Protofilaments

Protofilaments are the linear chains of tubulin heterodimers that form the longitudinal elements of microtubules, one of the main components of the cytoskeleton in eukaryotic cells. A microtubule is typically composed of about 13 protofilaments arranged in a hollow cylinder, though the exact number can vary from 11 to 15 in some species or conditions.

Each protofilament is built from alternating alpha- and beta-tubulin subunits in a head-to-tail arrangement, giving the filament a distinct polarity with a beta-tubulin-exposed plus end and an alpha-tubulin-exposed minus end. GTP binds at the exchangeable site of beta-tubulin; incorporation of dimers into the growing microtubule accompanies GTP hydrolysis, which weakens longitudinal interactions and contributes to dynamic instability.

Protofilaments interact laterally with neighboring protofilaments to form the hollow microtubule wall; these lateral contacts are important for stability but weaker than longitudinal contacts. Growth typically occurs at the plus ends, enabling rapid changes in length, shrinkage, and rescue events.

Microtubules serve as tracks for motor proteins such as kinesin and dynein, organize organelles, form the mitotic spindle, and build cilia and flagella. Protofilaments are thus foundational for processes such as chromosome segregation and intracellular transport.

Variation: while 13 protofilaments is common, some microtubules in certain organisms or organelles may have different numbers, and microtubule-associated proteins modulate assembly and stability.