AUTHOR=North Rachel A. , Wahlgren Weixiao Y. , Remus Daniela M. , Scalise Mariafrancesca , Kessans Sarah A. , Dunevall Elin , Claesson Elin , Soares da Costa Tatiana P. , Perugini Matthew A. , Ramaswamy S. , Allison Jane R. , Indiveri Cesare , Friemann Rosmarie , Dobson Renwick C. J. 

TITLE=The Sodium Sialic Acid Symporter From Staphylococcus aureus Has Altered Substrate Specificity

JOURNAL=Frontiers in Chemistry

VOLUME=6

YEAR=2018

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2018.00233

DOI=10.3389/fchem.2018.00233

ISSN=2296-2646

ABSTRACT=<p>Mammalian cell surfaces are decorated with complex glycoconjugates that terminate with negatively charged sialic acids. Commensal and pathogenic bacteria can use host-derived sialic acids for a competitive advantage, but require a functional sialic acid transporter to import the sugar into the cell. This work investigates the sodium sialic acid symporter (SiaT) from <italic>Staphylococcus aureus</italic> (<italic>Sa</italic>SiaT). We demonstrate that <italic>Sa</italic>SiaT rescues an <italic>Escherichia coli</italic> strain lacking its endogenous sialic acid transporter when grown on the sialic acids <italic>N</italic>-acetylneuraminic acid (Neu5Ac) or <italic>N</italic>-glycolylneuraminic acid (Neu5Gc). We then develop an expression, purification and detergent solubilization system for <italic>Sa</italic>SiaT and demonstrate that the protein is largely monodisperse in solution with a stable monomeric oligomeric state. Binding studies reveal that <italic>Sa</italic>SiaT has a higher affinity for Neu5Gc over Neu5Ac, which was unexpected and is not seen in another SiaT homolog. We develop a homology model and use comparative sequence analyses to identify substitutions in the substrate-binding site of <italic>Sa</italic>SiaT that may explain the altered specificity. <italic>Sa</italic>SiaT is shown to be electrogenic, and transport is dependent upon more than one Na<sup>+</sup> ion for every sialic acid molecule. A functional sialic acid transporter is essential for the uptake and utilization of sialic acid in a range of pathogenic bacteria, and developing new inhibitors that target these transporters is a valid mechanism for inhibiting bacterial growth. By demonstrating a route to functional recombinant <italic>Sa</italic>SiaT, and developing the <italic>in vivo</italic> and <italic>in vitro</italic> assay systems, our work underpins the design of inhibitors to this transporter.</p>