A group from the University of Gothenburg recently published a functional characterization of a bacterial membrane transporter protein that underscores the use of Monolith for the essential measurement of binding affinities (Weixiao et al. Nature Communications, 2018). This study was greatly enabled not only by MST’s tolerance for detergents (required for the investigation of transmembrane proteins such as transporters) but also by the ability of MST to provide data on the binding affinities of very small ligands, even ions, as in this case.
MST measurements typically involve fluorescent labeling of the protein,
but this is far from an absolute requirement.
Monolith NT.LabelFree system used in this study can deliver binding data using only the intrinsic fluorescence emitted by tryptophan and tyrosine residues. To top it all off, an orthogonal check of the MST-derived Kd using ITC showed great correlation, although it required considerably more target protein than MST for the same measurement.
Understanding how a bacterium exploits membrane-bound sialic acid reveals new Na+ binding site in the sialic acid symporter
The team of protein structure experts reported on the crystal structure and functional characteristics of a sialic acid transporter containing a novel Na+ binding domain from Proteus mirabilis, a soil bacterium that frequently colonizes the human urinary tract causing kidney stones or life-threatening urosepsis. Like many other pathogenic bacteria, P. mirabilis exploits the presence of sialic acid residues on mammalian cell surface glycoconjugates as N‑acetylneuraminic acid (Neu5Ac). Both commensal and pathogenic bacteria use neuraminidase enzymes to cleave these Neu5Ac moieties, then internalize them where they are used both to decorate the outer surface of the bacterial cell as protection against mammalian immune systems and as a source of carbon, nitrogen, and energy.
Transporters: The Achilles heel of bacteria
Because transporters are so crucial to the survival of bacteria, and because analogous mammalian transporters are not structurally similar to those in prokaryotes, thorough characterization of bacterial transporters can provide druggable targets for inhibiting bacterial growth in human digestive, respiratory, and urinary tracts. Some transporters employed by bacteria to import molecules across their membranes create an electrochemical gradient to facilitate transport. Known as secondary active transporters, these often bind one or two Na+ ions to generate a local gradient. The group working with P. mirabilis investigated a secondary active transporter of sialic acid from the sodium solute symporter (SSS) family called SiaT. In the process of generating a high-resolution crystal structure for SiaT they discovered a novel sodium binding domain that is conserved in many other SSS family members. The authors added that the discovery of this site might make it possible to pharmacologically exploit differences in this mechanism between SSS family members and other similar transporters.