Calcium Activates Erythrocyte AMP Deaminase [Isoform E (AMPD3)] through a Protein−Protein Interaction between Calmodulin and the N-Terminal Domain of the AMPD3 Polypeptide

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Abstract
Erythrocyte AMP deaminase [isoform E (AMPD3)] is activated in response to increased intracellular calcium levels in Tarui's disease, following exposure of ionophore-treated cells to extracellular calcium, and by the addition of calcium to freshly prepared hemolysates. However, the assumption that Ca(2+) is a positive effector of isoform E is inconsistent with the loss of sensitivity to this divalent cation following dilution of erythrocyte lysates or enzyme purification. Ca(2+) regulation of isoform E was studied by examining in vitro effects of calmodulin (CaM) on this enzyme and by monitoring the influence of CaM antagonists on purine catabolic flow in freshly prepared erythrocytes under various conditions of energy imbalance. Erythrocyte and recombinant isoform E both adsorb to immobilized Ca(2+)-CaM, and relative adsorption across a series of N-truncated recombinant enzymes localizes CaM binding determinants to within residues 65-89 of the AMPD3 polypeptide. Ca(2+)-CaM directly stimulates isoform E catalytic activity through a K(mapp) effect and also antagonizes the protein-lipid interaction between this enzyme and intracellular membranes that inhibits catalytic activity. AMP is the predominant purine catabolite in erythrocytes deprived of glucose or exposed to A23187 ionophore alone, whereas IMP accumulates when Ca(2+) is included under the latter conditions and also during autoincubation at 37 degrees C. Preincubation with a CaM antagonist significantly slows the accumulation of erythrocyte IMP under both conditions. The combined results reveal a protein-protein interaction between Ca(2+)-CaM and isoform E and identify a mechanism that advances our understanding of erythrocyte purine metabolism. Ca(2+)-CaM overcomes potent isoform E inhibitory mechanisms that function to maintain the total adenine nucleotide pool in mature erythrocytes, which are unable to synthesize AMP from IMP because of a developmental loss of adenylosuccinate synthetase. This may also explain why Tarui's disease erythrocytes exhibit accelerated adenine nucleotide depletion in response to an increase in intracellular Ca(2+) concentration. This regulatory mechanism could also play an important role in purine metabolism in other human tissues and cells where the AMPD3 gene is expressed.