αβ-tubulin grows by attachment of GTP, specifically to the E site, or the exchangeable site.

In αβ-tubulin, the N-site, or non-exchange site, is hidden within the binding region of the two monomers, and the E-site at β subunit is exposed on the surface.

The residues involved in binding of GTP (colored yellow) and GDP (colored orange) are highly conserved due to their important function. The residues directly involved in nucleotide binding are Asn-206 (colored cyan) and Asn-228 (colored pink), both of which are involved in specificity for GTP. α tubulin contains Glu-254 (colored red), which is an ideal residue for the hydrolysis of the nucleotide at the E-site, while β tubulin contains Lys-254 (colored green), which strengthens the monomer-monomer interaction by interaction with the GTP phosphate group at N-site.

1JFF (PDB)