Component of the purine nucleotide cycle (PNC), which interconverts IMP and AMP to regulate the nucleotide levels in various tissues, and which contributes to glycolysis and ammoniagenesis. Catalyzes the first committed step in the biosynthesis of AMP from IMP. GTP + IMP + L-aspartate = GDP + 2 H(+) + N(6)-(1,2-dicarboxyethyl)-AMP + phosphate Binds 1 Mg(2+) ion per subunit. Purine metabolism; AMP biosynthesis via de novo pathway; AMP from IMP: step 1/2. Homodimer. Belongs to the adenylosuccinate synthetase family.