On the move
Proteins keep cells together
Smaller structures inside cells are often made of microtubules (tube-shaped polymers of the protein tubulin). Microtubules are very versatile and have evolved to work in a range of different ways. For example, microtubules form the cytoskeleton, an internal scaffold in cells. Microtubule structures are dynamic and are constantly being built, pulled apart and rebuilt.
Cilia, thin rods that protrude from the surface of cells, are bundles of microtubules. The most basic cilia move in the ebb and flow of fluid outside the cell, sensing changes in the environment. Motile cilia can move fluid themselves instead of just being moved by it. Millions of cilia move to sweep mucus up out of the lungs into the throat, removing inhaled dust and dead cells.
Flagella, which are larger, are also tightly arranged bundles of microtubules. They help cells move and can also be involved in sensing the environment outside the cell. Eukaryotes (cells with nuclei) from any species have microtubule, cilia and flagella proteins with very similar sequences and secondary structures. As such, these proteins are ‘highly conserved’. In prokaryotic cells such as bacteria, which do not have a nuclear membrane, the flagellum is made from a different protein called flagellin.
Motile cilia and flagella are powered by tiny cellular machines, which are also made from proteins. Molecular motors like this move muscles, power cilia, shift cargo along networks of microtubules inside the cell and organise chromosomes for cell division.Lead image: