{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nMGAT4D, alternatively known as GnT1IP-L or GnT1IP-S, serves as a critical inhibitor of MGAT1—the N‐acetylglucosaminyltransferase that initiates the synthesis of hybrid and complex N‐glycans. Studies in mouse and cultured cell models have demonstrated that the inhibitory activity of MGAT4D resides within its luminal domain, which is effective when localized in the Golgi. In contrast, retention of MGAT4D in the endoplasmic reticulum (ER) reduces its inhibition of MGAT1. Furthermore, dynamic FRET and BiFC experiments revealed that MGAT4D forms homomeric complexes in the ER while establishing heteromeric interactions with MGAT1 in the Golgi; notably, this inhibitory interaction appears to be specific as it does not extend to other medial Golgi N‐glycan branching enzymes such as MGAT2, MGAT3, MGAT4B, or MGAT5. Expression patterns also suggest a physiological role in spermatogenesis, since MGAT4D transcripts are predominantly found in spermatocytes and spermatids and are diminished in the testes of men with impaired spermatogenesis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn addition to the above findings, further investigations using transfected mammalian and Drosophila cells have reinforced the role of the long isoform, MGAT4D-L, as an inhibitor of MGAT1. Expression of MGAT4D-L leads to the accumulation of the MGAT1 substrate (Man5GlcNAc2Asn) on glycoproteins, correlating with a reduction of complex N-glycans, as detected by lectin binding assays. Detailed structure–function analyses have identified a critical 25-amino-acid sequence at the C-terminus, and within this region the specific PSLFQ motif is essential for inhibitory activity. Although mutants lacking this motif still interact with MGAT1, they lose the capacity to inhibit its activity, highlighting the motif’s potential as a target for the development of small molecule inhibitors of MGAT1."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Hung-Hsiang Huang, Antti Hassinen, Subha Sundaram, et al. "}, {"type": "b", "children": [{"type": "t", "text": "GnT1IP-L specifically inhibits MGAT1 in the Golgi via its luminal domain."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Elife (2015)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.7554/eLife.08916"}], "href": "https://doi.org/10.7554/eLife.08916"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "26371870"}], "href": "https://pubmed.ncbi.nlm.nih.gov/26371870"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Ayodele Akintayo, Joshua Mayoral, Masahiro Asada, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Point mutations that inactivate MGAT4D-L, an inhibitor of MGAT1 and complex "}, {"type": "a", "children": [{"type": "t", "text": "i"}], "href": "i"}, {"type": "t", "text": "N"}, {"type": "a", "children": [{"type": "t", "text": "/i"}], "href": "/i"}, {"type": "t", "text": "-glycan synthesis."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2020)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.RA120.014784"}], "href": "https://doi.org/10.1074/jbc.RA120.014784"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "32763972"}], "href": "https://pubmed.ncbi.nlm.nih.gov/32763972"}]}]}]}