INTERACTION BETWEEN HUMAN TRANSFER-RNA SYNTHETASES INVOLVES REPEATED SEQUENCE ELEMENTS

Aminoacyl-tRNA synthetases (tRNA synthetases) of higher eukaryotes for m a multiprotein complex. Sequence elements that are responsible for t he protein assembly were searched by using a Feast two-hybrid system. Human cytoplasmic isoleucyl-tRNA synthetase is a component of the mult i-tRNA synthetase complex and it contains a unique C-terminal appendix . This part of the protein was used as bait to identify an interacting protein from a HeLa cDNA library. The selected sequence represented t he internal 317 amino acids of human bifunctional (glutamyl- and proly l-) tRNA synthetase, which is also known to be a component of the comp lex. Both the C-terminal appendix of the isoleucyl-tRNA synthetase and the internal region of bifunctional tRNA synthetase comprise repeatin g sequence units, two repeats of about 90 amino acids, and three repea ts of 57 amino acids, respectively, Each repeated motif of the two pro teins was responsible for the interaction, but the stronger interactio n was shown by the native structures containing multiple motifs. Inter estingly, the N-terminal extension of human glycyl-tRNA synthetase con taining a single motif homologous to those in the bifunctional tRNA sy nthetase also interacted with the C-terminal motif of the isoleucyl-tR NA synthetase although the enzyme is not a component of the complex. T he data indicate that the multiplicity of the binding motif in the tRN A synthetases is necessary for enhancing the interaction strength and may be one of the determining factors for the tRNA synthetases to be i nvolved in the formation of the multi-tRNA synthetase complex.