SMAD (protein)
Encyclopedia
SMADs are intracellular protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s that transduce extracellular signals from transforming growth factor beta ligands to the nucleus where they activate downstream TGF-β gene transcription.

The SMADs, which form a trimer of two receptor-regulated SMADs and one co-SMAD, act as transcription factors that regulate the expression of certain genes.

Classes

There are three classes of SMAD:
  1. The receptor-regulated Smads (R-SMAD
    R-SMAD
    R-SMAD stands for receptor-regulated SMAD. Smads are transcription factors that transduce extracellular TGF-ß superfamily ligand signaling from cell membrane bound TGF-ß receptors into the nucleus where they activate transcription TGF-ß target genes...

    ) which include SMAD1
    Mothers against decapentaplegic homolog 1
    Mothers against decapentaplegic homolog 1 also known as SMAD family member 1 or SMAD1 is a protein that in humans is encoded by the SMAD1 gene.-Nomenclature:...

    , SMAD2
    Mothers against decapentaplegic homolog 2
    Mothers against decapentaplegic homolog 2 also known as SMAD family member 2 or SMAD2 is a protein that in humans is encoded by the SMAD2 gene. MAD homolog 2 belongs to the SMAD, a family of proteins similar to the gene products of the Drosophila gene 'mothers against decapentaplegic' and the C....

    , SMAD3
    Mothers against decapentaplegic homolog 3
    Mothers against decapentaplegic homolog 3 also known as SMAD family member 3 or SMAD3 is a protein that in humans is encoded by the SMAD3 gene. SMAD3 is a member of the SMAD family of proteins.The human SMAD3 gene is located on chromosome 15...

    , SMAD5
    Mothers against decapentaplegic homolog 5
    Mothers against decapentaplegic homolog 5 also known as SMAD5 is a protein that in humans is encoded by the SAMD5 gene.SMAD5, as its name describes, is a homolog of the Drosophila gene: "Mothers against decapentaplegic". It belongs to the SMAD family of proteins, which belong to the TGFβ...

     and SMAD8/9
    Mothers against decapentaplegic homolog 9
    Mothers against decapentaplegic homolog 9 also known as SMAD9, SMAD8, and MADH6 is a protein that in humans is enocoded by the SMAD9 gene....

  2. The common-mediator Smad (co-SMAD) which includes only SMAD4
    Mothers against decapentaplegic homolog 4
    SMAD family member 4, also known as SMAD4, is a protein that in humans is encoded by the SMAD4 gene.SMAD4 is a 552-amino acid protein involved in cell signaling. It belongs to the Darfwin family of proteins that modulate members of the TGFβ protein superfamily...

    , which interacts with R-SMADs to participate in signaling
  3. The antagonistic or inhibitory Smads (I-SMAD) which include SMAD6
    Mothers against decapentaplegic homolog 6
    SMAD family member 6, also known as SMAD6, is a protein that in humans is encoded by the SMAD6 gene.SMAD6 is a protein that, as its name describes, is a homolog of the Drosophila gene "mothers against decapentaplegic". It belongs to the SMAD family of proteins, which belong to the TGFβ superfamily...

     and SMAD7
    Mothers against decapentaplegic homolog 7
    Mothers against decapentaplegic homolog 7 or SMAD7 is a protein that in humans is encoded by the SMAD7 gene.SMAD7 is a protein that, as its name describes, is a homolog of the Drosophila gene: "Mothers against decapentaplegic". It belongs to the SMAD family of proteins, which belong to the TGFβ...

    , which block the activation of R-SMADs and Co-SMADs.

Nomenclature

The SMAD proteins are homologs of both the drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

 protein, mothers against decapentaplegic
Mothers against decapentaplegic
Mothers against decapentaplegic is a protein from the SMAD family that was discovered in Drosophila.The SMAD proteins are homologs of both the Drosophila protein "mothers against decapentaplegic" and the C. elegans protein SMA. The name is a combination of the two...

 (MAD) and the Caenorhabditis elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

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
Decapentaplegic
Decapentaplegic is a key morphogen involved in the development of the fruit fly Drosophila melanogaster. It is known to be necessary for the correct patterning of the fifteen imaginal discs, which are tissues that will become limbs and other organs and structures in the adult fly. It has also been...

in the embryo.
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