are a class of proteins that modulate the activity of transforming growth factor beta ligands
The SMADs form complexes, often with other SMAD's, enter the nucleus and serve as transcription factors.
There are three classes of SMAD:
- 1)The receptor-regulated Smads (R-SMAD) which include SMAD1, SMAD2, SMAD3, SMAD5 and SMAD9
- 2) The common-mediator Smad (co-SMAD) which include only SMAD4, which interact with R-SMADs to participate in signaling
- 3) The antagonistic or inhibitory Smads (I-SMAD) which include SMAD6 and SMAD7, which block the activation of R-SMADs and Co-SMADs.
The SMAD proteins are homologs of both the drosophila
protein, mothers against decapentaplegic
(MAD) and the C. elegans
protein SMA. The name is a combination of the two.
During Drosophila research, it was found that a mutation in the gene, MAD, in the mother, repressed the gene decapentaplegic in the embryo. The phrase "Mothers against" was added since mothers often form organizations opposing various issues eg. Mothers Against Drunk Driving or (MADD).