June 25, 2013 | David F. Coppedge

Brain Cells Have a Twist-Open Gate

The structure of an important potassium-ion gate in the membrane of brain cells has been found to open like a camera iris.

In Nature, Eitan Reuveny described a paper in the same issue by Whorton and MacKinnon that reveals how an important molecular gate named GIRK2 works in the membranes of brain cells.  These gates control the passage of potassium ions (K+) to the outside of the cell, a process that changes the electrical charge between inside and outside, allowing conduction of electrical signals.  His description, “Ion channel twists to open,” compares the twisting motion of the components to a camera iris: “The conformational changes that open the inner helical gate are comparable to the widening of a lens aperture by hand-rotating the aperture ring.

But whose hands rotate the ring?  The cylindrical channel that allows passage of K+ ions is surrounded by four complex proteins that lock into the cell membrane.  Activation of these “hands” by G-protein coupled receptors makes them turn the channel, opening it just a bit wider, but not enough, to allow the ions through.  From there, random perturbations may permit the ions to “burst” through the narrow opening as observed in living cells.  The channel also contains a “selectivity filter” to ensure only K+ ions can make it through.

Reuveny began by explaining the importance of these channcels:

Ion channels are the main units responsible for the electrical activity in our body. They constitute a large family of some 400 proteins in humans. A subfamily of these proteins consists of four GIRK channels, which specialize in converting chemical signals — mostly those of neurotransmitter molecules such as acetylcholine, dopamine, serotonin and adrenaline — into electrical ones in heart cells and neurons. They are therefore essential for controlling heart rate and the activity of neural circuits.

Roderick MacKinnon received the Nobel Prize in Chemistry in 2003 for his work on ion channels (see “Wonders of the Salt Gate” 1/17/02, also 3/12/02, 5/29/02, 5/01/03).  This paper contributes to the “long-awaited crystal structure of the mammalian GIRK2 channel in complex with two subunits of a G protein (a dimer of the Gβ and Gγ subunits), providing information about their mechanism of opening,” Reuveny said.  Ion channels such as GIRK2 literally “pump” ions against the direction of osmosis in order to set up the voltage necessary for electrical transmission.

It’s nice to see MacKinnon still at work uncovering the secrets of these cellular gates a decade after receiving his Nobel.  In the past 10 years, has he seen the light of the Darwinists?  Has he found evolution essential to explain how these exquisite molecular gates work?  Nope.  Neither MacKinnon or Reuveny even mentioned it.

Thinking about the action of these gates in slow-motion is amazing enough.  Realize, though, that they act lightning-fast, allowing your heart rate to adjust and allowing chemicals and ions to speed through the brain at the speed of thought.  Just like in 2002, this is phenomenal evidence for intelligent design.

 

 

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