Figure 5.

Structural model explaining dynamic instability and its possible relationship with the GTP cap model. (a) Microtubules with two-dimensional sheets at their end are in a stable growing state. GTP-tubulin (in green) contained in the sheet could explain part of its stabilization capacity. When the tube closes, a few subunits in a GTP state at the extremity would be sufficient to maintain the microtubule in a growing state. (b) As soon as the GTP is hydrolyzed (c), the microtubule will explode because of the weak lateral interaction between GDP-tubulin (d). Blunt ends (b and c) are unstable because the loss of the GTP-tubulin subunits triggers them in a shrinking state, whereas sheets are always stable.