How do α-tubulin and β-tubulin assemble into protofilaments?

α-tubulin and β-tubulin first form a stable heterodimer through non-covalent interactions. These dimers then align longitudinally in a head-to-tail arrangement—α of one dimer binding to β of the next to produce a protofilament. 

The orientation gives the polymer polarity: the α-end (minus) and the β-end (plus). Around 13 of these protofilaments associate laterally through weak interactions, forming the microtubule wall. The process is powered by GTP binding at the β-tubulin site, which promotes polymerization. When β-tubulin’s GTP is hydrolyzed to GDP after incorporation, it weakens dimer bonding, leading to depolymerization if not stabilized by a GTP cap. 

Thus, microtubule assembly is a highly dynamic equilibrium between addition and loss of dimers. Cellular proteins like γ-tubulin rings and microtubule-associated proteins (MAPs) guide this process by nucleating and stabilizing protofilament formation.