{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nA large‐scale resequencing study of X‐linked synaptic genes in neurodevelopmental disorders provided an important context for exploring members of the transmembrane 4 L6 gene family, even though TM4SF20 was not directly implicated in these initial genetic analyses."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nSubsequent clinical genetic investigations identified a complex 4 kb deletion encompassing the penultimate exon of TM4SF20 in individuals predominantly from Southeast Asia. The deletion leads to a net loss of the exon, introducing a premature stop codon that yields a stable yet truncated protein. This aberrant form fails to target the plasma membrane and instead accumulates in the cytoplasm, a mislocalization linked to early childhood communication delays and prominent white matter hyperintensities—phenotypes that suggest a toxic mechanism associated with disrupted TM4SF20 function."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nAt the cellular and molecular level, TM4SF20 has been shown to modulate the proteolytic processing of CREB3L1, a membrane-bound transcription factor involved in inhibiting cell division and promoting collagen synthesis. Under basal conditions, TM4SF20 functions to inhibit CREB3L1 cleavage. However, upon exposure to ceramide, TM4SF20 undergoes a dramatic inversion of its membrane topology—a process termed regulated alternative translocation. This inversion exposes the N-terminal region of its first transmembrane helix, thereby stimulating CREB3L1 cleavage via a mechanism that initially requires the participation of TRAM2 but becomes independent of it once ceramide is present."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nFurther dissection of this mechanism has revealed that specific residues within the first transmembrane helix critically determine the efficiency of regulated alternative translocation. Mutational analyses have underscored the essential role of a conserved GXXXN motif—especially Asn-26—and the contribution of Pro-29, along with adjacent residues (such as Leu-25 and Val-17), in mediating the interactions necessary for TM4SF20’s topological inversion. These findings suggest that the proper display of these amino acids on the same helical face is vital for establishing correct interprotein contacts during translocation, thereby facilitating TM4SF20’s role as a dynamic regulator of membrane-associated signal processing."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "A Piton, J Gauthier, F F Hamdan, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Systematic resequencing of X-chromosome synaptic genes in autism spectrum disorder and schizophrenia."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Psychiatry (2011)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/mp.2010.54"}], "href": "https://doi.org/10.1038/mp.2010.54"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "20479760"}], "href": "https://pubmed.ncbi.nlm.nih.gov/20479760"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Wojciech Wiszniewski, Jill V Hunter, Neil A Hanchard, et al. "}, {"type": "b", "children": [{"type": "t", "text": "TM4SF20 ancestral deletion and susceptibility to a pediatric disorder of early language delay and cerebral white matter hyperintensities."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Am J Hum Genet (2013)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.ajhg.2013.05.027"}], "href": "https://doi.org/10.1016/j.ajhg.2013.05.027"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "23810381"}], "href": "https://pubmed.ncbi.nlm.nih.gov/23810381"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Qiuyue Chen, Bray Denard, Ching-En Lee, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Inverting the Topology of a Transmembrane Protein by Regulating the Translocation of the First Transmembrane Helix."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Cell (2016)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.molcel.2016.06.032"}], "href": "https://doi.org/10.1016/j.molcel.2016.06.032"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "27499293"}], "href": "https://pubmed.ncbi.nlm.nih.gov/27499293"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Jingcheng Wang, Lisa N Kinch, Bray Denard, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Identification of residues critical for topology inversion of the transmembrane protein TM4SF20 through regulated alternative translocation."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2019)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.RA119.007681"}], "href": "https://doi.org/10.1074/jbc.RA119.007681"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "30808712"}], "href": "https://pubmed.ncbi.nlm.nih.gov/30808712"}]}]}]}