SMAD gene family

Genes in the SMAD gene family provide instructions for producing proteins that are involved in cell signaling, regulating the activity of particular genes, and cell growth and division (proliferation). The SMAD proteins are part of the transforming growth factor beta (TGF-β) pathway, which transmits signals from the outside of the cell to the nucleus. This type of signaling pathway allows the environment outside the cell to affect how the cell produces other proteins.

The signaling process begins when a TGF-β protein attaches (binds) to a receptor on the surface of the cell, which then turns on (activates) a group of SMAD proteins (called receptor-regulated SMADs or R-SMADs). The R-SMADs include SMAD1, SMAD2, SMAD3, SMAD5, and SMAD8. These R-SMADs then bind together in multiple protein groups (or complexes) with another SMAD protein, SMAD4, also called the common mediator SMAD (or Co-SMAD). Once the SMAD protein complexes form, they are transported to the nucleus. In the nucleus, the SMAD complexes bind to specific areas of DNA, where they control the activity of particular genes and regulate cell proliferation.

When the signaling pathway needs to be turned off, two SMAD proteins, SMAD6 and SMAD7 (known as the inhibitory SMADs or I-SMADs), inactivate the receptors on the cell surface. The I-SMADs are also thought to interfere with the formation of SMAD-SMAD4 complexes and interfere with the interaction between the SMAD complex and DNA in the cell nucleus.

Mutations in SMAD genes are associated with disorders caused by abnormal gene activity and cell proliferation. For example, mutations in the SMAD4 gene cause some cases of juvenile polyposis syndrome, which is characterized by the development of multiple tumor-like growths (polyps) in the gastrointestinal tract due to uncontrolled cell growth and division.