Structural and Enzymatic characterization of the lactonase <em>Sis</em>Lac from <em>Sulfolobus islandicus</em>

<div><h3>Background</h3><p>A new member of the Phosphotriesterase-Like Lactonases (PLL) family from the hyperthermophilic archeon <em>Sulfolobus islandicus</em> (<em>Sis</em>Lac) has been characterized. <em>Sis</em>Lac is a native lactonase that exhibits a high promiscuous phosphotriesterase activity. <em>Sis</em>Lac thus represents a promising target for engineering studies, exhibiting both detoxification and bacterial <em>quorum</em> quenching abilities, including human pathogens such as <em>Pseudomonas aeruginosa</em>.</p> <h3>Methodology/Principal Findings</h3><p>Here, we describe the substrate specificity of <em>Sis</em>Lac, providing extensive kinetic studies performed with various phosphotriesters, esters, <em>N</em>-acyl-homoserine lactones (AHLs) and other lactones as substrates. Moreover, we solved the X-ray structure of <em>Sis</em>Lac and structural comparisons with the closely related <em>Sso</em>Pox structure highlighted differences in the surface salt bridge network and the dimerization interface. <em>Sis</em>Lac and <em>Sso</em>Pox being close homologues (91% sequence identity), we undertook a mutational study to decipher these structural differences and their putative consequences on the stability and the catalytic properties of these proteins.</p> <h3>Conclusions/Significance</h3><p>We show that <em>Sis</em>Lac is a very proficient lactonase against aroma lactones and AHLs as substrates. Hence, data herein emphasize the potential role of <em>Sis</em>Lac as <em>quorum</em> quenching agent in <em>Sulfolobus</em>. Moreover, despite the very high sequence homology with <em>Sso</em>Pox, we highlight key epistatic substitutions that influence the enzyme stability and activity.</p>