MVB12B

MVB12B is a dedicated subunit of the metazoan ESCRT-I complex, incorporating into the TSG101–VPS37 core through conserved C-terminal elements and contributing to membrane-associated cargo sorting and vesicle biogenesis (PMID:18005716). Its N-terminal MABP domain adopts a β-prism fold with a hydrophobic membrane-anchoring loop and an electropositive patch that binds acidic lipids in a charge-density-dependent manner, enabling autonomous membrane targeting (PMID:22232651). MVB12B is phosphorylated by TBK1 downstream of STING activation, which is essential for sorting bacterial DNA into extracellular vesicles during intracellular infection, linking ESCRT-I to innate immune signaling (PMID:30804548). MVB12B is also required for ESCRT-dependent unconventional secretion of CFTRΔF508 from the ER to the cell surface, and its perturbation impairs HIV-1 budding (PMID:29969945, PMID:18005716).

1. 2007 High

   The identity of the fourth ESCRT-I subunit class in metazoans was unknown; identification of MVB12A/MVB12B as stoichiometric ESCRT-I components established the heterotetrameric architecture and linked these subunits to HIV-1 budding.

   Evidence Reciprocal co-immunoprecipitation, hydrodynamic sedimentation, siRNA knockdown and overexpression with HIV infectivity readouts in human cells

   PMID:18005716

   Open questions at the time

   • Structural basis for MVB12B incorporation into the ESCRT-I core not resolved
   • Functional distinction between MVB12A and MVB12B paralogs unclear
   • Role in endogenous cargo sorting not tested
2. 2010 Medium

   How MVB12B stability is regulated post-translationally was unknown; EGF-stimulated ubiquitination at Lys264/Lys290, modulated by phosphorylation at Tyr241/Tyr243, was shown to destabilize MVB12B and promote inclusion body formation, establishing an EGFR-linked regulatory circuit.

   Evidence Mass spectrometry PTM mapping, site-directed mutagenesis, EGF stimulation in human cells

   PMID:20654576

   Open questions at the time

   • The E3 ubiquitin ligase responsible was not identified
   • Kinase phosphorylating Tyr241/Tyr243 not determined
   • Physiological consequence of MVB12B destabilization on cargo sorting not measured
3. 2012 High

   How MVB12B engages membranes was structurally undefined; the 1.3-Å crystal structure of the MABP domain revealed a β-prism fold with a hydrophobic anchoring loop and an electropositive patch, establishing a charge-density-dependent (not stereoselective) lipid-binding mechanism.

   Evidence X-ray crystallography at 1.3 Å, in vitro liposome-binding assays, fluorescence microscopy localization

   PMID:22232651

   Open questions at the time

   • Which specific endosomal phosphoinositide species are preferred in vivo not determined
   • Whether MABP-mediated membrane binding is required for ESCRT-I function in cells not tested
4. 2013 Medium

   Whether MVB12A/MVB12B and UBAP1 subunits are functionally interchangeable within ESCRT-I was unclear; depletion experiments showed that UBAP1, but not MVB12A/MVB12B, is required for ubiquitin-dependent EGF receptor sorting, revealing functional specialization among ESCRT-I fourth-subunit variants.

   Evidence siRNA knockdown of individual subunits with EGF receptor degradation as cargo-sorting readout

   PMID:24284069

   Open questions at the time

   • The specific cargo class sorted by MVB12B-containing ESCRT-I complexes remained unidentified
   • Whether MVB12A and MVB12B are truly redundant or have distinct cargo selectivities was not resolved
5. 2018 Medium

   Whether ESCRT-I participates in unconventional ER-to-plasma-membrane secretion was unknown; MVB12B was shown to be required for stress-induced, Golgi-bypassing surface trafficking of CFTRΔF508, with its overexpression partially rescuing chloride channel function.

   Evidence siRNA screen of ESCRT components, overexpression rescue, cell-surface expression and Cl⁻ channel functional assays

   PMID:29969945

   Open questions at the time

   • Mechanism by which MVB12B recognizes CFTRΔF508 as cargo not defined
   • Whether this pathway operates for wild-type CFTR or other misfolded clients not tested
6. 2019 High

   How innate immune signaling intersects with ESCRT-mediated vesicle biogenesis was undefined; TBK1-dependent phosphorylation of MVB12B was shown to be essential for packaging bacterial DNA into extracellular vesicles, which activate cGAS-STING in bystander cells and suppress T-cell proliferation.

   Evidence Phosphoproteomics, siRNA knockdown, EV isolation, bystander cGAS-STING activation, T-cell proliferation assays across three bacterial pathogens

   PMID:30804548

   Open questions at the time

   • Specific phosphorylation site(s) on MVB12B required for this function not mapped
   • Whether MVB12B directly binds DNA cargo or acts indirectly through adaptor proteins unknown
   • Relevance to viral DNA sorting into EVs not tested
7. 2025 Medium

   Whether MVB12B has tumor-suppressive activity was unexplored; overexpression in bladder cancer cell lines suppressed proliferation and migration, suggesting a growth-inhibitory role linked to ESCRT function.

   Evidence Overexpression in T24 and UM-UC-3 bladder cancer lines, proliferation and migration assays, GSEA pathway analysis

   PMID:41510117

   Open questions at the time

   • Mechanism by which MVB12B suppresses proliferation not elucidated
   • No in vivo tumor model used
   • Loss-of-function genetic evidence (knockout, mutations in tumors) not provided

• Key unresolved questions include: which specific cargo molecules are selectively sorted by MVB12B-containing versus MVB12A-containing ESCRT-I complexes; the identity of the TBK1 phosphorylation site(s) on MVB12B; whether the MABP domain is required for ESCRT-I function in vivo; and how EGF-regulated ubiquitination feeds back onto MVB12B's sorting activities.
• No cargo-selectivity model distinguishing MVB12B- from MVB12A-containing ESCRT-I
• No reconstitution of TBK1–MVB12B–DNA sorting in a minimal system
• No structural model of full-length MVB12B within the ESCRT-I heterotetramer