Structural basis for oligomerization of the prokaryotic peptide transporter PepT.

Proton-dependent oligopeptide transporters (POTs) belong to the major facilitator superfamily (MFS) and transport dipeptides and tripeptides from the extracellular environment into the target cell. The human POTs PepT1 and PepT2 are also involved in the absorption of various orally ingested drugs. Previously reported structures revealed that the bacterial POTs possess 14 helices, of which H1-H6 and H7-H12 constitute the typical MFS fold and the residual two helices are involved in the cytoplasmic linker. PepT from Shewanella oneidensis is a unique POT which reportedly assembles as a 200 kDa tetramer. Although the previously reported structures suggested the importance of H12 for tetramer formation, the structural basis for the PepT-specific oligomerization remains unclear owing to the lack of a high-resolution tetrameric structure. In this study, the expression and purification conditions for tetrameric PepT were optimized. A single-particle cryo-EM analysis revealed the tetrameric structure of PepT incorporated into Salipro nanoparticles at 4.1 Å resolution. Furthermore, a combination of lipidic cubic phase (LCP) crystallization and an automated data-processing system for multiple microcrystals enabled crystal structures of PepT to be determined at resolutions of 3.5 and 3.9 Å. The present structures in a lipid bilayer revealed the detailed mechanism for the tetrameric assembly of PepT, in which a characteristic extracellular loop (ECL) interacts with two asparagine residues on H12 which were reported to be important for tetramerization and plays an essential role in oligomeric assembly. This study provides valuable insights into the oligomerization mechanism of this MFS-type transporter, which will further pave the way for understanding other oligomeric membrane proteins.