Primer dependency of glycogen synthetase during differentiation in Dictyostelium discoideum

The cellular slime mold, Dictyostelium discoideum, has a life cycle in which the limited number of cell types and easily recognizable stages of development offer a unique model to relate biochemical events to differentiation. Ultramicrochemical techniques were employed to assay enzyme activity and product levels in cell samples as small as 0.02 mug of dry weight in reaction volumes of 0.1 mug. The techniques utilized an amplification procedure employing the enzymatic cycling of pyridine nucleotides. Glycogen synthetase (glucose 6-phophate independent form) was assayed in individual organisms over the time course of development. From aggregation to culmination, activity decreased and was dependent on soluble glycogen primer. From culmination to sorocarp stage, enzyme activity was independent of soluble glycogen primer. Further, the enzyme and its glycogen product were recovered in a low-spin (2000g) pellet fraction from sorocarp homogenates. The change in primer requirements and solubility of enzyme and product occurred during culmination. Localization studies in developing spore cells revealed trends in enzyme activities and solubilities of enzyme and product similar to those in whole organisms. Possible models of cell-specific biochemical events in D. discoideum are discussed.