Like the petals of a flower.

Like the petals of a flower. Ion channels open and shut on a millisecond time scale and send thousands of charged ions flowing in or out of the cell to trigger or modulate nerve cell electrical activity, muscle contraction or tuning of “hair cells” in our inner ear.

Ion channels made by microbes as part of their competitive arsenal can be stunningly simple and highly effective: valinomycin, secreted by Streptomyces , is a circular ring of only 12 amino acids. (see:

But ion channels in our cells are enormously large and complex because they need to be regulated in sophisticated ways, in response to small molecules, voltage or pH. One such type of potassium channel opens when calcium binds and closes when calcium comes off. To understand how this works, researchers from Rod McKinnon’s group crystallized and solved for the structure of these membrane proteins in multiple forms and put the story together in this movie.

First, you will see a ring of 4 blue protein domains, sitting on top of 4 red protein domains, which form the circular gate that regulates the opening of the channel. Morphing between the calcium bound and unbound structures shows how the ring changes shape.

Next, we zoom in on the calcium (yellow sphere) site. Finally, we see the gating ring assembled with the ion channel itself (green helices) which forms the K+ conducting pore through the membrane. Watch how the gating ring opens “like the petals of a flower” to tweak the channel open or closed. Thus, the free energy from calcium binding is converted into mechanical energy of opening (gating) the ion channel. This is poetry in motion!


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12 Responses to Like the petals of a flower.

  1. I love ion channels. My favorite protein — is it weird that I have a favorite protein? — is the GABA(A)R receptor. It’s an incredibly elegant piece of design.

  2. Its like every cell in our body is constantly breathing in and breathing out, a three dimensional fractal of some insane complexity. I have been learning all about osmosis today and the role of ions in its workings.

  3. Rajini Rao says:

    Andreas Schou , of course! We all have our favorites..I hope to show case mine here at regular intervals like a proud parent 🙂 Suhail Manzoor , osmosis first, ion pumps and channels next!

  4. Rajini Rao How about some love for kinesins?

  5. DaFreak says:

    You should have saved this one for science sunday. :p

    I am getting lost in that wiki link you threw in there. Wiki always does this to me, start reading about something and you end up clicking through a 100 different links, each one more interesting than the last one… There goes my evening. ^^

    This binding of calcium to a potassium channel, what function does this have for the cell? I guess it changes the shape which allows something else to happen? How do you get from this to controlling a muscle? How do they communicate with each other? Yeah I really need to get a book on this. ^^

  6. Rajini Rao says:

    Andreas Schou , kinesins coming your way soon 🙂

  7. DaFreak says:

    So this ion door changes electrical potential by flooding the cells with ions but what function does this have? I would assume that a whole bunch of cells next to each other would all have to do this in lockstep to pass an electrical signal down the line but how does that happen? Do they all attract calcium to open their pores, one after another? How does this calcium show up at the right time?

  8. Rajini Rao says:

    Koen De Paus , calcium is the premier second messenger in the cell. You probably already know that calcium has a structural role in building bone and teeth..relatively uninteresting to me. Much more important is that it carries a signal that can be sensed by other calcium binding proteins. So it can trigger muscle contraction, synaptic vesicle fusion (to release neurotransmitter), cell division, turn on genes, etc. Ca2+ triggers muscle contraction by binding to a protein called troponin C which otherwise blocks the working of the contractile apparatus (actin fibers + myosin motors). Once troponin C binds Ca2+ and gets out of the way, the muscle can contract! Not for long, though, since calcium pumps quickly move the calcium out and the troponin C goes back to blocking the contractile apparatus. So it is all about calcium (one of my favorite ions)!

    Ca2+ activated K channels allow the Ca (chemical) signals to modify electrical excitability, which is controlled by the K channels. The channels described here are opened by voltage (inside gets more positive) and high calcium ..the let K flow out of the cell to make the inside more negative. As a result, they dampen the firing potential of neurons. (FYI, excitable cells fire action potentials when inside gets more if they become more negative instead, they are said to hyperpolarize and harder to fire). They are used to regulate muscle tone and fine tune hearing. If something I said does not make sense, please ask 🙂

  9. Rajini Rao says:

    Oops, we crossed posts, Koen De Paus ! Calcium goes in and out through pumps and channels..another story altogether. The ion door you mention, of potassium flowing out of the cell, is very important and a basic property of all cells. Because there is a 10x gradient of K+ ions (high inside, lower outside), when a K+ channel opens, K+ goes out until the chemical gradient is exactly balanced by the opposing electrical gradient (the negative charges left behind retard the movement of more K+). The result is an equilibrium, known as Nernst potential. This is also called the resting potential. Any change in this level, alters the ability to fire an action potential. I’ll try to cover what an action potential really is at some point 🙂 Basically, the charge reverses so the inside briefly becomes positive due to sodium ions rushing in through sodium channels. Now that I’ve thoroughly confused you, I should stop…


    Overall rating

  10. DaFreak says:

    Lol, I was expecting a specific function… Calcium seems to be quite the workhorse. :p

    I Can’t say I can form a detailed mental picture of what is really going on inside our cells or how they communicate but I assume that building such a picture takes a lifetime of study. ^^ Makes a bit more sense though, thanks for the explanation!

  11. Rajini Rao says:

    Koen De Paus , stick around and we biochemists will soon suck you into a vortex of dazzling cellular/molecular science! All we ask for in return, is some meager tidbit on the arcane physical world around us. Yes, calcium is all that important and more 🙂

  12. really good………collections reflects that u r a science lover

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