 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 . 
 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 <a href='drop_aux' title='1'><mark>are</mark></a>globular and soluble as monomers , but polymerize to form long , stiff fibers <a href='null_relcl' title='2'><mark>that</mark></a>make up the cytoskeleton , which <a href='uninflect' title='3'><mark>allows</mark></a> the cell to maintain its shape and size . 
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 globular and soluble as monomers , but polymerize to form long , stiff fibers make up da cytoskeleton , which allow the cell to maintain its shape and size .