Transporter oligomerization: form and function

Abstract

Transporters are integral membrane proteins with central roles in the efficient movement of molecules across biological membranes. Many transporters exist as oligomers in the membrane. Depending on the individual transport protein, oligomerization can have roles in membrane trafficking, function, regulation and turnover. For example, our recent studies on UapA, a nucleobase ascorbate transporter, from Aspergillus nidulans, have revealed both that dimerization of this protein is essential for correct trafficking to the membrane and the structural basis of how one UapA protomer can affect the function of the closely associated adjacent protomer. Here, we review the roles of oligomerization in many particularly well-studied transporters and transporter families.

Keywords: integral membrane transporter; oligomerization; regulation of transporter function; trafficking; transporter function.

Figure 1.. Role of oligomerization of UapA.

(A) Co-expression of WT UapA and a non-functional mutant Q408E in a UapA knockout strain of A. nidulans. Both forms traffic effectively to the membrane as indicated by GFP fluorescence, but the fungi only grow effectively in the presence of xanthine when expressing from one or two copies of the WT uapa gene. (B) Comparison of xanthine uptake in A. nidulans cells expressing the WT and mutant forms of UapA. UapA⁻ indicates the UapA knockout strain. (C) Structure of the dimer of UapA from A. nidulans (PDB: 5I6C) shown looking through the membrane and (D) from the intracellular side of the protein. In each case, one protomer is shown in light green and one in light pink with TM13 shown in bright green and magenta, respectively. Xanthine is shown in cyan space-filling model, and R481 is shown in bright green and magenta space-filling model. For clarity, TMs 6 and 7 have been removed. (E) Growth of the UapA⁻ strain expressing WT or substrate selectivity mutant UapA on a range of native (uric acid or xanthine) or non-native (hypoxanthine or adenine) substrates. (F) Transport characteristics of these mutant proteins. Note that the R481G mutant allows transport of non-native substrates.

Figure 2.. Monomer versus oligomer transporter structures.

(A) Structure of the dopamine transporter from Drosophila melanogaster (PDB: 4M48). For clarity, only the transmembrane regions are shown. Each transmembrane region is individually coloured and labelled. The residues of the predicted leucine zipper motif and the GXXXG motif are shown in white and wheat coloured space-filling model, respectively. (B) Key ionic interactions between protomers of the BetP trimer from Corynebacterium glutamicum (PDB: 3P03). Only two of the protomers are shown for clarity, one in light green and one in light pink. The residues involved are coloured according to the protomer, and are shown in space-filling representation.

Figure 3.. Phosphorylation-dependent regulation of NRT1.1 from Arabidopsis thaliana (PDB: 4OH3).

In the dimer form, T101, shown in space-filling representation, is unphosphorylated, and the protein functions as a low-affinity transporter. Upon phosphorylation, the dimer dissociates and the individual protomers function independently as high-affinity transporters.