Truncation of Medicago truncatula Auxin Conjugate Hydrolases Alters Substrate Specificity

We have previously isolated and characterized a family of auxin amino acid conjugate hydrolases from the legume Medicago truncatula. All characterized members of this family possess a conserved second methionine within the predicted hydrolase domain. We therefore constructed 5′-truncated clones of the hydrolases to investigate whether this methionine could have a potential function. Overall, the hydrolases exhibited altered substrate specificities towards a variety of auxin conjugates tested, with a somewhat broadened substrate range. In vitro hydrolase activity increased over wild-type in several of the shortened proteins, but only for some substrates. The 5′ "head" domain may be serving a regulatory function in the full-length versions of the enzymes or could provide a mechanism to broaden the substrate range in vivo.