The relationship between the two forms of glycogen phosphorylase in Dictyostelium discoideum

The cellular slime mold, Dictyostelium disoideum, provides an ideal model system to study eukaryotic cell differentiation. In D. discoideum, glycogen degradation provides precursors for the synthesis of developmentally regulated structural products. The enzyme responsible for glycogen degradation, glycogen phosphorylase, exists in active and inactive forms. The active, or 'a' form, is independent of 5'adenosine monophosphate (5'AMP) while the inactive, or 'b' form, is 5'AMP-dependent. The activity of the 'b' form predominates early in development, while the activity of the 'a' form peaks in mid-late development; their combined specific activities remain constant at any point. Polyclonal antibodies raised to the purified forms of this enzyme showed low cross-reactivity. The anti-'a' serum reacted with a 104-kDa protein that was associated with phosphorylase 'a' activity; the anti-'b' serum reacted with a 92-kDa protein that was associated with phosphorylase 'b' activity and weakly cross-reacted with the 104-kDa protein. Immunoblots of peptide maps of the purified enzyme forms showed that each antibody was specific for the proteolytic fragments of its respective antigen. We also demonstrated in vitro phosphorylation of the 'b' form by an endogenous protein kinase. Cyclic AMP perturbation of intact cells caused induction of both phosphorylase-'a' activity and the 104-kDa protein. Immunotitration data suggested that the 'a' form accumulates due to de novo protein synthesis, although this result must be interpreted with caution.