Plays an important role in the salvage pathway for purine nucleotide biosynthesis. Catalyzes the first committed step in the biosynthesis of AMP from IMP (By similarity). 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. Parasitic protozoa lack the de novo purine biosynthesis pathway and rely exclusively on the salvage pathway for their purine nucleotide requirements. Belongs to the adenylosuccinate synthetase family.