What are the tubulin subunits that form microtubule strands?

Microtubules are long, cylindrical structures in eukaryotic cells composed of tubulin dimers. Each dimer consists of two closely related protein subunits—α-tubulin (alpha-tubulin) and β-tubulin (beta-tubulin). 

These subunits bind head-to-tail to create linear protofilaments that align side-by-side to form a hollow tube. Both α and β tubulins bind guanine nucleotides (GTP or GDP). In α-tubulin, the GTP is non-exchangeable and remains trapped, providing structural stability. In β-tubulin, GTP can be hydrolyzed to GDP, driving dynamic assembly and disassembly of the microtubule. 

These dimers are highly conserved among species and serve as scaffolds for intracellular transport, chromosome movement, and cell shape maintenance. The combination of α and β subunits gives microtubules their characteristic polarity—α-tubulin forms the minus end and β-tubulin the plus end—allowing directional growth and interaction with motor proteins such as kinesin and dynein.