Eight of the mammalian aminoacyl-tRNA synthetases associate as a multienzyme complex, whereas prokaryotic and low eukaryotic synthetases occur only as free soluble enzymes. Association of the synthetases may result in effective compartmentalization of synthetases and suggests the association of the entire protein biosynthetic machinery. To elucidate the structural elements and the nature of the molecular interactions involved in the association of the synthetases, we have cloned and sequenced the complementary DNA coding human aspartyl-tRNA synthetase. The full length cDNA encodes an open reading frame of 500 amino acids with 56% identity with yeast aspartyl-tRNA synthetase. The similarity with yeast aspartyl-tRNA synthetase is unevenly distributed with a high percent of identity at the C-terminus and relatively low identity at the N-terminus. The N-terminal sequence strongly prefers an alpha-helical secondary structure and shows amphiphilic characteristics. Further comparison with the yeast synthetases showed that the basic positively charged helixes in yeast synthetases are evolved to a neutral amphiphilic helix in this mammalian synthetase. The mammalian neutral amphiphilic helix is so far unique among all known sequences of bacterial, yeast, and mammalian synthetases and may account for the association of synthetases in the synthetase complex.