Microtubules structure

Microtubules are polymers of a-dimer containing 1 molecule of a-tubulin and 1 molecule of b-tubulin. Both the a-tubulin and b-tubulin subunits of the free a,b-tubulin dimer bind GTP (as the conc. of GTP in the cell is much higher than GDP). GTP bound a,b- tubulin dimer is incorporated into existing microtubules by binding to an exposed a,b-tubulin dimer at the end of the polymer so each protofilament has alternate a-tubulin and b-tubulin subunits. Incorporation of the dimer causes hydrolysis of GTP bound b-tubulin to GDP bound b-tubulin. a-tubulin remains bound to GTP. The a-tubulin and b-tubulin subunits also make side contacts with other a-tubulin and b-tubulin subunits in adjacent protofilaments to make sheets of ~13 parallel protofilaments which zipper together to form a microtubule. This leaves a hole down the centre of the tubule - which is refered to as the lumen.