Involved in the biosynthesis of L-aspartate-beta-semialdehyde which is a central intermediate in the biosynthesis of different amino acids (L-lysine, L-methionine, L-threonine). Catalyzes the phosphorylation of the beta-carboxyl group of L-aspartate to yield 4-phospho-L-aspartate. ATP + L-aspartate = 4-phospho-L-aspartate + ADP Activated by L-lysine, L-methionine, and L-isoleucine. L-threonine, at low concentrations, is a mild activator and has a weak inhibitory effect only at concentrations over 10 mM. Strongly feedback inhibited by the concerted combination of L-lysine and L-threonine and slightly feedback inhibited by the concerted combination of L-threonine and L-methionine. Activated by the combination of L-methionine and L-lysine, L-methionine and L-isoleucine and L-lysine and L-isoleucine. Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; (S)-tetrahydrodipicolinate from L-aspartate: step 1/4. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homoserine from L-aspartate: step 1/3. Amino-acid biosynthesis; L-threonine biosynthesis; L-threonine from L-aspartate: step 1/5. Homotrimer (Probable). In the presence of inhibitory amino acids the Stokes radius of the protein increases, suggesting its oligomeric state may change (PubMed:4357741). Belongs to the aspartokinase family.