With the identification of mutations in the monocarboxylate transporter 8 (MCT8) gene in patients afflicted with the AllanHerndonDudley syndrome (AHDS), the concept of transporter-mediated transmembrane transport of thyroid hormones was finally accepted. Impaired thyroid hormone transport into neurons and pituitary cells is believed to cause severe psychomotor retardation and altered thyroid hormone function tests. MCT8 is a specific thyroid hormone transporter able to transport T4, T3, rT3, and 3,3′-T2, but does not transport D-enantiomers or related compounds lacking amino or carboxyl functional groups. MCT10 is a highly related thyroid hormone transport protein which is also known as TAT1 or T-type amino acid transporter. MCT10 transports T3 and aromatic amino acids, but not T4.
Based on a homology model of MCT8, we have identified 8 amino acid differences between MCT8 and MCT10 that are located along the presumed substrate translocation channel. We hypothesized that creation of chimeric transporters by exchanging these amino acids should alter MCT10 substrate preferences in MCT10MCT8 chimeric proteins. If successful, we expect that MCT10MCT8 would expand its substrate spectrum to include T4. An interesting question would then be whether the mutant MCT10MCT8 would still be able to transport Trp or Tyr or whether T4 and aromatic amino acid transport are mutually exclusive.
We created several single and compound mutants of MCT10 with amino acid exchanges as found in MCT8 and tested their ability to transport T4 and Tyr in transfected MDCK-1 cells and Xenopus oocytes respectively. We will present data on a MCT10MCT8 that is able to transport T4 and show its activity towards Tyr.
In summary, our data demonstrates that structure-guided site-directed mutagenesis can serve to identify amino acids important for substrate recognition. In turn, these findings support the usefulness of our model.