Microtubule polarity arises from the head-to-tail orientation of alpha-beta tubulin heterodimers in the microtubule lattice. The identity of the polypeptide at each end of the microtubule is unknown, but structural models predict that the beta-tubulin end contains an exchangeable guanosine triphosphate (GTP) binding site. When GTP-coated fluorescent beads were incubated with microtubules, they bound specifically to plus ends, suggesting that tubulin is oriented in microtubules with beta-tubulin toward the plus end.