SignificanceTransporters isomerize between conformations to shuttle cargo across membranes, but the mechanism is not understood. Double electron–electron resonance measurements on the sodium-dependent sugar transporter (vSGLT) were used to explore the conformational state of the transporter under specific ligand conditions. Although sugar transport by vSGLT is driven by sodium gradients, vSGLT adopts an inward-open conformation irrespective of the presence of sodium. In the presence of sodium and galactose, the transporter transitions to an occluded conformation. We propose that the cell’s negative membrane potential aids in driving vSGLT toward the outward-facing state to bind sugar and begin the transport cycle. These findings could be applicable to other transporters whereby the inherent cellular membrane potential is integrated into the transport cycle.
We show that a combinatorial library constructed by random pairwise assembly of low affinity binders can efficiently generate binders with increased affinity. Such a library based on the Sso7d scaffold, from a pool of low affinity binders subjected to random mutagenesis, contained putative high affinity clones for a model target (lysozyme) at higher frequency than a library of monovalent mutants generated by random mutagenesis alone. Increased binding affinity was due to intramolecular avidity generated by linking binders targeting nonoverlapping epitopes; individual binders of K ∼ 1.3 μM and 250 nM produced a bivalent binder with apparent K ∼ 2 nM. Furthermore, the bivalent protein retained thermal stability (T = 84.5 °C) and high recombinant expression yields in E. coli. Finally, when binders comprising the bivalent protein are fused to two of the three fragments of tripartite split-green fluorescent protein (GFP), target-dependent reconstitution of fluorescence occurs, thereby enabling a "mix-and-read" assay for target quantification.
Background: Although the solute-sodium symporter (SSS) vSGLT and the neurotransmitter-sodium symporter (NSS) LeuT have similar structural folds, their crystallographically identified substrate sites diverge in location and composition. Results: We identified second substrate sites in two SSSs that align with the crystallographically identified site in LeuT. Conclusion: Substrate transport by SSSs involves two substrate sites. Significance: NSS and SSS share common mechanistic features.
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