Membrane-traversing mechanism of thyroid hormone transport by monocarboxylate transporter 8

Abstract

Monocarboxylate transporter 8 (MCT8) mediates thyroid hormone (TH) transport across the plasma membrane in many cell types. In order to better understand its mechanism, we have generated three new MCT8 homology models based on sugar transporters XylE in the intracellular opened (PDB ID: 4aj4) and the extracellular partly occluded (PDB ID: 4gby) conformations as well as FucP (PDB ID: 3o7q) and GLUT3 (PDB ID: 4zwc) in the fully extracellular opened conformation. T-docking studies from both sides revealed interactions with His192, His415, Arg445 and Asp498 as previously identified. Selected mutations revealed further transport-sensitive positions mainly at the discontinuous transmembrane helices TMH7 and 10. Lys418 is potentially involved in neutralising the charge of the TH substrate because it can be replaced by charged, but not by uncharged, amino acids. The side chain of Thr503 was hypothesised to stabilise a helix break at TMH10 that undergoes a prominent local shift during the transport cycle. A T503V mutation accordingly affected transport. The aromatic Tyr419, the polar Ser313 and Ser314 as well as the charged Glu422 and Glu423 lining the transport channel have been studied. Based on related sugar transporters, we suggest an alternating access mechanism for MCT8 involving a series of amino acid positions previously and newly identified as critical for transport.

Thyroid Hormone Transporters

“…For a long time, the generation of homology models in transporter conformations other than the inward-open conformation had been limited by the paucity of available crystal structure templates with sufficient homology to MCT8 (242). By combining several template crystal structures, Protze et al (2017) recently established 3 MCT8 homology models in different conformations allowing further delineation of putative mechanisms by which substrate molecules transit the substrate pore (252). These studies further confirmed that Arg445, Asp498, and His192 are located at the center of the substrate pore and are key players in the docking of substrate in the inward-open and outward-open conformation.…”

“…For a long time, a major gap in the field had been the absence of crystal structures of thyroid hormone transporters or any homologous proteins. Although structural homology models have broadened our understanding of the mechanism by which thyroid hormone transporters translocate the various iodothyronine substrates (eg, (77,153,252,253,276), the sequence similarity to template structures is generally low, preventing accurate molecular predictions. In this context, thorough functional characterization of the WT and mutant thyroid hormone transporters will be a prerequisite to interpret the impact of any genetic variant and prove pathogenicity in case of MCT8 mutations.…”

Molecular Mechanisms of Thyroid Hormone Transport by l-Type Amino Acid Transporter

“…Therefore, further investigations are necessary to explain the negative effect of LAT3 on the TH import by LAT2. However, recent studies [2,9,18] verified the previous hypothesis that the known TH transporter types (MCT8, 10, OATP1C1, LAT1, 2) differ and are transferring distinct TH-derivatives with different preferences and by different molecular recognition and translating mechanisms. The outcome of these structure-function studies is very relevant for our molecular understanding of the pathogenesis of diseases associated with TH transporter proteins themselves or with other deregulations of TH.…”

“…For MCT8 that belongs to the major facilitator superfamily (MSF), we together with others recently elucidated a molecular mechanism for TH recognition and traversing through the transporter [9]. Since T3 uptake into primary neurons does not com-…”

The Protein Translocation Defect of MCT8L291R Is Rescued by Sodium Phenylbutyrate

“…Detailed investigation of the distribution pattern of MCT8 L291R expressed in human-derived JEG1 cells revealed that the mutant is able to escape from the ER but is routed to the lysosomal compartment. Our homology models [38] predict that Leu291 is located at the end of transmembrane helix four facing the intracellular environment. In general, an unstable plasma membrane protein is sent for lysosomal degradation after its recognition by the cytosolic C-terminus of HSC70-interacting protein (CHIP) [39].…”