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Direct evidence for extracellular adenosine 3':5'-monophosphate phosphodiesterase induction and phosphodiesterase inhibitor repression by exogenous adenosine 3':5'-monophosphate in Dictyostelium purpureum.

The phosphodiesterase inhibitor of Dictyostelium purpureum was purified from buffered suspensions of starved amoebae and identified on two-dimensional electrophoretic gels. The polypeptide associated with the inhibitor activity has an apparent molecular weight of 50000 and an isoelectric point of about pH 4.0. 
 
 
 
To determine whether or not exogenous adenosine 3′: 5′-monophosphate (cAMP) stimulates the synthesis de novo of one or both of the extracellular phosphodiesterases of D. purpureum [1], pre-aggregative cells were incubated with 14C-labelled protein hydrolysate in the presence and in the absence of 1 mM cAMP. The extracellular proteins of the labelled suspensions were then separated by two-dimensional gel electrophoresis and the relative amount of label incorporated into the polypeptides was analyzed by autoradiography. It was found that the polypeptides associated with the phosphodiesterase-2 activity were labelled in the cAMP-treated suspension, but not in the control. Moreover, the polypeptide associated with the phosphodiesterase-1 activity was not labelled under these conditions. These results suggest that exogenous cAMP induces the synthesis de novo of phosphodiesterase-2, but not phosphodiesterase-1, during the early development of D. purpureum. 
 
 
 
Under the same conditions, there was little or no incorporation of labelled amino acids into the polypeptide associated with the phosphodiesterase inhibitor activity either in the presence or in the absence of cAMP. However, when the extracellular proteins, which accumulated in the cell suspensions during the first 10 h of starvation, were separated on two-dimensional gels and then stained with Coomassie blue, the inhibitor polypeptide was found to be missing from the cAMP-treated suspension. These results suggest that exogenous cAMP either represses the synthesis of the phosphodiesterase inhibitor, or, perhaps, interfers with the processing of precursor inhibitor molecules.