Catalyzes the conversion of gamma-aminobutyrate and L-beta-aminoisobutyrate to succinate semialdehyde and methylmalonate semialdehyde, respectively. Can also convert delta-aminovalerate and beta-alanine. 2-oxoglutarate + 4-aminobutanoate = L-glutamate + succinate semialdehyde (S)-3-amino-2-methylpropanoate + 2-oxoglutarate = 2-methyl-3-oxopropanoate + L-glutamate Binds 1 [2Fe-2S] cluster per homodimer. Homodimer; disulfide-linked. Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family.