Forisomes are proteins occurring in the sieve tubes of Fabaceae. Their molecules are about 1-3 µm wide and 10-30 µm long. They expand and contract anisotropically in response to changes of electric field, pH, or concentration of Ca ions. Unlike most other moving proteins, the change is not dependent on ATP.
Forisomes function as valves in sieve tubes of the phloem system, by reversibly changing shape between low-volume ordered crystalloid spindles and high-volume disordered spherical conformations. The change from ordered to disordered conformation involves tripling of the protein's volume, loss of birefringence present in the crystalline phase, 120% radial expansion and 30% longitudinal shrinkage. In Vicia it was shown that forisomes are associated to the endoplasmic reticulum at sieve plates. There are evidences that the forisomes's behavior could depend on Ca changes provoked by Ca-permeable ion channels, located on the endoplasmic reticulum and plasma membrane of sieve elements.