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PLoS One. 2016 May 17;11(5):e0154875. doi: 10.1371/journal.pone.0154875. eCollection 2016.

Pycnosomes: Condensed Endosomal Structures Secreted by Dictyostelium Amoebae.

Sabra A1, Leiba J1, Mas L2,3,4, Louwagie M2,3,4, Couté Y2,3,4, Journet A2,3,4, Cosson P1, Aubry L2,3,4.

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Department of Cell Physiology and Metabolism, Faculty of Medicine, University of Geneva, 1 rue Michel Servet, CH-1211, Geneva 4, Switzerland.
Univ. Grenoble Alpes, F-38000 Grenoble, France.
CEA, BIG, Laboratoire Biologie à Grande Echelle, F-38054 Grenoble, France.
INSERM, U1038, F-38054 Grenoble, France.


Dictyostelium discoideum has been used largely as a model organism to study the organization and function of the endocytic pathway. Here we describe dense structures present in D. discoideum endocytic compartments, which we named pycnosomes. Pycnosomes are constitutively secreted in the extracellular medium, from which they can be recovered by differential centrifugation. We identified the most abundant protein present in secreted pycnosomes, that we designated SctA. SctA defines a new family of proteins with four members in D. discoideum, and homologous proteins in other protists and eumetazoa. We developed a monoclonal antibody specific for SctA and used it to further characterize secreted and intracellular pycnosomes. Within cells, immunofluorescence as well as electron microscopy identified pycnosomes as SctA-enriched dense structures in the lumen of endocytic compartments. Pycnosomes are occasionally seen in continuity with intra-endosomal membranes, particularly in U18666A-treated cells where intraluminal budding is highly enhanced. While the exact nature, origin and cellular function of pycnosomes remain to be established, this study provides a first description of these structures as well as a characterization of reagents that can be used for further studies.

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