Vol. 13, Issue 4, 1390-1407, April 2002

Morphology and Dynamics of the Endocytic Pathway in Dictyostelium discoideum

Eva M. Neuhaus,^* Wolfhard Almers,^* and Thierry Soldati^*§

 ^*Department of Molecular Cell Research, Max-Planck-Institute for Medical Research, D-69120 Heidelberg, Germany;  Vollum Institute, Portland, Oregon 97201; and  Department of Biological Sciences, Imperial College of Science Technology and Medicine, London SW7 2AZ, United Kingdom

Dictyostelium discoideum is a genetically and biochemically tractable social amoeba belonging to the crown group of eukaryotes. It performs some of the tasks characteristic of a leukocyte such as chemotactic motility, macropinocytosis, and phagocytosis that are not performed by other model organisms or are difficult to study. D. discoideum is becoming a popular system to study molecular mechanisms of endocytosis, but the morphological characterization of the organelles along this pathway and the comparison with equivalent and/or different organelles in animal cells and yeasts were lagging. Herein, we used a combination of evanescent wave microscopy and electron microscopy of rapidly frozen samples to visualize primary endocytic vesicles, vesicular-tubular structures of the early and late endo-lysosomal system, such as multivesicular bodies, and the specialized secretory lysosomes. In addition, we present biochemical and morphological evidence for the existence of a micropinocytic pathway, which contributes to the uptake of membrane along side macropinocytosis, which is the major fluid phase uptake process. This complex endosomal compartment underwent continuous cycles of tubulation/vesiculation as well as homo- and heterotypic fusions, in a way reminiscent of mechanisms and structures documented in leukocytes. Finally, egestion of fluid phase from the secretory lysosomes was directly observed.