Side Chain IndependentRecognition of Aminoacyl Adenylates by the Hint1 Transcription Suppressor.

Human Hint1 suppresses specific gene transcription byinteracting with the transcription factor MITF in mast cells. Hint1activity is connected to lysyl-tRNA synthetase (LysRS), a member ofthe universal aminoacyl tRNA synthetase family that catalyzes specificaminoacylation of their cognate tRNAs, through an aminoacyl adenylate(aa-AMP) intermediate. During immune activation, LysRS produces aside-product diadenosine tetraphosphate (Ap4A) from thecondensation of Lys-AMP with ATP. The pleiotropic signaling moleculeAp4A then binds Hint1 to promote activation of MITF-targetgene transcription. Earlier work showed that Hint1 can also bind andhydrolyze Lys-AMP, possibly to constrain Ap4A production.Because Ap4A can result from condensation of other aa-AMP'swith ATP, the specificity of the Hint1 aa-AMP–hydrolysis activityis of interest. Here we show that Hint1 has broad specificity foradenylate hydrolysis, whose structural basis we revealed through high-resolutionstructures of Hint1 in complex with three different aa-AMP analogues.Hint1 recognizes only the common main chain of the aminoacyl moiety,and has no contact with the aa side chain. The α-amino groupis anchored by a cation-pi interaction with Trp123 at the C-terminusof Hint1. These results reveal the structural basis for the remarkableadenylate surveillance activity of Hint1, to potentially control Ap4A levels in the cell. [ABSTRACT FROM AUTHOR]