Trends in Biochemical Sciences
OpinionTransporter Specificity: A Tale of Loosened Elevator-Sliding
Section snippets
Unique and Shared Themes in Transporter Functioning
Plasma membrane (PM) transporters are ubiquitous transmembrane proteins (see Glossary) that mediate the selective uptake or efflux of nutrients, ions, metabolites, and drugs (reviewed in [1]). Their role is essential for cell growth, homoeostasis, detoxification, signaling, and generally for communication with the environment. Their biological importance is not only reflected in a plethora of diseases and pathologies related to their malfunction (e.g., cystic fibrosis, type II diabetes,
Elevator-Type Transporters:An Emerging Mechanism of Substrate Translocation
Despite structural differences, all elevator-type transporters (symporters and antiporters) possess the common characteristic of a motile substrate binding site included in a core/elevator domain that moves through the membrane as a rigid body, thus translocating the substrate(s) from one side of the membrane to the other (Box 1). Briefly, movement of the core/elevator domain takes place against a much more rigid scaffold (or oligomerization) domain, anchored in the PM via specific interactions
UapA as a Model Elevator Transporter for Understanding Specificity
The issue of substrate specificity has been previously addressed by a genetic dissection of the elevator-type purine transporter UapA of the model ascomycete Aspergillus nidulans (see review [6]). UapA is high-affinity xanthine or uric acid/H+ symporter that historically defined the ubiquitous Nucleobase Ascorbate Transporter (NAT) family. Evidence for the essentiality of H+ symport by UapA, as well as other microbial NATs, is supported by abolishment of transport activity by the protonophore
Shaping a Hypothesis on How UapA Specificity Is Modified
As discussed earlier, a major conclusion from the work of Sauer et al. [5] on DASS transporters is that a control point restricting leaky sliding of the elevator (i.e., uncontrolled slippage), before the binding site is loaded with substrate, is imposed by electrostatic incompatibility of the empty binding site with the interface of the scaffold domain. Loading of the carboxylate ion onto the binding site creates the necessary electrostatic compatibility to unlock sliding. This mechanism of
Concluding Remarks
If the basis of UapA specificity enlargement is a direct consequence of loosening of the sliding mechanism, as proposed herein, other elevator-type transporters might also be easily accessible to engineering, or prone to evolution of new substrate specificities. Any mutation that allows increased, less-controlled sliding, permitting the transport of additional weakly-binding ligands, will probably behave as a neutral mutation in respect to the original major transport function of the protein.
Acknowledgments
I am grateful to Anezia Kourkoulou, Iliana Zantza, and Emmanuel Mikros for sharing unpublished data and Claudio Scazzocchio, Bernadette Byrne, and Ariadne Vassilaki for critically reading this article. Work in my laboratory related to UapA specificity has been supported by the ‘Stavros Niarchos Foundation’.
Declaration of Interests
No interests are declared.
Glossary
- Affinity
- the affinity a protein (e.g., enzyme or transporter) binds its substrate(s). Kinetically defined by the so-called Michaelis constant Km, which equals the concentration of substrate at which the reaction/transport rate is at half-maximum (Vm/2).
- Antiporter
- a cotransporter of two or more different molecules or ions moving across a cell membrane in opposite directions.
- Cryogenic electron microscopy (cryo-EM)
- an electron microscopy technique used for determining the molecular structure of
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2021, Journal of Molecular BiologyCitation Excerpt :It is reasonable to assume that, by tilting TM12 (Figure 8), S377G affects this elevator-like motion of the core domain and modulates the kinetics of transport. Several specific mutations at the elevator-scaffold interface in the fungal UapA (including mutations at TM12, at positions corresponding to M375, I379, R385 in XanQ) can broaden the specificity of this highly selective xanthine and uric acid transporter to allow recognition and/or transport of most purines or, even, uracil, and the effects were proposed to reflect changes in the dynamics of sliding of the core (elevator) domain, which includes the binding site, rather than changes in residues of the binding site per se.19 S377G in XanQ might also affect specificity via a similar route.
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