 Structural proteins confer <unk> and <unk> to otherwise @-@ fluid biological components . Most structural proteins are fibrous proteins ; for example , <unk> and <unk> are critical components of <unk> tissue such as cartilage , and keratin is found in hard or <unk> structures such as hair , nails , feathers , hooves , and some animal shells . Some globular proteins can also play structural functions , for example , actin and <unk> are globular and soluble as monomers , but <unk> to form long , stiff fibers that make up the cytoskeleton , which allows the cell to maintain its shape and size . 
