User:Andy Maloney/Notebook/Lab Notebook of Andy Maloney/2009/06/02/Measurement of the Force-Velocity Relation for Growing Microtubules

Disclaimer

These are my notes on this paper. If I've said something wrong, please point it out on the talk page. Please do not take my notes as the end all to the paper. Read it yourself.

Paper

Measurement of the Force Velocity Relation for Growing Microtubules

Review

• Goal - They aim to investigate forces produced by microtubules from their polymerization. In other words, they are investigating how much force a polymerizing microtubule can exert on something.
  • Note: This is important to investigate because it may help to determine the forces necessary to move cells. I vaguely recall talking to Evan Evans about some interesting things about cellular motion and the breakdown and formation of actin.
• They measured the forces a microtubule will exert as it polymerizes by measuring their buckling strength. This is an indirect observation and my first inclination is to not trust it in its entirety but, I will allow this statement until after I read the article they reference on the subject.
• One cool thing about this paper is how they seeded and then grew microtubules. My previous post talked about microtubule seeded growth. I never thought to do this. This is what they did:
  • Etch 30 µm wide by 1µm deep channels in glass with an overhang, see fig. 1.
  • Coat the channels with streptavidin.
  • Load the channels with "short" (they don't describe what short is) stabilized microtubules and then after attachment, allow polymerization to occur.
• They talk about a critical buckling force twice before they define what it is. They did reference a paper the first time mentioning it but, that looses style points in my mind but there is nothing wrong with it.
• They note that the force on a buckling microtubule is greater for small guys than it is for big ones. I think this relates to the way tubulin bends as the microtubule curves.
• There is a typo on page 4. It should read "α = the on rate" not "αc = the on rate".
• They note that as a microtubule encounters a wall, they slow polymerization. It does so in an exponential way.
  • They argue this thermodynamically by saying the ratio of on/off rates should be equation 1. I think what they call "critical tubulin concentration" is really the "free" tubulin concentration. Equation 1 is nothing more than a way to define the Boltzman factors used in equation 2.
• In equation 2, I think q is related to the normalized ratio of β(f<sub<p)/β(0). Now since α and β are basically free and bound tubulin in solution, the total concentration is α + β = q. Hence the factors of q and (1-q) in the exponential. I really should derive these equations...

Take home

• The importance of this article that they were able to make theoretical lower and upper bounds for microtubule forces and then show through experiment that the forces fell within these bounds.
• They also show that as the microtubules encounter a wall, they grow slower. This slowing may induce something in a cell that tells it it has hit a wall. Oh so cool...
• I hope they read this because this is one bad ass experiment. I think that they could make a photoetched piece of glass that has the microtubule polymerizing in only one direction. Use a laser tweezer to place a bead in front of the polymerizing microtubule and measure forces on the bead directly.
  • Steve Koch 16:29, 3 June 2009 (EDT): Yup, you are onto something! http://www.citeulike.org/user/skoch3/article/3273642
• They modeled microtubules as a homogeneous elastic rod. I wonder if using a Worm Like Chain description would do anything to their results.
• Ha! These guys reference BRB80 buffer as Brinkley's reassembly buffer.