Quaternary structure of the yeast Arc1p-aminoacyl-tRNA synthetase complex in solution and its compaction upon binding of tRNAs

In the yeast Saccharomyces cerevisiae, the aminoacyl-tRNA synthetases (aaRS) GluRS and MetRS form a complex with the auxiliary protein cofactor Arc1p. The latter binds the N-terminal domains of both synthetases increasing their affinity for the transfer-RNA (tRNA) substrates tRNA^Met and tRNA^Glu. Until now, structural information was available only on the enzymatic domains of the individual aaRSs but not on their complexes with associated cofactors. We have analysed the yeast Arc1p-complexes in solution by small-angle X-ray scattering (SAXS). The ternary complex of MetRS and GluRS with Arc1p, displays a peculiar extended star-like shape, implying possible flexibility of the complex. We reconstituted in vitro a pentameric complex and demonstrated by electrophoretic mobility shift assay that the complex is active and contains tRNA^Met and tRNA^Glu, in addition to the three protein partners. SAXS reveals that binding of the tRNAs leads to a dramatic compaction of the pentameric complex compared to the ternary one. A hybrid low-resolution model of the pentameric complex is constructed rationalizing the compaction effect by the interactions of negatively charged tRNA backbones with the positively charged tRNA-binding domains of the synthetases.