The induction of spore cells in vitro by membranes of Dictyostelium discoideum

A mutant of Dictyostelium discoideum, HM18, will differentiate into both stalk and spore cells when plated at high cell density (10⁵ cells/cm²) as a monolayer on non-nutrient agar containing 5 mM cAMP 6]. At low cell density (10³ cells/cm²) neither stalk nor spore cells are produced, but the addition of a cytosol fraction leads to stalk cell formation, and the addition of a membrane fraction leads to spore cell formation. The spore cell-inducing activity of the cell membranes is developmentally regulated; it is first detectable during late aggregation and increases to a maximum level in the pseudoplasmodial stage of development. The activity is sensitive to proteolysis and insensitive to periodate treatment. It is partially inactivated by incubation at 100 °C for 5 min. Variable amounts of the activity can be removed from the membrane by washing, suggesting that at least part of the activity is loosely membrane-bound. Activity is enriched in plasma membrane fractions, suggesting that the inducing factor is located at the cell surface. It is possible that the membranes are replacing a cell-cell contact requirement for spore formation.