A developmentally controlled change in the post-translational modifications on the lysosomal alpha-mannosidase of the cellular slime mould Dictyostelium discoideum

Developing Dictyostelium cells contain the lysosomal enzyme alpha-mannosidase in a secretory granule

The prespore vesicle (PSV) is an organelle which secretes spore coat proteins and gal/galNAc polysaccharides from prespore cells of Dictyostelium. By combining the techniques of protein A-gold immunocytochemistry and ricin-gold affinity cytochemistry we have demonstrated colocalization of the lysosomal enzyme alpha-mannosidase with gal/galNAc polysaccharides in prespore vesicles and the spore coat. To determine the origin of prespore vesicles a series of pulse-chase experiments were performed. Cells were labeled with [35S]methionine or [35S]sulfate at different times during development and allowed to differentiate in the presence of unlabeled methionine or sulfate for various periods of time. The cells were homogenized and intracellular organelles were separated using Percoll density gradient centrifugation. The distribution of [35S]methionine-labeled alpha-mannosidase and [35S]sulfate-labeled glycoproteins in the Percoll gradients was determined. It was found that prespore vesicles contained protein which was previously found in lysosomes. Newly labeled protein also entered these vesicles. The data suggest that developing Dictyostelium cells either restructure preexisting lysosomes into prespore vesicles or transport protein between these two organelles. We propose that secretory granules and lysosomes may have a common biosynthetic origin and may be evolutionarily related.

Developing Dictyostelium discoideum cells contain two distinct acid hydrolase-containing vesicles

Two distinct populations of acid hydrolase-containing vesicles have been found in developing Dictyostelium discoideum cells. Percoll gradient centrifugation revealed these vesicle populations have densities of 1.07 and 1.13 g/ml. The 1.13 g/ml vesicle populations arose during the aggregation stage of differentiation. Although both vesicle populations contained an array of acid hydrolases, they could be shown to differ by several criteria. Electron micrographs of prespore cells showed they contained two types of vesicles with distinct acid phosphatase-staining patterns. One of these vesicle types appeared identical to the lysosomes found in vegetative cells. The second vesicle type had a morphology similar to that of a previously identified organelle, the prespore vesicle. The prespore vesicle is known to contain spore coat proteins which are exocytosed during the final stages of spore differentiation. The higher density acid hydrolase-containing vesicle population was found to contain spore coat proteins. Electron micrographs of the higher density vesicle population showed the presence of acid phosphatase-staining vesicles with a morphology similar to that of prespore vesicles. These data suggest that the higher density acid hydrolase-containing vesicles represent a subpopulation of lysosomes which appear during development and which may be identical to prespore vesicles.

A bacterial factor induces changes in cysteine proteinase forms in the cellular slime mould Dictyostelium discoideum

The electrophoretic pattern of cysteine proteinases in axenically grown myxamoebae of Dictyostelium discoideum can be altered by the addition of either Gram-negative (Klebsiella aerogenes, Escherichia coli) or Gram-positive (Micrococcus lysodeikticus, Bacillus subtilis) bacteria to the culture. No changes occurred, however, if either yeast or latex beads were used in place of bacteria. The changes involved the simultaneous loss of proteinases characteristic of the axenic cells (the A-forms) and the acquisition of those found in cells which have been grown on bacteria (the B-forms). Using K. aerogenes the conversion was complete within 4 h. Extracellular proteinase activity was unaffected during this period. After the D. discoideum cells had been lysed, no equivalent change in proteinase band pattern could be produced either by prolonged incubation of cell extracts or by treatment with proteinases. An identical conversion could be induced in cultures of myxamoebae by a factor, cysteine proteinase converting factor (CPCF), present in the 15,000 g supernatant of a sonicated suspension of K. aerogenes. CPCF was macromolecular, as demonstrated by both ultrafiltration and gel filtration, acid-precipitable, but was soluble in ethanol or alkali. Its activity was unaffected by treatment with trypsin. The results suggested that CPCF might be a component of the bacterial cell wall, and since its activity was affected by lysozyme treatment, peptidoglycan is implicated. The results can be interpreted in terms of a novel nutrient-dependent post-translational change which affected most of the cysteine proteinases present in D. discoideum myxamoebae.