Glycogen phosphorylase 'b' in Dictyostelium: stability and endogenous phosphorylation

The slime mold Dictyostelium discoideum has two forms of the enzyme glycogen phosphorylase. The inactive phosphorylase 'b' form requires 5' AMP for activity and is present in early development. The active phosphorylase 'a' form is 5' AMP independent and occurs during later development. We here show that the 92 kd 'b' enzyme subunit exists either as a singlet or a doublet upon SDS-PAGE, depending on the method of sample extraction. In the presence of exogenously added Mn2+ and ATP, the phosphorylase 'b' shows apparent conversion into a 5' AMP independent form as measured by enzyme activity. In addition, Mn2+ and ATP also support an in vitro phosphorylation of the 92 kd phosphorylase 'b' subunit. We also demonstrate phosphorylation of the 'b' enzyme subunit in vivo by 32-P incorporation into the enzyme protein. A protein kinase responsible for the observed in vitro phosphorylation of the phosphorylase 'b' subunit is characterized.