{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nSERINC5 is a multipass transmembrane protein that serves as an intrinsic host restriction factor by impairing the infectivity of HIV‐1 particles. When expressed in virus‐producing cells, SERINC5 is normally present within the plasma membrane and becomes incorporated into nascent virions where it interferes with Env‐mediated membrane fusion, thereby blocking the formation of fusion pores and decreasing viral core delivery into target cells. In several studies, HIV‐1 Nef (and analogous proteins such as glycoGag) has been shown to counteract SERINC5 by preventing its virion incorporation through clathrin‐ and AP-2–dependent endocytic mechanisms, resulting in dramatic enhancement of viral infectivity. These findings underscore the fundamental role of SERINC5 in diminishing virus–cell fusion and highlight the potency of viral countermeasures in overcoming host restriction mechanisms."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "7"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nAt the molecular level, SERINC5’s antiviral activity is closely linked to its structure and membrane localization. Detailed analyses have revealed that the predominant antiviral isoform (SERINC5-001) is stabilized by a full complement of ten transmembrane domains and specific C-terminal sequences critical for its plasma membrane targeting. Structural work using cryo–electron microscopy has uncovered a novel fold with a lipid-binding groove, while mutagenesis—targeting features such as a conserved EDTEE motif and an N-linked glycosylation site at N294—demonstrated that modifications in these regions affect protein stability and sensitivity to Nef antagonism. In addition, studies have reported that upon incorporation into virions, SERINC5 perturbs the conformation of the HIV-1 envelope glycoprotein, disrupts Env cluster formation, and in some cases differentially inactivates “open” versus “closed” Env trimers. Complementary approaches including domain-mapping and the development of specific monoclonal antibodies have further advanced our understanding of both SERINC5’s intrinsic functions—including its putative role in serine incorporation for lipid biosynthesis—and the structural determinants that define its antiviral mechanisms, as exemplified by Env cytoplasmic tail truncation conferring resistance to SERINC5‐mediated inhibition."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "8", "end_ref": "17"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nViral counteraction of SERINC5 is multifaceted. Nef employs mechanisms that include homodimerization, recruitment of clathrin adaptor proteins, and modulation of endosomal trafficking to effectively downregulate SERINC5 from the cell surface, thereby diminishing its antiviral incorporation. In certain T-cell lines, however, knockout studies indicate that Nef’s enhancement of viral replication cannot be fully explained by its antagonism of SERINC5 alone, hinting at the existence of additional, as yet unidentified, antiviral factors. Moreover, emerging evidence points to broader roles for SERINC5 beyond retroviral restriction – for example, genetic associations link SERINC5 to neurobiological processes such as myelination, and immune-escape polymorphisms in Nef that affect SERINC5 counteraction have been correlated with host HLA-B allele–mediated control of HIV-1 infection. Innovative tools such as CRISPR/Cas9-mediated gene editing to generate knock-in reporters and novel monoclonal antibodies have further refined our insights into endogenous SERINC5 expression and its modulation—for instance, by interferon-α—in primary target cells. Collectively, these studies emphasize that the interplay between SERINC5 and viral antagonists like Nef is critical not only for determining viral infectivity and Env conformation but also for influencing overall viral fitness and immune control."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "18", "end_ref": "23"}]}, {"type": "t", "text": ""}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Yoshiko Usami, Yuanfei Wu, Heinrich G Göttlinger "}, {"type": "b", "children": [{"type": "t", "text": "SERINC3 and SERINC5 restrict HIV-1 infectivity and are counteracted by Nef."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Nature (2015)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/nature15400"}], "href": "https://doi.org/10.1038/nature15400"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "26416733"}], "href": "https://pubmed.ncbi.nlm.nih.gov/26416733"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Annachiara Rosa, Ajit Chande, Serena Ziglio, et al. 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