Bifunctional aspartate kinase and homoserine dehydrogenase that catalyzes the first and the third steps toward the synthesis of the essential amino acids threonine, isoleucine, and methionine. L-homoserine + NADP(+) = H(+) + L-aspartate 4-semialdehyde + NADPH L-homoserine + NAD(+) = H(+) + L-aspartate 4-semialdehyde + NADH ATP + L-aspartate = 4-phospho-L-aspartate + ADP Inhibition of aspartate kinase activity by threonine and leucine and 3-fold activation by cysteine, isoleucine, valine, serine and alanine at 2.5 mM. Partial inhibition of homoserine dehydrogenase activity by threonine and cysteine (14% of activity remaining at saturation with either amino acid). No synergy between the effectors for both activation or inhibition. 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-methionine biosynthesis via de novo pathway; L-homoserine from L-aspartate: step 3/3. Amino-acid biosynthesis; L-threonine biosynthesis; L-threonine from L-aspartate: step 1/5. Amino-acid biosynthesis; L-threonine biosynthesis; L-threonine from L-aspartate: step 3/5. Homo- or heterodimer. In the N-terminal section; belongs to the aspartokinase family. In the C-terminal section; belongs to the homoserine dehydrogenase family.