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 (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. This protein may be expected to contain an N-terminal transit peptide but none has been predicted. Belongs to the adenylosuccinate synthetase family.