Protofilament

Protofilament is a linear polymer of tubulin heterodimers (alpha-tubulin and beta-tubulin) that stacks end to end to form one of the longitudinal strands of a microtubule. In most eukaryotes, microtubules are hollow cylinders comprised of about 13 protofilaments arranged around a central lumen; these protofilaments interact laterally to create the cylindrical wall and a seam where the lattice has a slightly different alignment, known as a B-lattice arrangement.

Protofilaments are polar, with a distinct plus end that preferentially elongates by the addition of GTP-tubulin and a minus end that is often anchored at microtubule organizing centers. Subunits within a protofilament contain bound GTP on the beta-tubulin that is hydrolyzed after incorporation, contributing to dynamic instability of microtubules. The GTP cap at the growing plus end promotes stability; loss of this cap can trigger rapid depolymerization, during which protofilaments peel outward as curved sheets from the microtubule end.

Variation exists in the number of protofilaments per microtubule across species and cell types, typically 11 to 16, with 13 being most common in many mitotic and cytoplasmic microtubules.

Protofilaments serve as tracks for motor proteins such as kinesins and dyneins and play essential roles in cell shape, organelle positioning, and chromosome segregation during cell division. They can be studied by electron and cryo-electron microscopy, which reveal their staggered lateral contacts and the seam in the lattice.