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ABSTRACT
Membrane transport proteins are often classified into two groups: channels and transporters. While channels may open and close, at some stage they create a continuous pore allowing for the rapid, often selective, diffusion of solutes down their electrochemical gradient. In contrast, transporters never have a continuous pathway for substrate translocation; instead, they undergo conformational changes after substrate binding to shuttle substrates across the membrane - often against their concentration gradients.  While the distinction between channels and transporters is usually clear, there is increasing evidence that the boundary is not always distinct. 

Throughout my PhD, I determined the transport mechanisms of two proteins that we show break the standard channel / transporter definitions and instead function as anion “chansporters” - proteins that exist somewhere on the spectrum between transporters and channels.  These are the Plasmodium falciparum Formate-Nitrite Transporter (PfFNT), which mediates the essential export of lactate and H+ from the malaria parasite cytosol, and the human Xenotropic and Polytropic Retrovirus Receptor 1 (XPR1), the only protein known to mediate inorganic phosphate export from our cells.  In addition, we have also investigated mutations in PfFNT associated with resistance to the known inhibitor MMV007839, as well as the selectivity of XPR1 for inorganic phosphate over chloride ions.  

PfFNT and XPR1 are examples of the 30–50% of proteins that form homo‑oligomeric complexes, in which subunit architecture is critical for dictating substrate specificity, transport kinetics, and regulatory mechanisms.  As experimental determination of oligomeric states is often challenging, we investigated whether oligomeric states can instead be predicted computationally.  We show that the artificial intelligence tools AlphaFold2 and AlphaFold3 can accurately predict protein oligomeric states, but struggle when proteins lack similarity to structures in the training set.   

Together, this work expands our knowledge of proteins borderline between channels and transporters and suggests a new method for predicting protein oligomeric states.