4.16: Structural Protein Function
Structural proteins are a category of proteins responsible for functions ranging from cell shape and movement to providing support to major structures such as bones, cartilage, hair, and muscles. This group includes proteins such as collagen, actin, myosin, and keratin.
Collagen, the most abundant protein in mammals, is found throughout the body. In connective tissue, such as skin, ligaments, and tendons, it provides tensile strength and elasticity. In bones and teeth, it mineralizes to form hard tissues and contributes to their load-bearing capacity. In addition to structural support, collagen can also interact with cell surface receptors and other intermediate molecules to regulate cellular processes, such as growth and migration, which involve changes in cell and tissue shape.
Structural proteins form the basic framework of the cell cytoskeleton. The cytoskeleton is made up of three types of filaments, microfilaments, intermediate filaments, and microtubules, and each is composed of different structural proteins. The microfilament is formed when actin self-polymerizes into long repetitive structures. These actin filaments contribute to cell shape and organization; additionally, microfilaments can also contribute to cell movement and division, when it acts in conjunction with myosin. The composition of intermediate filaments varies based on cell type. There are around 70 different genes that code for various intermediate filaments. Intermediate filaments in epithelial cells contain keratin, peripheral neurons contain peripherin, and the sarcomere in muscle cells contains desmin. The primary structural function of these filaments is to reinforce cells and organize them into tissues. Microtubules are made up of structural proteins called tubulins. Tubulins self-assemble to form microtubules that contribute to the organization of the cytoplasm, including the location of the organelles. Microtubules are also essential for mitosis and cell division.
As structural proteins are widespread, a mutation in a gene that codes for any of these proteins can have severe detrimental effects. For example, a mutation in a gene coding for collagen can result in a condition known as osteogenesis imperfecta, which is characterized by weak bones and deformities in connective tissues. Different mutations in a collagen gene can result in Alport syndrome, which is characterized by problems in organs such as kidneys, eyes, and ears.