Human tryptophanyl-tRNA synthetase binds with heme to enhance its aminoacylation activity.

Mammalian tryptophanyl-tRNA synthetases (TrpRSs) are Zn2+-binding proteins that catalyze the aminoacylation of tRNATrp. The cellular expression level of human TrpRS is highly upregulated by interferon-gamma (IFN-gamma). In this study, a heme biosynthesis inhibitor, succinylacetone (SA), was found to inhibit cellular TrpRS activity in IFN-gamma-activated cells without affecting TrpRS protein expression. In addition, supplementation of lysates from the SA-treated cells with hemin fully restored TrpRS activity to control levels. Biochemical analyses using purified TrpRS demonstrated that heme can interact strongly with Zn2+-depleted human full-length TrpRS with a stoichiometric heme:protein ratio of 1:1 to enhance the aminoacylation activity significantly. In contrast, the Zn2+-bound form of TrpRS did not bind heme. Further studies using site-directed mutagenesis clarified that the Zn2+-unbound human H130R mutant cannot bind heme. These results provide the first evidence of the involvement of heme in regulation of TrpRS aminoacylation activity. The regulation mechanism and its physiological roles are discussed.