Tubulin is a globular protein that serves as the fundamental building block for microtubules, which are major components of the cell’s internal support structure, the cytoskeleton. These hollow, cylindrical polymers maintain cell shape, facilitate cell movement, and organize the internal space of the cell. Microtubules are dynamic polymers that constantly grow and shrink. They perform functions ranging from intracellular transport to the precise segregation of chromosomes during cell division. The structure of tubulin enables this dynamic instability and mechanical strength.
The Core Tubulin Dimer
The fundamental unit of the microtubule is the tubulin dimer, composed of two distinct but closely related globular protein subunits: alpha (\(\alpha\))-tubulin and beta (\(\beta\))-tubulin. These subunits are tightly bound in a stable heterodimer that acts as the basic construction component. Although they share high amino acid sequence identity, the \(\alpha\)– and \(\beta\)-subunits have functionally distinct regions, especially concerning nucleotide binding.
Each tubulin monomer possesses a binding site for a guanosine triphosphate (GTP) molecule. The GTP bound to \(\alpha\)-tubulin is at the non-exchangeable site (N-site) and is permanently buried within the interface, meaning it is not hydrolyzed. Conversely, the GTP bound to \(\beta\)-tubulin occupies the exchangeable site (E-site) and is exposed to the solvent when the dimer is free. This E-site nucleotide can be hydrolyzed from GTP to GDP, a difference that dictates the protein’s assembly dynamics.
Protofilament Assembly
The first step in building a microtubule involves the linear association of tubulin dimers into long, single strands called protofilaments. This polymerization occurs through a head-to-tail stacking mechanism. The \(\alpha\)-tubulin subunit of one dimer links longitudinally to the \(\beta\)-tubulin subunit of the adjacent dimer.
This stacking creates a continuous chain of alternating \(\alpha\) and \(\beta\) subunits along the protofilament. The longitudinal contact between dimers is similar to the bond holding the \(\alpha\) and \(\beta\) subunits within a single dimer. This consistent, directional linkage imparts a distinct structural orientation to the protofilament. The protofilament is therefore a polar structure, with one end chemically and structurally distinct from the other.
Microtubule Wall Architecture
Multiple protofilaments associate laterally to form the complete, hollow, cylindrical structure of the microtubule. Most microtubules in mammalian cells are constructed from 13 parallel protofilaments. The resulting structure has a precise geometry, typically exhibiting an outer diameter of 25 nanometers and an inner diameter, or lumen, of about 17 nanometers.
The lateral contacts between adjacent protofilaments hold the cylindrical wall together. Throughout most of the wall, protofilaments align to create a seamless helical lattice, known as a B-lattice. In this arrangement, \(\alpha\)-tubulin neighbors \(\alpha\)-tubulin and \(\beta\)-tubulin neighbors \(\beta\)-tubulin across the lateral interface, forming a gentle, left-handed pseudo-helix.
Closing a 13-protofilament cylinder requires a single structural discontinuity called the seam. At the seam, the lateral association breaks the regular B-lattice pattern, resulting in a unique \(\alpha\)-tubulin to \(\beta\)-tubulin contact between the two adjacent protofilaments. Despite this different type of lateral bond, the overall stability of the microtubule is maintained across this boundary.
Structural Polarity and Nucleotide Binding
The head-to-tail assembly of the tubulin dimers gives the entire microtubule a distinct structural polarity, fundamental to its cellular function. Since all protofilaments align in the same direction, the microtubule has two chemically and kinetically different ends. The end where \(\alpha\)-tubulin is exposed is the Minus (-) end, and the end exposing \(\beta\)-tubulin is the Plus (+) end.
The Plus end is the site of faster growth and shrinkage, making it the more dynamic end. This behavior is linked to the nucleotide bound at the \(\beta\)-tubulin’s exchangeable E-site. When free dimers incorporate into the growing end, they carry GTP, which stabilizes the protofilament structure.
After incorporation, the \(\beta\)-tubulin subunit hydrolyzes its bound GTP into GDP and inorganic phosphate. This hydrolysis is not instantaneous, creating a temporary region of GTP-bound tubulin dimers at the Plus end, known as the GTP cap. The GTP cap stabilizes the microtubule by forcing protofilaments into a straight conformation with strong lateral contacts.
The loss of this stabilizing cap exposes the underlying GDP-tubulin lattice. GDP-bound tubulin is structurally less stable, causing a conformational change that promotes the protofilaments to curve outward. This instability leads to rapid depolymerization, a process known as catastrophe, where the microtubule shrinks by shedding curved GDP-tubulin dimers.