Plays an important role in the de novo pathway and in the salvage pathway of purine nucleotide biosynthesis. 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. Inhibited competitively by AMP and IMP and non-competitively by fructose 1,6-bisphosphate. Purine metabolism; AMP biosynthesis via de novo pathway; AMP from IMP: step 1/2. Homodimer. Belongs to the adenylosuccinate synthetase family.