Structural proteins grant hardness and stiffness to otherwise-fluid biological components. The majority structural proteins are fibrous proteins; for instance, actin and tubulin are globular and soluble like monomers, but polymerize to form long, rigid fibers that encompass the cytoskeleton, which permits the cell to preserve its contour and amount. Collagen and elastin are dangerous components of connective tissue such as cartilage, and keratin is found in hard or filamentous structures such as hair, nails, feathers, hooves, and some animal shells.

Other proteins that provide structural tasks are motor proteins such as myosin, kinesin, and dynein, which are able to generate mechanical forces. These proteins are critical for cellular motility of unicell organisms and the sperm of many sexually reproducing multicellular organisms. They also generate the forces exerted by contracting muscles.