How do bacteria make decisions? Part 2

bacterial, motility, chemotaxis
The path bacterial cells swim tracked using motion tracking software

Yesterday, we looked at one of the first factors (or protein families specifically) that guide bacteria in their decisions. Which direction to travel is not the only decision needed to be made by bacteria (more coming soon). I wrote about methyl-accepting chemotaxis proteins (MCPs) briefly (more visuals to come). These proteins interact directly or indirectly by sensing changes caused by chemical compounds from the bacterium’s environment. Today I will briefly write about the next step towards decision making in a bacterial cell in regards to direction of travel. MCPs by themselves would be useless if they did not interact with some other cellular machinery. Lucky for bacteria, the MCPs are only the first of many specialized proteins for regulating direction of travel. If a chemical binds to an MCP outside the cell, how does the inside of the cell get the message? When chemicals bind to an MCP or stop binding to it, it slightly changes the structure of the MCP. It is thought association or dissociation of chemicals to MCPs causes a rotation like a slight turn of a door knob. Depending on if the MCPs are rotated or not changes the activity of another enzyme that interacts with MCPs, a protein called CheA (pronounced, ‘key A’). When CheA is active, it converts two other proteins into an active form, CheB and CheY. Confused yet? When CheY is in its active form, it interacts with the base of the flagellum and causes the flagellum to switch direction of rotation and causes the bacterium to change the direction it is moving. The take home message for the mechanism is this: when the MCPs are not interacting with chemicals (nutrients), CheA and CheY are active, and the flagellum switches rotation to make the cell go in a different direction. Hopefully for the cell, the new direction it is traveling will have more nutrients that will interact with the MCPs and block more changes in direction by CheA/CheY activity.

You might ask, if chemical compounds are always bound to the MCPs, what if the cell begins going to passed the best environment and needs to turn around? Good question. That is where the other protein activated by CheA comes in, CheB. What makes this system unique is the ability to adapt to current conditions (nutrient chemical levels) so the cell can respond to new information. A set of protein enzymes act upon part of the MCPs that can cause a change in how rotated (think door knob again) the MCP is. If the MCP is always interacting with a nutrient, a protein called CheR changes the MCP structure and causes rotation back towards the non-interacting form of the MCP. When CheA is active, it can activate CheB which reverses the changes caused by CheR. Ultimately, the whole system remains sensitive to new information over a wide range of nutrient concentrations (a 1000-fold range).

This whole system and its parts and regulation took several years for me to understand. I’m sure I did not do it justice, but hopefully you can get a small glimpse into the truth about “simple” bacteria.

In Part 3, I will discuss other decisions that bacteria make besides which direction to go in search of the promised land of milk and honey, or in this case, carbon and nitrogen sources.

A Gram-negative bacterial flagellum
A Gram-negative bacterial flagellum (Photo credit: Wikipedia)