title: SM protein Munc18-2 facilitates transition of Syntaxin 11-mediated lipid mixing to complete fusion for T-lymphocyte cytotoxicity.
abstract: The atypical lipid-anchored Syntaxin 11 (STX11) and its binding partner, the Sec/Munc (SM) protein Munc18-2, facilitate cytolytic granule release by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Patients carrying mutations in these genes develop familial hemophagocytic lymphohistiocytosis, a primary immunodeficiency characterized by impaired lytic granule exocytosis. However, whether a SNARE such as STX11, which lacks a transmembrane domain, can support membrane fusion in vivo is uncertain, as is the precise role of Munc18-2 during lytic granule exocytosis. Here, using a reconstituted "flipped" cell-cell fusion assay, we show that lipid-anchored STX11 and its cognate SNARE proteins mainly support exchange of lipids but not cytoplasmic content between cells, resembling hemifusion. Strikingly, complete fusion is stimulated by addition of wild-type Munc18-2 to the assay, but not of Munc18-2 mutants with abnormal STX11 binding. Our data reveal that Munc18-2 is not just a chaperone of STX11 but also directly contributes to complete membrane merging by promoting SNARE complex assembly. These results further support the concept that SM proteins in general are part of the core fusion machinery. This fusion mechanism likely contributes to other cell-type-specific exocytic processes such as platelet secretion.
authors.affiliation: Division of Cell Pathology, Department of Pathology and Laboratory Medicine, University of Pennsylvania-Children's Hospital of Philadelphia, Philadelphia PA 19104.
authors.forename: Waldo A
authors.initials: WA
authors.lastname: Spessott
authors.affiliation: Division of Cell Pathology, Department of Pathology and Laboratory Medicine, University of Pennsylvania-Children's Hospital of Philadelphia, Philadelphia PA 19104.
authors.forename: Maria L
authors.initials: ML
authors.lastname: Sanmillan
authors.affiliation: Division of Cell Pathology, Department of Pathology and Laboratory Medicine, University of Pennsylvania-Children's Hospital of Philadelphia, Philadelphia PA 19104.
authors.forename: Margaret E
authors.initials: ME
authors.lastname: McCormick
authors.affiliation: Division of Cell Pathology, Department of Pathology and Laboratory Medicine, University of Pennsylvania-Children's Hospital of Philadelphia, Philadelphia PA 19104.|Biomedical Graduate Studies, University of Pennsylvania, Philadelphia, PA 19104.
authors.forename: Vineet V
authors.initials: VV
authors.lastname: Kulkarni
authors.affiliation: Division of Cell Pathology, Department of Pathology and Laboratory Medicine, University of Pennsylvania-Children's Hospital of Philadelphia, Philadelphia PA 19104; giraudoc@mail.med.upenn.edu.
authors.forename: Claudio G
authors.identifier: 0000-0002-9124-2229
authors.initials: CG
authors.lastname: Giraudo
mesh_terms: D016475:3T3 Cells; D000818:Animals; D016466:CHO Cells; D002352:Carrier Proteins / Q000378:metabolism; D003412:Cricetulus; D003602:Cytotoxicity, Immunologic; D006801:Humans; D007694:Killer Cells, Natural / Q000276:immunology / Q000378:metabolism; D008561:Membrane Fusion; D008563:Membrane Lipids / Q000378:metabolism*; D051379:Mice; D046912:Multiprotein Complexes / Q000378:metabolism; D051938:Munc18 Proteins / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D054730:Protein Interaction Domains and Motifs; D050765:Qa-SNARE Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D050600:SNARE Proteins / Q000378:metabolism; D013601:T-Lymphocytes / Q000276:immunology* / Q000378:metabolism*; D013602:T-Lymphocytes, Cytotoxic / Q000276:immunology / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D008563:Membrane Lipids; D046912:Multiprotein Complexes; D051938:Munc18 Proteins; D050765:Qa-SNARE Proteins; D050600:SNARE Proteins
keywords: CTL; Munc18-2; SM protein; SNARE; Syntaxin 11