Abstact

An increasing number of esterases is being revealed by (meta) genomic sequencing projects, but few of them are functionally/structurally characterized, especially enzymes of fungal origin. Starting from a three-member gene family of secreted putative "lipases/esterases" preferentially expressed in the symbiotic phase of the mycorrhizal fungus Tuber melanosporum ("black truffle"), we show here that these enzymes (TmelEST1-3) are dimeric, heat-resistant carboxylesterases capable of hydrolyzing various short/medium chain p-nitrophenyl esters. TmelEST2 was the most active (kcat = 2302 s-1 for p-nitrophenyl-butyrate) and thermally stable (T50 = 68.3 °C), while TmelEST3 was the only one displaying some activity on tertiary alcohol esters. X-ray diffraction analysis of TmelEST2 revealed a classical α/β hydrolase-fold structure, with a network of dimer-stabilizing intermolecular interactions typical of archaea esterases. The predicted structures of TmelEST1 and 3 are overall quite similar to that of TmelEST2 but with some important differences. Most notably, the much smaller volume of the substrate-binding pocket and the more acidic electrostatic surface profile of TmelEST1. This was also the only TmelEST capable of hydrolyzing feruloyl-esters, suggestinng a possible role in root cell-wall deconstruction during symbiosis establishment. In addition to their potential biotechnological interest, TmelESTs raise important questions regarding the evolutionary recruitment of archaea-like enzymes into mesophilic subterranean fungi such as truffles.