Microtubule - Structure

Structure

Microtubules are long, hollow cylinders made up of polymerised α- and β-tubulin dimers.

Tubulin dimers polymerize end to end in protofilaments which are the building block for the microtubule structure. Thirteen protofilaments associate laterally to form a single microtubule and this structure can then extend by addition of more protofilaments.

The lateral association of the protofilaments generates an imperfect helix with one turn of the helix containing 13 tubulin dimers, each from a different protofilament. The image above illustrates a small section of microtubule, a few αβ dimers in length. The number of protofilaments can vary; microtubules made up of 14 protofilaments have been seen in vitro.

Microtubules have a distinct polarity which is important for their biological function. Tubulin polymerizes end to end with the α subunit of one tubulin dimer contacting the β subunit of the next. Therefore, in a protofilament, one end will have the α subunit exposed while the other end will have the β subunit exposed. These ends are designated the (−) and (+) ends, respectively. The protofilaments bundle parallel to one another, so, in a microtubule, there is one end, the (+) end, with only β subunits exposed, while the other end, the (−) end, has only α subunits exposed. Elongation of microtubules typically only occurs from the (+) end.