switch II rearrangement releases α7 (PMID:21059949). These GTP-loaded oligomers act as scaffolds at the surface of lipid droplets and the endoplasmic reticulum (PMID:21059949, PMID:32306002). GIMAP2's own GTPase activity is intrinsically weak and is stimulated in trans by heterodimerization with GIMAP7, which supplies a catalytic arginine, establishing that GTP turnover in the GIMAP family is governed by homo- and heterodimerization (PMID:23454188). Beyond this nucleotide-cycling and scaffolding mechanism, the downstream cellular consequences of GIMAP2 activity remain largely uncharacterized in the available corpus.

1. 2010 High

   Resolved how nucleotide state controls GIMAP2 conformation and self-assembly, defining it as a GTP-switch scaffold rather than a conventional signaling GTPase.

   Evidence X-ray crystallography of GIMAP2 in nucleotide-free, GDP-, and GTP-bound states with mutagenesis of oligomerization interfaces

   PMID:21059949

   Open questions at the time

   • Does not identify what cargo or effectors the oligomeric scaffold recruits
   • Intrinsic GTPase rate and the trigger for nucleotide exchange in cells not defined
2. 2010 Medium

   Placed the GTP-dependent oligomer in a cellular context, showing GIMAP2 assembles on lipid droplet surfaces in T cells.

   Evidence Cellular localization and biochemical fractionation in Jurkat T cells linked to structural data

   PMID:21059949

   Open questions at the time

   • Functional consequence of lipid droplet scaffolding not established
   • Single cell type; generality across tissues untested
3. 2013 High

   Explained how GIMAP2 GTPase activity is regulated, showing GIMAP7 acts as a trans-activating partner that supplies a catalytic arginine.

   Evidence Crystal structure of GTP-bound GIMAP7 homodimer plus in vitro GTP hydrolysis assays of GIMAP7-stimulated GIMAP2 turnover

   PMID:23454188

   Open questions at the time

   • Whether GIMAP2-GIMAP7 heterodimers form at physiological levels in cells not shown
   • Upstream signals controlling heterodimer formation unknown
4. 2020 Medium

   Confirmed GIMAP2 membrane localization in a non-lymphoid (breast cancer) cell line, extending the scaffold model beyond T cells.

   Evidence Super-resolution immunofluorescence of endogenous and mCherry-tagged GIMAP2 co-stained with ER and lipid droplet markers in MDA-MB-436 cells

   PMID:32306002

   Open questions at the time

   • Co-localization does not establish functional role at the ER
   • No interaction partners identified in this system
5. 2021 Low

   First attempt to connect GIMAP2 to a cellular phenotype, linking its loss to cell cycle arrest and apoptotic pathway changes in cancer cells.

   Evidence siRNA knockdown in oral squamous cell carcinoma lines with immunoblotting of cell cycle and apoptosis markers

   PMID:34992682

   Open questions at the time

   • Correlative knockdown without rescue; off-target effects not excluded
   • No mechanistic link between the scaffolding/GTPase activity and the observed CDK4/6, p53/p21, or Bcl-2/Bax changes
   • Effects observed in a single tumor context

• The effectors and physiological substrates recruited by the GIMAP2 GTP-dependent oligomeric scaffold, and how its membrane scaffolding connects to cell cycle or apoptotic outcomes, remain unknown.
• No identified downstream effector bound by the oligomer
• No mechanistic bridge between GTPase cycling and the cancer-cell phenotypes
• Physiological function in T cells not directly tested