Role of Arc1p in the Modulation of Yeast Glutamyl-tRNA Synthetase Activity

Yeast methionyl-tRNA synthetase (MetRS) and glutamyl-tRNA synthetase (GluRS) possess N-terminal extensions that bind the cofactor Arc1p in trans. The strength of GluRS−Arc1p interaction is high enough to allow copurification of the two macromolecules in a 1:1 ratio, in contrast to MetRS. Deletion analysis from the C-terminal end of the GluRS appendix combined with previous N-terminal deletions of GluRS allows restriction of the Arc1p binding site to the 110−170 amino acid region of GluRS. This region has been shown to correspond to a novel protein−protein interaction domain present in both GluRS and Arc1p but not in MetRS [Galani, K., Grosshans, H., Deinert, K., Hurt, E. C., and Simos, G. (2001) EMBO J. 20, 6889−6898]. The GluRS apoenzyme fails to show significant kinetics of tRNA aminoacylation and charges unfractionated yeast tRNA at a level 10-fold reduced compared to Arc1p-bound GluRS. The K m values for tRNAGlu measured in the ATP−PPi exchange were similar for the two forms of GluRS, whereas k cat is increased 2-fold in the presence of Arc1p. Band-shift analysis revealed a 100-fold increase in tRNA binding affinity when Arc1p is bound to GluRS. This increase requires the RNA binding properties of the full-length Arc1p since Arc1p N domain leaves the K d of GluRS for tRNA unchanged. Transcripts of yeast tRNAGlu were poor substrates for measuring tRNA aminoacylation and could not be used to clarify whether Arc1p has a specific effect on the tRNA charging reaction.