Fibrous proteins are either long and narrow proteins or assemble to form long and thin structures. They contain repetitive units and usually consist of either alpha helices or beta sheets and, in rare cases, a mix of both. The amino acids in the primary structure often consist of repeating amino acid sequences. The role of fibrous proteins is primarily structural. Many are located in the extracellular matrix and are present in connective tissues to impart strength and joint mobility. They are not typically soluble in water but may be soluble in strong acids or bases. Collagen, keratin, elastin, silk, and fibrin are examples of fibrous proteins.

Collagen

Collagen fiber is made from fibrous protein subunits linked together to form a long, straight fiber. Collagen fibers, while flexible, have significant tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the body's movement.

Elastic Fiber

Elastic fiber contains elastin— a fibrous protein and lesser amounts of other proteins and glycoproteins. The main property of elastin is that it will return to its original shape after being stretched or compressed. Elastic fibers are prominent in elastic tissues found in the skin and the elastic ligaments of the vertebral column.

Reticular Fiber

Reticular fibers are also formed from the type III collagen proteins; however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body but are most abundant in the reticular tissue of soft organs, such as the liver and spleen, where they anchor and provide structural support to the organs' functional cells, blood vessels, and nerves.

This text is partially adapted from Openstax, Anatomy and Physiology, Section 4.3: Connective Tissue Fibers and Ground Substance