any of several fibrous proteins of cells and tissues once thought to be insoluble but now known to be dissolved by dilute solutions of acids such as citric and acetic.

The two most important classes of scleroproteins are the collagens and the keratins. Others include fibroin, which forms about 67 percent of the content of natural silk (the remainder is the protein sericin); elastin, a structural protein of elastic fibres that occurs together with collagen in many tissues; certain proteins of marine sponges (spongin) and corals (gorgonin, antipathin); flagellin, a structural protein in the whiplike structures (flagella) of certain bacteria; and reticulin, found with elastin and collagen in mammalian skin. See also collagen; keratin.