 Structural proteins confer stiffness and rigidity to otherwise @-@ fluid biological components . Most structural proteins are fibrous proteins ; for example , collagen and elastin are critical 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 . Some globular proteins can also play structural functions , for example , actin and tubulin are globular and soluble as monomers , but polymerize to form long , stiff fibers that make up the cytoskeleton , which allows the cell to maintain its shape and size . 
