GTP-tubulin draft

While I know GTP-tubulin is what tells the microtubule to polymerize and GDP-tubulin tells the microtubules to depolymerize, I would assume something must initiate/signal the GTP or GDP to bind to tubulin. I am curious how this signaling system works and how the microtubule knows whether or not it wants to lengthen or shorten. Microtubules have a variety of important functions in the cell. They help with chromosome segregation, cell structure, cell movement, and directionality. The article does not say exactly how microtubules influence directionality, but I would think that the motor proteins, especially kinesin and dynein, play a large role in direction and movement.