Collisions cause spontaneous organization of microtubules
The specific growth direction of plant cells is determined to a large extent by the orientation of what are known as 'microtubules' in the cells. These sturdy little tubes of tubulin proteins form a highly-ordered structure perpendicular to the direction of growth. Researchers at the Foundation for Fundamental Research on Matter’s Institute for Atomic and Molecular Physics (FOM AMOLF) have shown how this structure organizes and maintains itself. Although the tubes can grow and shrink in all directions, a form of Darwinian selection takes place. Collisions cause microtubules that are wrongly oriented to disappear, whereas those with the correct orientation survive. AMOLF group leader Bela Mulder is jointly publishing these results with his former post-doctoral researcher Rhoda Hawkins and PhD-student Simon Tindemans, who obtained his degree in 2009. The publication will be appearing in the authoritative journal Physical Review Letters on Monday 1 February 2010.
Living cells are well-organized, dynamic structures in which microtubules play an important role. These thin, stiff protein tubes have a diameter of 25 nanometres and can grow to several micrometres in length. Their lengths change continually because they can switch between periods of growth and shrinkage at random moments. Sometimes they disappear due to shrinkage, but new microtubules are also constantly being formed.
Microtubules have different functions. They facilitate cell division, they hold the cell nucleus in its place and they help plant cells to determine and maintain their direction of growth. So-called cortical microtubules, which are anchored to the inside of the cell membrane, are responsible for this latter function. Shortly after a cell division, almost all of these microtubules align themselves perpendicularly to the growth direction of the cell. The transverse cortical array which arises in this way is an orderly structure that serves as ‘rails’ for the construction of the cell wall in the right direction.
The researchers at AMOLF were looking for the underlying mechanism for this organized structure of microtubules in plant cells. Because microtubules are anchored to the cell membrane, they can only move by growth and shrinkage at the ends. Earlier research has shown that a growing end, or tip, can collide with another microtubule that is differently oriented. What happens when a collision like this takes place depends on the angle at which the two microtubules encounter each other. If this angle is less than about 40 degrees, 'zippering' is most probable: the incoming microtubule changes its trajectory and continues to grow in alignment (i.e. parallel to) with the other microtubule. If, on the other hand, the encounter angle is steeper, there is more likely to be a 'catastrophe': the incoming microtubule switches from growth to shrinkage. Another possibility, irrespective of the angle of collision, is a 'crossover': the incoming microtubule slides over the obstacle and continues in its original growth direction.
Mulder and his colleagues formulated a mathematical model for calculating the effect of repeated collisions. Surprisingly, although zippering ensures that colliding microtubules become co-aligned, this appears to have hardly any effect on the formation of the transversal cortical array. It is the collisions in which a growing microtubule switches to a shrinking one that determine the co-alignment. The microtubule in question is, as it were, sent on early retirement: its life is considerably shorter than that of a microtubule that is not subjected to any collisions. The microtubules that grow at right angles to the majority collide often and ultimately come off worst. A preferred orientation therefore arises spontaneously by means of a Darwinian selection principle. There must, however, be sufficient microtubules present to achieve the characteristic ordered structure. This corresponds with experimental observations: certain protein complexes in the cell ensure that this required minimum number of microtubules is indeed generated. Using specially-developed computer simulations, the researchers can track large numbers of microtubules individually for a long period of time. These simulations also show the microtubules that are arranged in parallel to be the survivors.