Scientists at St. Jude Children’s Research Hospital are studying voltage-gated ion channels (VGICs). Their work revealed a previously unknown inactivation mechanism for one of these channels that plays an important role in how neurons and muscles respond to electrical signals sent by the nervous system. An article about work appeared today in Molecular cell.
VGICs are transmembrane proteins that form a pore that opens and closes to allow ions to pass into or out of a cell. Cells such as neurons and muscle cells respond to electrical signals by opening (turning on) and closing their VGICs. Proper activation and closure of VGICs allows these cells to properly coordinate their functions.
The researchers used cryogenic electron microscopy (cryo-EM), biochemistry and electrophysiology approaches to study a VGIC called Kv4. Kv4 mutations are linked to neurological and cardiac conditions. Understanding how Kv4 works can help researchers identify therapeutic strategies to treat these disorders.
“Neural communication is based on how electro-signals are transmitted, which is mediated by the action of proteins in the neuronal membrane. Many researchers are interested in studying this process, but it has been difficult to capture,” corresponding author Chia-Hsueh said. Lee, Ph.D., St. Jude Department of Structural Biology. “We were able to capture multiple states of this specific ion channel to get a better picture of how this protein works in molecular detail. We were pleased to find that Kv4 works in a way that is distinct from other VGIC types.
The researchers were able to complement and validate the structural findings with electrophysiology work from collaborators at the University of California, San Francisco.
Finding out why the car won’t start: understanding Kv4 inactivation
VGICs occupy different states to function. Channels can transition from idle/closed state to active/open state. Think of a car: when it’s off, it’s like the VGIC in idle/closed state. When you have it on and driving, it’s like the VGIC in the on/open state. However, Kv4 can also enter an inactivated state, where the pore is closed and unresponsive. Imagine a car with the engine on and you step on the accelerator, but the car won’t start because the handbrake is on.
The researchers wanted to understand how Kv4 switches between these different states. Using cryo-EM, they initially captured the channel in three different conformations (shapes), corresponding to activated/open, inactivated and intermediate states. These structures revealed the mechanisms behind Kv4 inactivation, which featured an unexpected symmetry breaking from quadruple to double symmetry.
Like other VGICs, Kv4 is made up of four identical copies of a protein (imagine a four-leaf clover), and in the activated/open and intermediate states, all four copies adopt the same conformation. On the other hand, in the inactivated state, the two pairs facing each other have different conformations. To capture Kv4 in its quiescent/closed state, researchers had to “lock” the channel in this position, using protein engineering and certain reagents.
This is the first time researchers have identified the gated-state inactivation mechanism, and the approaches used here could be applied to other ion channels.
“I think our study is quite exciting for the field because we were able to obtain several structures related to functional states for an ion channel,” said first author Hongtu Zhao, Ph.D., St. Jude Structural Biology. “Being able to determine multiple structures of the same protein and in a single study allowed us to derive a lot of information by comparing them. It really reflects the power of cryo-EM.”
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