Spectrin

Spectrin is a flexible cytoskeletal protein that forms a meshwork beneath the cell membrane, often through assemblies of alpha and beta chains. Connections to membrane-associated components and actin help distribute mechanical stress. The red blood cell provides a striking example: its membrane skeleton supports repeated deformation through narrow vessels. Spectrin is also present in other cells, where related assemblies help organize membrane domains. ^[ref]

A modular protein can release additional contour length when a folded domain crosses a mechanical unfolding barrier. At almost fixed end separation this lowers fractional extension and tension, producing a saw-tooth force record. Successive worm-like chain fits can share backbone persistence length while their contour lengths increase. Rupture force and hysteresis depend on loading history; peak force alone is not an equilibrium unfolding free energy.