Chapter 6 Endocytosis and Recognition Mechanisms in Dictyostelium discoideum

Abstract

Publisher Summary Endocytosis is the uptake of fluid (pinocytosis) or particles (phagocytosis) by a eukaryotic cell from the extracellular environment into the cytoplasm through plasma membrane-derived vesicles. Dictyostelium discoideum is particularly well suited as an experimental system to study mechanisms of phagocytosis and cell recognition. The major experimental problem in measuring initial rates of endocytosis is to separate cells containing ingested material from the bulk of noningested material. This problem is overcome by centrifuging the cell suspension through a column of highly viscous polyethylene glycol. Extracellular fluid and small particles with a high surface/volume ratio remain on top of the column, whereas the large phagocytes are found on the bottom. Fluorescently labeled dextrans are found to be suitable fluid-phase markers to measure pinocytosis. Analysis of the Phag mutant phenotype reveals the presence of at least two membrane receptors that recognize different features of a foreign surface. A “lectin-type” receptor recognizes glucose residues and a second nonspecific receptor recognizes the relative hydrophobicity of a surface, which may be a phagocytosable particle, a solid surface, or the surface of another amoeba. Mutants defective in the nonspecific receptor can be isolated using various selection procedures, and all of the mutants independently derived display a similar phenotype.