Conserved hydrophilic checkpoints tune FocA-mediated formate:H+ symport
- Christian Tüting
- Kevin Janson
- Michelle Kammel
- Christian Ihling
- Jana Lorenz
- Fotis L. Kyrilis
- Farzad Hamdi
- Christopher Erdmann
- Andrea Sinz
- R. Gary Sawers
- Panagiotis L. Kastritis
2025-10-27
FocA belongs to the widespread, evolutionarily ancient formate-nitrite transporter (FNT) family of pentameric anion channels and translocates formic acid bidirectionally. Here, we identify compartmentalized polarity distribution across the complete FocA pore structure – resolved at 2.56 Å – mirrored against a two-fold axis with H209 at its center. A FocA-H209N variant that exhibits an efflux-only channel-like function in vivo reveals a density consistent with formate located directly at N209, abolishing the channel’s amphiphilicity. Pyruvate formate-lyase, which generates formate, orients at the cytoplasmic face where formate delivery is regulated by conformational changes in the FocA vestibule. Comparisons with other FNTs suggest a tuning mechanism of formate-specific transport via checkpoints enriched in hydrophilic residues.