TrkH Potassium Ion Transporter
Ion TrafficTo control the concentration of ions, cells have evolved a complex collection of transporters and channels that control a constant traffic of ions across cell membranes. Some of these proteins, such as bacterial porins, are simple holes through the membrane, allowing anything smaller than a given size to pass. Others, however, are highly specific, selecting and transporting only one particular type of ion. Researchers at NYCOMPS have solved the structure of a new transporter for potassium ions, and discovered the atomic basis for its selectivity.
Potassium vs. SodiumIt is particularly challenging to build a channel that is specific for potassium, that doesn't also allow sodium to pass. Sodium and potassium both have a similar charge, so this doesn't provide a way to discriminate between the two. In addition, sodium ions are smaller than potassium, so you can't simply create a pore that measures the size. Previous structures of potassium channels have shown that cells use a trickier method to choose potassium ions by mimicking the way potassium ions interact with water.
Selecting PotassiumPotassium channels contain a "selectivity filter" with a very specific arrangement of oxygen atoms, as shown in the Jmol image below. These oxygen atoms perfectly mimic the shell of waters that normally surround the ion when it is free in solution. The water shells are quite different between sodium and potassium ions, so by making the arrangement in the pore similar to the potassium hydration shell, the protein can favor the desolvation and passage of potassium rather than sodium. By taking advantage of this difference, some potassium channels allow only a single sodium ion to pass for every 10,000 potassium ions that flow through.
Structures for SpecificityThe TrkH structure, available in PDB entry 3pjz, adds several new wrinkles to the story of potassium transport. The previously studied channels are composed of four identical subunits that together form a symmetrical pore, with each subunit providing one fourth of the selectivity filter. The sequence of this filter is also highly constrained and the slightest mutation will abolish the selectivity. The channel in TrkH, on the other hand, is formed by a single protein chain. This chain forms all of the familiar functional structures needed for potassium passage, including a cage of oxygen atoms to act as the selectivity filter, and a set of pore helices angled to point their negative ends towards the potassium site. Surprisingly, the sequence of amino acids has several changes from the symmetrical channel proteins, showing that there are alternative ways to achieve specificity for potassium. However, although TrkH is known to select potassium ions over smaller ones like sodium and lithium, researchers are still testing whether it is as selective as the symmetrical potassium channels.
Transport of PotassiumIn cells, TrkH forms a dimer, with two side-by-side pores, and works along with several other proteins to transport potassium across the cell membrane. The structure of TrkH suggests its role as a potassium-specific channel, although biochemical studies have shown that the rate of passage is significantly slower than in other potassium channels. This may be due to a small loop that partially blocks the pore. It is still a mystery, however, whether TrkH is the engine that performs potassium transport, or simply works as part of a larger machine, acting as the filter that sorts potassium ions from other ions.
Potassium Ion Transporter TrkH (PDB entry 3pjz)
The selectivity filter of TrkH includes a precise array of oxygen atoms that mimic the water structure around a free potassium ion. In this Jmol image, the potassium ion is shown in magenta, surrounded by the eight oxygens provided by the protein peptide groups. Use the buttons to zoom out to view the entire protein, to highlight the four pore helices that point their negative ends towards the ion, and to highlight the small intramembrane loop that partially blocks the pore and may be important for controlling the flow of ions through the transporter.
- Cao, Y. et al. Crystal structure of a potassium ion transporter, TrkH. Nature doi:10.1038/nature09731
- Doyle, D. A. et al. The structure of the potassium channel: molecular basis of K+ conduction and specificity. Science 280, 69-77 (1998).