MGAT4B

MGAT4B (GnT-IVb) is a Golgi-resident glycosyltransferase that catalyzes addition of a GlcNAcβ1-4 branch to the GlcNAcβ1-2Manα1-3 arm of N-glycan core structures, displaying lower donor and acceptor affinity than its isozyme MGAT4A (PMID:17006639). Genetic ablation studies established that MGAT4B and MGAT4A together account for all GnT-IV activity and the Manα1-3 arm GlcNAcβ1-4 branch, with MGAT4B broadly expressed across organs and capable of triggering compensatory MGAT4A upregulation in its absence (PMID:20015870). Its activity toward glycoprotein substrates depends on a C-terminal lectin domain that adopts a CBM32-like β-sandwich fold and binds β-N-acetylglucosamine (PMID:36106687); a single nonconserved residue within this domain dictates the distinct glycoprotein substrate preference of MGAT4B versus MGAT4A, biasing it toward N-glycans already modified by GnT-IV (PMID:35988645). This lectin domain also carries a self-ligand N-glycan whose interaction with the domain's binding site provides a glycan-structure-dependent self-regulatory mechanism that tunes activity toward glycoproteins (PMID:39669865). Within the Golgi membrane MGAT4B assembles into multi-enzyme and enzyme-transporter complexes with other MGATs, the α-mannosidase MAN2A2, and nucleotide-sugar transporters (PMID:30737517, PMID:25944901). In vivo, MGAT4B controls N-glycosylation of GPNMB, KIT, and TYRP1 and governs junctional plakoglobin (JUP) localization to direct melanocyte progenitor migration, stem cell pool establishment, and BRAFV600E melanoma initiation, a role not compensated by MGAT4A (PMID:40424122).

1. 2006 High

   Defined the core catalytic reaction of MGAT4B and showed it is a lower-affinity counterpart to MGAT4A, establishing that the two isozymes are kinetically distinct despite similar acceptor specificity.

   Evidence In vitro enzyme assays with recombinant flag-tagged enzymes and kinetic analysis across 14 PA-sugar chain acceptors

   PMID:17006639

   Open questions at the time

   • Did not address glycoprotein (versus free glycan) substrate selectivity
   • No structural basis for the affinity difference
2. 2009 High

   Genetic knockout established the physiological scope of MGAT4B activity and revealed a compensatory MGAT4A upregulation circuit, distinguishing the broad-tissue MGAT4B from gastrointestinal-restricted MGAT4A.

   Evidence GnT-IVb single and GnT-IVa/IVb double-deficient mice with MALDI-TOF/GC-MS glycomics and Ets-1 expression analysis

   PMID:20015870

   Open questions at the time

   • Did not identify specific endogenous glycoprotein substrates
   • Mechanism of Ets-1-driven compensation not dissected at the regulatory level
3. 2015 Medium

   Showed MGAT4B is physically organized with specific nucleotide-sugar transporters in the Golgi, implicating supramolecular assembly in channeling its activity, with selective proximity to UGT2 distinguishing it from other MGATs.

   Evidence In situ proximity ligation assay and FLIM-FRET at endogenous and overexpressed levels

   PMID:25944901

   Open questions at the time

   • Proximity does not establish functional substrate channeling
   • Stoichiometry and architecture of assemblies undefined
4. 2015 Medium

   Demonstrated by negative result that MGAT4B does not engage the MGAT1 inhibitor GnT1IP-L, separating MGAT4B from the GnT1IP-mediated regulatory axis acting on MGAT1.

   Evidence Dynamic FRET and BiFC assays in transfected cells

   PMID:26371870

   Open questions at the time

   • Negative interaction result from a single lab/method pair
   • Does not address other potential regulators of MGAT4B
5. 2019 Medium

   Mapped MGAT4B into a defined Golgi multi-enzyme network organized around MAN2A2 as a hub, placing it within the spatial logic of N-glycan branch processing.

   Evidence High-throughput FRET- and BiFC-based interaction screens in live cells

   PMID:30737517

   Open questions at the time

   • Functional consequence of each interaction for glycan output not measured
   • Single-lab screen without orthogonal biochemical isolation of complexes
6. 2020 Medium

   Excluded MGAT4B as an MGAT1 inhibitor despite sharing the PSLFQ motif of MGAT4D-L, establishing that sequence-motif sharing does not confer inhibitory function and reinforcing MGAT4B's dedicated catalytic role.

   Evidence MGAT4B transfection into CHO cells with GNA lectin binding assay for MGAT1 inhibition

   PMID:32763972

   Open questions at the time

   • Negative functional result; structural reason for lack of inhibition not resolved
   • Single method readout
7. 2022 High

   Identified the C-terminal lectin domain, and specifically a single nonconserved residue within it, as the determinant of the distinct glycoprotein substrate preference that separates MGAT4B from MGAT4A.

   Evidence UDP-Glo in vitro and cellular glycosylation assays with site-directed mutagenesis of the lectin domain

   PMID:35988645

   Open questions at the time

   • Structural mechanism by which the residue alters preference not solved
   • Full repertoire of preferred glycoprotein substrates not enumerated
8. 2022 Medium

   Provided the structural framework for the lectin domain as a CBM32-like β-sandwich that binds β-GlcNAc, with binding residues conserved across the GnT-IV family including MGAT4B.

   Evidence Crystal structures of human GnT-IVa CBML and Bombyx mori CBML with β-GlcNAc plus sugar-binding assays

   PMID:36106687

   Open questions at the time

   • MGAT4B lectin domain not directly crystallized; assignment is by conservation
   • No full-length enzyme structure
9. 2024 High

   Revealed a lectin-assisted self-regulatory mechanism in which the lectin domain's own N-glycan acts as a self-ligand to modulate activity toward glycoprotein substrates, linking the lectin domain's requirement for glycoprotein activity to autoregulation.

   Evidence UDP-Glo assays with glycan-remodeled enzymes plus domain deletion and glycan remodeling experiments

   PMID:39669865

   Open questions at the time

   • Self-regulation directly demonstrated for MGAT4A; extent to which it tunes MGAT4B in vivo not fully resolved
   • Physiological glycan states triggering autoregulation unknown
10. 2025 High

    Established the non-redundant in vivo role of MGAT4B in melanocyte biology, identifying its glycoprotein substrates (GPNMB, KIT, TYRP1) and the JUP-mislocalization mechanism underlying defective migration and melanoma initiation.

    Evidence Zebrafish mgat4b disruption with scRNA-seq, lectin-affinity proteomics, JUP confocal localization, and a BRAFV600E MAZERATI tumor model, with MGAT4A comparison

    PMID:40424122

    Open questions at the time

    • Causal glycosylation site(s) on JUP regulators not pinpointed
    • Conservation of the melanoma role in mammalian systems untested
11. 2026 Medium

    Showed MGAT4B is dispensable for ATRA-induced myeloid differentiation, in contrast to MGAT4A, demonstrating functional divergence of the isozymes in a hematopoietic context.

    Evidence CRISPR/Cas9 MGAT4B knockout in NB4 leukemia cells with flow cytometry for CD11b/CD11c

    PMID:42194104

    Open questions at the time

    • Negative result in a single cell line
    • Does not exclude MGAT4B roles in other myeloid or differentiation programs

• How the Golgi multi-enzyme/transporter assemblies, the lectin-domain self-regulation, and tissue-specific substrate selection are integrated to determine which glycoproteins MGAT4B modifies in a given cell remains unresolved.
• No structure of full-length MGAT4B or its complexes
• Mammalian in vivo substrate map beyond melanocyte proteins undefined
• Quantitative link between complex assembly and catalytic output not established