Structural proteins can form relatively rigid biological components. Most structural proteins are fibrin; collagen and elastin are key components in connective tissues (such as cartilage), while keratin is found in hair, nails, feathers, hooves and some shells. In addition, actin and tubulin are spherical soluble proteins as monomers, but once they are multimerized, they form long rigid fibers to compose the cytoskeleton to maintain the size and shape of the cells.