Other proteins transport molecules across cell membranes. Structural Proteins Structural proteins are very important for support. Collagen and elastin provide a fibrous framework in animal connective tissues, such as tendons and ligaments. Keratin is the protein of hair, horns, feathers, quills, and other skin appendages of animals. These proteins store amino acids.
Ovalbumin is the protein of egg white, used as an amino acid source for the developing embryo.
Casein, the protein of milk, is the major source of amino acids for baby mammals. Plants stor proteins in seeds. Hormonal Proteins Hormonal proteins coordinate the bodily activities. Insulin, ahormone secreted by the pancreas, helps regulate the concentration of sugar in the blood. Receptor Proteins Receptor proteins are built into the membrane of a nerve cell and they detect chemical signals released by other nerve cells.