One-Atom Linchpin

One-Atom Linchpin

A single calcium atom can make or break a bacterium's movement—and infectiousness.


One-atom linchpin. I'm Bob Hirshon and this is Science Update.

In order to move across a solid surface—and infect a host—some bacteria form tiny legs called pili. The pili stick to the surface like grappling hooks, then retract to pull the bacteria along. Now, University of North Carolina biochemist Matthew Redinbo and his colleagues have found that all this hinges on a single calcium atom. The atom binds to a protein in tiny molecular motors that control the legs. 

And if you take that atom away—if you change the protein so that it can't bind to the calcium—the bacteria can't make any legs.

Making the calcium bind permanently, on the other hand, prevented the bacteria from retracting their legs. Either way, the bacteria couldn't move. Redinbo says exploiting this would be medically valuable, but hard to do, since calcium is vital to so many other cellular functions. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Just as a car won't run without gasoline, many cellular functions require certain materials in order to work. Sometimes, the essential materials include elements like calcium, magnesium, or copper. Rarely, though, does one lone atom have the dramatic impact that it has in this study. If nothing else, the work shows that complex biological systems can be extremely sensitive and precisely designed.

To be clear, a single calcium atom—or lack of it—can't paralyze an entire bacterium. But one calcium atom makes the difference in each little molecular motor that creates and retracts the bacterium's pili. These pili, or "legs," are really just extensions of the bacterium's outer membrane. The calcium atom acts as a sort of on/off switch for each of the pili: when the calcium binds to the motor, the leg extends, and when the calcium detaches, the leg retracts. 

By coordinating the extension and retraction of many pili, the bacterium can drag itself along a solid surface, usually the surface of a host cell. It also relies on this ability to infect a host; after all, if the bacterium can't even attach itself to a host cell, it certainly can't get inside it. Redinbo's team found that when they altered the molecular motors so they couldn't bind to calcium, the bacterium couldn't make pili anymore. And when they altered the motor to make calcium bind to it permanently, the bacterium did make pili, but couldn't retract them, which means it couldn't pull itself along.

The obvious response to this finding would be to try and use it to fight disease. A number of significant disease-causing bacteria move and infect hosts in this way, including those that cause potentially deadly meningitis. However, because calcium serves a critical role in many human biological functions, it's very difficult to target this calcium-binding site without potentially harming the patient. Still, at least the goal is extremely simple, even if finding a means to that end is not.

Now try and answer these questions:

  1. What is the one-atom linchpin in this study?
  2. How does the calcium atom allow the bacterium to move?
  3. Making calcium bind permanently to the molecular motor makes the bacterium unable to move. So does preventing calcium from binding in the first place. Explain why, including the reasons for each.
  4. What does this study tell you about biological systems in general?

For Educators

How the Poliovirus Works, from the Smithsonian Institution, describes the way another infectious agent infiltrates cells.

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Toxicology 3: Toxicology and Human Health
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