Cell electrophoresis of the cellular slime mould Dictyostelium discoideum. II. Relevance of the changes in cell surface charge density to cell aggregation and morphogenesis.

The reduction in the cell surface charge density of Dictyostelium discoideum during differentiation has been studied by the technique of cell electrophoresis. It was abolished under conditions in which cell aggregation was inhibited (e.g. low temperature, the presence of actinomycin D or cycloheximide). In the presence of nutrients incapable of supporting growth, cell aggregation occurred without a reduction in surface charge density. Cell adhesion in these aggregates was impaired, and a reduction in surface charge density appeared to be necessary for further development. Brief treatment of exponential phase and aggregating cells with agents which disaggregate slugs failed to alter their electrophoretic mobilities. Low concentrations of magnesium chloride caused extensive agglutination, especially in aggregating cell suspensions, but little change in their electrophoretic mobilities. Magnesium chloride could agglutinate cells by association with cell surface components undetectable by cell electrophoresis. This, together with immunological evidence from other workers, supports the possibility of involvement of specific surface macromolecules in cellular slime mould aggregation. It was concluded that changes in surface charge density, though important for cell adhesion and morphogenesis, cannot account for all aspects of cell interactions in D. discoideum.