Amino acids in transmembrane helix 1 confer major functional differences between human and mouse orthologs of the polyspecific membrane transporter OCT1

J Biol Chem. 2022 Jun;298(6):101974. doi: 10.1016/j.jbc.2022.101974. Epub 2022 Apr 22.

Abstract

Organic cation transporter 1 (OCT1) is a membrane transporter that affects hepatic uptake of cationic and weakly basic drugs. OCT1 transports structurally highly diverse substrates. The mechanisms conferring this polyspecificity are unknown. Here, we analyzed differences in transport kinetics between human and mouse OCT1 orthologs to identify amino acids that contribute to the polyspecificity of OCT1. Following stable transfection of HEK293 cells, we observed more than twofold differences in the transport kinetics of 22 out of 28 tested substrates. We found that the β2-adrenergic drug fenoterol was transported with eightfold higher affinity but at ninefold lower capacity by human OCT1. In contrast, the anticholinergic drug trospium was transported with 11-fold higher affinity but at ninefold lower capacity by mouse Oct1. Using human-mouse chimeric constructs and site-directed mutagenesis, we identified nonconserved amino acids Cys36 and Phe32 as responsible for the species-specific differences in fenoterol and trospium uptake. Substitution of Cys36 (human) to Tyr36 (mouse) caused a reversal of the affinity and capacity of fenoterol but not trospium uptake. Substitution of Phe32 to Leu32 caused reversal of trospium but not fenoterol uptake kinetics. Comparison of the uptake of structurally similar β2-adrenergics and molecular docking analyses indicated the second phenol ring, 3.3 to 4.8 Å from the protonated amino group, as essential for the affinity for fenoterol conferred by Cys36. This is the first study to report single amino acids as determinants of OCT1 polyspecificity. Our findings suggest that structure-function data of OCT1 is not directly transferrable between substrates or species.

Keywords: SLC22A1; drug transport; membrane transport; molecular modeling; organic cation transporter 1; species differences; structure-function; transmembrane domain; transporter.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Catecholamine Plasma Membrane Transport Proteins / chemistry*
  • Catecholamine Plasma Membrane Transport Proteins / metabolism
  • Fenoterol
  • HEK293 Cells
  • Humans
  • Mice
  • Molecular Docking Simulation
  • Organic Cation Transporter 1* / chemistry
  • Organic Cation Transporter 1* / metabolism

Substances

  • Catecholamine Plasma Membrane Transport Proteins
  • Organic Cation Transporter 1
  • Slc22a1 protein, mouse
  • Fenoterol