Plays an important role in the de novo 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. Purine metabolism; AMP biosynthesis via de novo pathway; AMP from IMP: step 1/2. Homodimer. Belongs to the adenylosuccinate synthetase family.