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A 20-nm Step toward the Cell Membrane Preceding Exocytosis May Correspond to Docking of Tethered Granules

Erdem Karatekin ^*, Viet Samuel Tran ^*, Sébastien Huet ^*, Isabelle Fanget ^*, Sophie Cribier  and Jean-Pierre Henry ^*

^* Institut de Biologie Physico-Chimique, Centre National de la Recherche Scientifique UPR 1929, Université Paris 7 Denis Diderot, Paris, France; and Centre National de la Recherche Scientifique UMR 7099, Institut de Biologie Physico-Chimique and Université Pierre et Marie Curie, Paris, France

Correspondence: Address reprint requests to Erdem Karatekin, E-mail: erdem.karatekin{at}ibpc.fr; or to Jean-Pierre Henry, E-mail: jean-pierre.henry{at}univ-paris-diderot.fr.

In endocrine cells, plasma membrane (PM)-bound secretory granules must undergo a number of maturation stages (i.e., priming) to become fusion-competent. Despite identification of several molecules involved in binding granules to the PM and priming them, the exact nature of events occurring at the PM still largely remains a mystery. In stimulated BON cells, we used evanescent wave microscopy to study trajectories of granules shortly before their exocytoses, which provided a physical description of vesicle-PM interactions at an unprecedented level of detail, and directly lead to an original mechanistic model. In these cells, tethered (T), nonfusogenic, vesicles are prevented from converting to fusogenic, docked (D) ones in resting conditions. Upon elevation of calcium, T-vesicles perform a 21-nm step toward the PM to become D, and fuse 3 s thereafter. Our ability to directly visualize different modes of PM-attachment paves the way for clarifying the exact role of various molecules implicated in attachment and priming of granules in future studies.