Immunoelectron microscopy of the intracellular compartments of prespore cells of dictyostelium discoideum

Abstract

In the amoeboid cellular slime molds two cell types are formed during differentiation, prestalk cells and prespore cells which ultimately become the stalk and spore cells of the mature sorocarp. The prespore cells can be identified ultrastructurally by the prespore vesicles (PSV), which have a dense layer lining the inside of the compartment (Fig. 1). The PSV's are known to contain several glycoproteins and a galactose polysaccharide that are released to the cell surface to become major components of the spore coat. Further investigations in this laboratory, by three dimensional reconstruction from serial thin sections and by confocal fluorescence microscopy have shown that the PSV's are not a collection of separate vesicles, but rather a tubular reticulum asymmetrically disposed in the cell. Similar geometry has been shown for some lysosomal systems. A recent report has shown that the lysosomal enzyme, α-mannosidase, can be localized to the PSV. These pieces of evidence suggest a possible lysosomal function or lysosomal origin for the prespore secretory system. In an effort to better characterize the PSV compartment we have chosen two monoclonal antibodies, 5G7 which recognizes a common lysosomal epitope and 81.2 which recognizes a carbohydrate epitope on the plasma membrane and on compartments of phagocytotic and autophagic origin. We further chose to characterize the distribution of acidic compartments using the DAMP method of Anderson, which would give us an indication of functional lysosomal activity.