Catalyzes reversively the conversion of L-aspartate beta-semialdehyde (ASA) to L-2,4-diaminobutyrate (DABA) by transamination with L-glutamate. Seems to use L-glutamate specifically as the amino group donor to ASA, as it is not active with L-alanine, L-glutamine, L-aspartate and L-lysine, and is only poorly active with L-homoserine. In the reverse reaction, gamma-aminobutyric acid (GABA) and L-ornithine can also be used as amino group donors to 2-oxoglutarate, but with a reduced activity compared to that with DABA. 2-oxoglutarate + L-2,4-diaminobutanoate = L-aspartate 4-semialdehyde + L-glutamate Optimum pH is 8.6-8.7. Optimum temperature is 25 degrees Celsius. Amine and polyamine biosynthesis; ectoine biosynthesis; L-ectoine from L-aspartate 4-semialdehyde: step 1/3. Homohexamer. Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family.