integrin

Imagine a climber who must both grip the rock face and tie that grip to a harness on their own body, so the cliff and the climber become one mechanical system. A cell sitting on the matrix outside it has the same need: it must grab the matrix and connect that grip to its own internal framework, both to hold on and to feel what the surface is doing. Integrins are the molecules that do this — surface proteins that clamp onto the extracellular matrix on the outside and tie into the cytoskeleton on the inside.

An integrin is a protein made of two parts (an alpha and a beta subunit) that span the cell membrane. The outside end binds to matrix proteins such as fibronectin, laminin, and collagen, often recognizing a short three-amino-acid tag in them. The inside end connects, through a cluster of adaptor proteins, to the actin cytoskeleton. Crucially, an integrin can switch shape between a low-affinity, off state and a high-affinity, gripping state, and it passes information both ways: the cell can decide to make integrins grip harder, and the grip itself reports back to the cell about how stiff or stretched the matrix is. This two-way signaling is called inside-out and outside-in.

Integrins are the main link in cell-matrix adhesion, the counterpart to cadherins in cell-cell adhesion. They let cells crawl by gripping the matrix at the front and letting go at the back, they hold skin onto its base through hemidesmosomes, and they let immune cells stop and exit blood vessels. Because they sense force, they are central to mechanotransduction. They also figure in disease and medicine: anti-clotting and anti-inflammatory drugs work by blocking specific integrins. A common misconception is that integrins are passive anchors; in fact they are active, switchable, signal-carrying machines that the cell tunes moment to moment.