title: Munc13-4 functions as a Ca2+sensor for homotypic secretory granule fusion to generate endosomal exocytic vacuoles.
abstract: Munc13-4 is a Ca2+-dependent SNARE (solubleN-ethylmaleimide-sensitive factor attachment protein receptor)- and phospholipid-binding protein that localizes to and primes secretory granules (SGs) for Ca2+-evoked secretion in various secretory cells. Studies in mast cell-like RBL-2H3 cells provide direct evidence that Munc13-4 with its two Ca2+-binding C2 domains functions as a Ca2+sensor for SG exocytosis. Unexpectedly, Ca2+stimulation also generated large (>2.4 μm in diameter) Munc13-4+/Rab7+/Rab11+endosomal vacuoles. Vacuole generation involved the homotypic fusion of Munc13-4+/Rab7+SGs, followed by a merge with Rab11+endosomes, and depended on Ca2+binding to Munc13-4. Munc13-4 promoted the Ca2+-stimulated fusion of VAMP8-containing liposomes with liposomes containing exocytic or endosomal Q-SNAREs and directly interacted with late endosomal SNARE complexes. Thus Munc13-4 is a tethering/priming factor and Ca2+sensor for both heterotypic SG-plasma membrane and homotypic SG-SG fusion. Total internal reflection fluorescence microscopy imaging revealed that vacuoles were exocytic and mediated secretion of β-hexosaminidase and cytokines accompanied by Munc13-4 diffusion onto the plasma membrane. The results provide new molecular insights into the mechanism of multigranular compound exocytosis commonly observed in various secretory cells.
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53706.
authors.forename: Sang Su
authors.initials: SS
authors.lastname: Woo
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53706.
authors.forename: Declan J
authors.initials: DJ
authors.lastname: James
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53706 tfmartin@wisc.edu.
authors.forename: Thomas F J
authors.initials: TF
authors.lastname: Martin
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D011992:Endosomes / Q000378:metabolism / Q000502:physiology; D005089:Exocytosis / Q000502:physiology; D008561:Membrane Fusion / Q000502:physiology; D021381:Protein Transport; D011506:Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D051381:Rats; D050600:SNARE Proteins / Q000378:metabolism; D022142:Secretory Vesicles / Q000502:physiology; D052067:Soluble N-Ethylmaleimide-Sensitive Factor Attachment Proteins / Q000378:metabolism; D014617:Vacuoles / Q000502:physiology
chemical_list: D002352:Carrier Proteins; D011506:Proteins; D050600:SNARE Proteins; D052067:Soluble N-Ethylmaleimide-Sensitive Factor Attachment Proteins; C430713:Unc13d protein, rat; D002118:Calcium