Defying one of biology’s more persistent dogmas, a potassium channel combines functions once thought to be invariably asunder. This channel, which is found in common bacteria, incorporates a subunit that accomplishes passive transport, as well as a subunit that accomplishes active transport. In fact, the subunits work together, allowing bacteria to import potassium ions against extreme concentration gradients.
To detail the workings of the chimeric potassium-importing complex, an international scientific team did a little synergizing of its own. Specifically, the team combined findings derived from x-ray diffraction studies and cryogenic electron microscopy (cryo-EM) studies. The x-ray work, which was completed last year, established that the complex, which is called KdpFABC, contained one subunit with a channel architecture (KdpA), and another with an active transporter architecture (KdpB). “However, we felt that the transport mechanism that was proposed based on this structure was not convincing,” said Cristina Paulino, a researcher at the University of Groningen. “Something was missing.”
The missing something has now been found, thanks to cyro-EM, which revealed that the complex, while shifty in terms of its conformation, tends to settle into a couple of states more frequently than any other. Details about these states—and a discussion of their significance—appeared November 26 in the journal Nature Communications, in an article titled, “Cryo-EM structures of KdpFABC suggest a K+ transport mechanism via two inter-subunit half-channels.”
“Unexpectedly, the structures suggest a translocation pathway through two half-channels along KdpA and KdpB, uniting the alternating-access mechanism of actively pumping P-type ATPases with the high affinity and selectivity of K+ channels,” the article’s authors indicated. “This way, KdpFABC would function as a true chimeric complex, synergizing the best features of otherwise separately evolved transport mechanisms.”
Conformational changes during transport in KdpFABC. Green: channel-like subunit KdpA. Brown: p-type ATPase subunit KdpB. Purple spheres: potassium ions. Pink densities: entrance and exit tunnels as pink densities. [University of Groningen]
Paulino, one of the article’s corresponding authors, exclaimed, “This could very well become textbook stuff!
“The novel transport system combines two worlds that were seen as separate in many ways,” she continued. “Passive pores and active transporters use different mechanisms, have a different evolutionary history, and are mostly studied by different groups of scientists, each with their own nomenclature and conferences.”
The two transport worlds, passive and active, have long been thought to be mutually exclusive. “We have shown that it is not always so black and white,” Paulino asserted.
Paulino worked with Goethe University’s Inga Hänelt to lead the new investigation of KdpFABC, which is activated only in extreme cases, such as when the concentration inside the cells is about 10,000 times higher than the concentration outside. “In this situation,” explained Paulino, “where the concentration outside is so very low, you need extremely high affinity to bind the potassium ions, and the system needs to overcome a very high energy barrier to import the potassium ions.”
Essentially, KdpFABC contains a set of tunnels that allow potassium ions to enter the channel from the outside, bridge to the transporter unit, and then pass through another tunnel into the cell.
According to Paulino, the transport mechanism that was derived from the structure is unique. “The pore binds potassium ions on the outside with very high affinity. These potassium ions flow into the transport complex, which causes a change in conformation and is driven by the use of energy.”
The transport subunit uses ATP, the general energy carrier of the cells, to “fuel” the conformational change, which opens up a tunnel to the interior of the membrane. The change in conformation also reduces the affinity for potassium ions, so the ions can be released inside the cell.
“The system uses a channel which evolved to have a very high selective affinity for potassium ions with a pump that evolved for transport against a very high energy barrier,” summarized Paulino. Thus, the system blurs the boundaries between pores and active transporters.
