A microtubule protofilament is formed by the: lateral association of only γ-tubulin subunits. head-to-tail association of only γ-tubulin subunits. lateral association of tubulin dimers. head-to-tail association of tubulin dimers.

head-to-tail association of tubulin dimers.

Explanation:

Microtubules are made up of a single type of globular protein, called tubulin. Tubulin is made up of two 55 kD polypeptides, α-tubulin and β-tubulin. Tubulin dimers are polymerized into microtubules, consisting of 13 linear protofilaments coupled around a hollow core. These protofilaments are composed of head-to-tail matrices of tubulin dimers, which are arranged in parallel.