FIGURE 6.

Structural model of assembling tubulin heterodimers. The model shows that Ser¹⁶⁵ (green) in the α-tubulin subunit (blue) lies at the interface of two polymerizing α/β-heterodimers. Ser¹⁶⁵ participates in longitudinal contacts with a β-tubulin subunit at the plus end of a growing polymer and may influence GTP hydrolysis or GDP/GTP exchange on β-tubulin. The exchangeable GTP/GDP on β-tubulin and the nonexchangeable GTP on α-tubulin are indicated. Highlighted in brown is the site in β-tubulin phosphorylated by Cdk1 (Ser¹⁷⁴ in this model) (33). Molecular modeling of the α/β-heterodimer (Protein Data Bank entry 1TUB) was performed with Protein Explorer, version 2.80.