title: Fusion between phagosomes, early and late endosomes: a role for actin in fusion between late, but not early endocytic organelles.
abstract: Actin is implicated in membrane fusion, but the precise mechanisms remain unclear. We showed earlier that membrane organelles catalyze the de novo assembly of F-actin that then facilitates the fusion between latex bead phagosomes and a mixture of early and late endocytic organelles. Here, we correlated the polymerization and organization of F-actin with phagosome and endocytic organelle fusion processes in vitro by using biochemistry and light and electron microscopy. When membrane organelles and cytosol were incubated at 37 degrees C with ATP, cytosolic actin polymerized rapidly and became organized into bundles and networks adjacent to membrane organelles. By 30-min incubation, a gel-like state was formed with little further polymerization of actin thereafter. Also during this time, the bulk of in vitro fusion events occurred between phagosomes/endocytic organelles. The fusion between latex bead phagosomes and late endocytic organelles, or between late endocytic organelles themselves was facilitated by actin, but we failed to detect any effect of perturbing F-actin polymerization on early endosome fusion. Consistent with this, late endosomes, like phagosomes, could nucleate F-actin, whereas early endosomes could not. We propose that actin assembled by phagosomes or late endocytic organelles can provide tracks for fusion-partner organelles to move vectorially toward them, via membrane-bound myosins, to facilitate fusion.
authors: European Molecular Biology Laboratory, D-69117 Heidelberg, Germany. kjeken@bio.uio.no
authors: Rune
authors: R
authors: Kjeken
authors: Morten
authors: M
authors: Egeberg
authors: Anja
authors: A
authors: Habermann
authors: Mark
authors: M
authors: Kuehnel
authors: Pascale
authors: P
authors: Peyron
authors: Matthias
authors: M
authors: Floetenmeyer
authors: Paul
authors: P
authors: Walther
authors: Andrea
authors: A
authors: Jahraus
authors: Hélène
authors: H
authors: Defacque
authors: Sergei A
authors: SA
authors: Kuznetsov
authors: Gareth
authors: G
authors: Griffiths
mesh_terms: D000199:Actins / Q000378:metabolism*; D000255:Adenosine Triphosphate; D000818:Animals; D002478:Cells, Cultured; D020285:Cryoelectron Microscopy; D003599:Cytoskeleton / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004705:Endocytosis / Q000502:physiology; D011992:Endosomes / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism*; D008561:Membrane Fusion / Q000502:physiology*; D051379:Mice; D018613:Microscopy, Confocal; D008958:Models, Molecular; D015388:Organelles / Q000378:metabolism; D010588:Phagosomes / Q000378:metabolism*; D013947:Thymosin / Q000378:metabolism; D000099201:Thymosin beta(4)
chemical_list: D000199:Actins; D000255:Adenosine Triphosphate; D013947:Thymosin; D000099201:Thymosin beta(4)