Mechanism of LAT1 amino acid antiport: a molecular dynamics simulation of the behaviour of a solute and of an inhibitor

The L-type amino acid transporter 1 (LAT1) is an ubiquitous Na+- and pH-independent antiporter, involved in the cellular uptake of essential amino acids. Being over-expressed in many human cancers, LAT1 was recently acknowledged as a novel target for cancer therapy (Scalise et al., Front Chem 2018, 6:243). Here, the unknown mechanism of transport mediated by LAT1 was investigated by molecular modelling approaches, by comparing the transport of its solute tyrosine (Tyr) with the behavior of the well-known inhibitor 3,5-diiodo-L-tyrosine (DIT). First, the outward-facing (OF) and the inward-facing (IF) LAT1 structures were built by comparative modelling. Then, a series of targeted molecular dynamics simulations (tMD) were carried out in an explicit membrane-like bilayer, driving LAT1 structure from the initial to the final state of the transport. A different behaviour was observed between Tyr and DIT. Only at the highest spring value both Tyr and DIT could pass through the LAT1 transport channel, the solute Tyr being faster that the inhibitor DIT. Under these simulation conditions, an additional putative inner gate was identified. Decreasing the spring constant, Tyr and DIT progressively lost the ability to pass across the LAT1 transport channel. Moreover, Tyr appeared to interact with some specific residues; conversely, DIT established only transient interactions with residues located in the external part of the transport channel. Overall, these tMD simulations allowed us to describe for the first time the amino acid transport mechanism of LAT1. The recent publication of LAT1 structure at atomic resolution in its IF conformation will provide further insights into the transport mechanism of these superfamily of transporters. This approach can thus be proposed for developing novel inhibitors, useful for further pharmacological applications in cancer therapy (Singh and Ecker, Int J Mol Sci 2018, 19:1278).