How many strands make a microtubule, and why it matters

Every cell in your body has a skeleton. It is not made of bone but of protein filaments that hold the cell’s shape, carry cargo from one place to another, help cells move, and pull the chromosomes apart when a cell divides. The stiffest of these filaments are microtubules: hollow tubes thousands of times thinner than a hair, which work both as scaffolding and as tracks.

A tube made of strands

A microtubule is built from a protein called tubulin. Pairs of tubulin units stack head to tail into long strands, called protofilaments, and the strands sit side by side to close into a tube. Most microtubules have 13 strands, but this is not a fixed rule: depending on the organism and the cell type, the number can range from 9 to 16. Microtubules assembled in the lab also come out with different numbers of strands, depending on which tubulin is used and on the molecules added to stabilise them.

That variety raises a simple question. If cells can build tubes with more or fewer strands, does the choice change how the tube behaves? One idea in the field is that cells might tune the number of strands to adjust the mechanics of their microtubules.

Listening to the tube vibrate

Proteins are never still. Their atoms jiggle constantly, and those small movements spread from one part of a structure to another, a bit like a tremor passing through a scaffold. In an earlier study we followed how these vibrations travel through the wall of a microtubule. We found that one of the two tubulin units, beta-tubulin, does most of the work of passing mechanical information from one strand to the next, and that this pathway is interrupted at the seam, the one line along the tube where neighbouring strands meet in a different way.

In the new study we asked what happens when the number of strands changes. We combined three computational methods: molecular dynamics simulations, which follow the motion of every atom; network analysis, which turns those motions into a map of the routes along which vibrations travel; and elastic network models, which describe the whole tube as a set of connected springs so that its stiffness can be estimated.

What changed with more strands

Adding strands did more than make the tube wider. It changed how neighbouring tubulins touch one another, changed how water-repelling their side surfaces are, and changed the motion of a small, flexible loop of tubulin known as the M-loop.

The clearest result was a trade-off. As the number of strands grew, the routes that carry vibrations through the wall changed, and so did the tube’s resistance to bending. Wider microtubules passed mechanical information along less efficiently, but they offered more structural support.

Why it matters

These results suggest that the number of strands is not a detail: it may act as a design parameter that sets how a microtubule balances strength against the ability to carry mechanical signals. The same building block, arranged differently, gives a different material.

This matters for understanding how cells sense and respond to forces, including nerve cells, which rely on microtubules along their long extensions. It is also a useful lesson for anyone who designs materials inspired by biology. A natural next step is to test these predictions against experiments on microtubules with known numbers of strands.

Read the paper: Zizzi, Cannariato, Miceli, Morbiducci, Deriu, Journal of the Royal Society Interface 22(233), 20250556 (2025). doi:10.1098/rsif.2025.0556

Related: Cannariato, Zizzi, Pallante, Miceli, Deriu, Biomechanics and Modeling in Mechanobiology 23(2), 569–579 (2024). doi:10.1007/s10237-023-01792-5