A channel profile report of the unusual K+ channel KtrB
| Autor: | Natalie Kolesova, Marina Schrecker, Miyer Patiño-Ruiz, Vedrana Mikusevic, Klaus Fendler, Inga Hänelt |
|---|---|
| Rok vydání: | 2019 |
| Předmět: |
0301 basic medicine
Potassium Channels Physiology Sodium Dimer Protein subunit 030106 microbiology chemistry.chemical_element Gating 03 medical and health sciences chemistry.chemical_compound Bacterial Proteins Cation Transport Proteins Research Articles Vibrio alginolyticus Ion channel Ions Cell Membrane Membrane Proteins Biological Transport Isothermal titration calorimetry Protein Subunits 030104 developmental biology Membrane chemistry Potassium Biophysics Selectivity Research Article |
| Zdroj: | The Journal of General Physiology |
| ISSN: | 1540-7748 0022-1295 |
| DOI: | 10.1085/jgp.201912384 |
| Popis: | KtrAB is a bacterial ion channel with a noncanonical selectivity filter. Consequently, it was assumed to be rather unselective for monovalent cations. Mikušević et al. show that in the presence of Na+, KtrAB very selectively translocates K+, which results from a high binding affinity and facilitated K+ gating. KtrAB is a key player in bacterial K+ uptake required for K+ homeostasis and osmoadaptation. The system is unique in structure and function. It consists of the K+-translocating channel subunit KtrB, which forms a dimer in the membrane, and the soluble regulatory subunit KtrA, which attaches to the cytoplasmic side of the dimer as an octameric ring conferring Na+ and ATP dependency to the system. Unlike most K+ channels, KtrB lacks the highly conserved T(X)GYG selectivity filter sequence. Instead, only a single glycine residue is found in each pore loop, which raises the question of how selective the ion channel is. Here, we characterized the KtrB subunit from the Gram-negative pathogen Vibrio alginolyticus by isothermal titration calorimetry, solid-supported membrane–based electrophysiology, whole-cell K+ uptake, and ACMA-based transport assays. We found that, despite its simple selectivity filter, KtrB selectively binds K+ with micromolar affinity. Rb+ and Cs+ bind with millimolar affinities. However, only K+ and the poorly binding Na+ are efficiently translocated, based on size exclusion by the gating loop. Importantly, the physiologically required K+ over Na+ selectivity is provided by the channel’s high affinity for potassium, which interestingly results from the presence of the sodium ions themselves. In the presence of the KtrA subunit, sodium ions further decrease the Michaelis–Menten constant for K+ uptake from milli- to micromolar concentrations and increase the Vmax, suggesting that Na+ also facilitates channel gating. In conclusion, high binding affinity and facilitated K+ gating allow KtrAB to function as a selective K+ channel. |
| Databáze: | OpenAIRE |
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