Orthogonal use of a human tRNA synthetase active site to achieve multifunctionality

Abstract

TRNA synthetases (RSs) have been implicated in functions beyond tRNA synthesis, and human TrpRS has been implicated in inhibiting angiogenesis. Tryptophan residues implicated in homotypic interactions on VE-cadherin are now shown to bind the active site of a naturally truncated TrpRS, indicating how this enzyme has been repurposed. Protein multifunctionality is an emerging explanation for the complexity of higher organisms. In this regard, aminoacyl tRNA synthetases catalyze amino acid activation for protein synthesis, but some also act in pathways for inflammation, angiogenesis and apoptosis. It is unclear how these multiple functions evolved and how they relate to the active site. Here structural modeling analysis, mutagenesis and cell-based functional studies show that the potent angiostatic, natural fragment of human tryptophanyl-tRNA synthetase (TrpRS) associates via tryptophan side chains that protrude from its cognate cellular receptor vascular endothelial cadherin (VE-cadherin). VE-cadherin's tryptophan side chains fit into the tryptophan-specific active site of the synthetase. Thus, specific side chains of the receptor mimic amino acid substrates and expand the functionality of the active site of the synthetase. We propose that orthogonal use of the same active site may be a general way to develop multifunctionality of human tRNA synthetases and other proteins.