Opinion
Transporter Specificity: A Tale of Loosened Elevator-Sliding

https://doi.org/10.1016/j.tibs.2021.03.007Get rights and content

Highlights

  • Transporters are plasma membrane proteins that mediate the selective uptake or efflux of nutrients, ions, metabolites, and drugs in all cells.

  • Transporters are essential for well-functioning cells and organisms and consequently their malfunction is reflected in several genetic diseases and pathologies.

  • Over the past 8 years, X-ray crystallography and cryogenic electron microscopy methodologies have led to the identification of a new group of transporters that seem to function via a sliding elevator-type mechanism.

  • The molecular basis that determines how different elevator-type transporters recognize and transport distinct substrates remains elusive.

  • Understanding how specificity of transporters is determined or changed in the course of evolution might lead to targeted exploitation of transporters in pharmacological control of pathologies and against microbial infections.

Elevator-type transporters are a group of proteins translocating nutrients and metabolites across cell membranes. Despite structural and functional differences, elevator-type transporters use a common mechanism of substrate translocation via reversible movements of a mobile core domain (the elevator), which includes the substrate binding site, along a rigid scaffold domain, stably anchored in the plasma membrane. How substrate specificity is determined in elevator transporters remains elusive. Here, I discuss how a recent report on the sliding elevator mechanism, seen under the context of genetic analysis of a prototype fungal transporter, sheds light on how specificity might be genetically modified. I propose that flexible specificity alterations might occur by ‘loosening’ of the sliding mechanism from tight coupling to substrate binding.

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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