tubulin and actin subunits
/ TOOB-yoo-lin and AK-tin SUB-yoo-nits /
Think of a brick wall. The wall can be tall, long, or curved, but it is always made of the same simple bricks. The cell builds its two most dynamic filaments — microtubules and microfilaments — out of just such repeating bricks. The brick for microtubules is the protein tubulin; the brick for microfilaments is the protein actin. Understanding these subunits explains why these fibers can be built and torn down so quickly.
Actin is a single small, roughly ball-shaped protein. Hundreds of actin proteins link end to end (and two such strands twist together) to form one microfilament. Tubulin comes as a tight pair of two closely related proteins, alpha-tubulin and beta-tubulin; these pairs stack to form the wall of a microtubule. Crucially, both kinds of subunit can carry a small fuel molecule (ATP in the case of actin, GTP in the case of tubulin) that is slowly spent after the subunit joins the filament. That spent fuel acts like a timer that destabilizes older parts of the fiber, driving the constant building-up and tearing-down that makes the cytoskeleton dynamic.
Because the cell keeps a large pool of free, unassembled subunits floating in the cytoplasm, it can grow a filament wherever and whenever it is needed simply by letting subunits snap into place, then recover them by letting the filament dissolve. This reusable-brick design is what lets one cell rapidly switch from a resting shape to a crawling shape to a dividing shape, all using the same molecules over and over.
The drug colchicine, used for gout, binds free tubulin and stops it from joining microtubules — a useful reminder that filaments only exist as long as their subunits keep snapping together.
Block the brick and the wall cannot stand.
The fuel molecules (GTP for tubulin, ATP for actin) are not used to power motion directly; they act as a clock that marks how long a subunit has been in the filament, controlling when it falls apart.