The L3 loop: a structural motif determining specific interactions between SMAD proteins and TGF-beta receptors

EMBO J. 1998 Feb 16;17(4):996-1005. doi: 10.1093/emboj/17.4.996.


Signal transduction specificity in the transforming growth factor-beta (TGF-beta) system is determined by ligand activation of a receptor complex which then recruits and phosphorylates a subset of SMAD proteins including Smads 1 and 2. These then associate with Smad4 and move into the nucleus where they regulate transcription. We have identified a discrete surface structure in Smads 1 and 2 that mediates and specifies their receptor interactions. This structure is the L3 loop, a 17 amino acid region that protrudes from the core of the conserved SMAD C-terminal domain. The L3 loop sequence is invariant among TGF-beta- and bone morphogenetic protein (BMP)-activated SMADS, but differs at two positions between these two groups. Swapping these two amino acids in Smads 1 and 2 induces a gain or loss, respectively, in their ability to associate with the TGF-beta receptor complex and causes a switch in the phosphorylation of Smads 1 and 2 by the BMP and TGF-beta receptors, respectively. A full switch in phosphorylation and activation of Smads 1 and 2 is obtained by swapping both these two amino acids and four amino acids near the C-terminal receptor phosphorylation sites. These studies identify the L3 loop as a determinant of specific SMAD-receptor interactions, and indicate that the L3 loop, together with the C-terminal tail, specifies SMAD activation.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Basic Helix-Loop-Helix Leucine Zipper Transcription Factors
  • DNA-Binding Proteins / chemistry*
  • DNA-Binding Proteins / metabolism*
  • Humans
  • Models, Molecular
  • Molecular Sequence Data
  • Phosphorylation
  • Protein Structure, Tertiary
  • Receptors, Transforming Growth Factor beta / metabolism*
  • Repressor Proteins*
  • Signal Transduction*
  • Smad2 Protein
  • Structure-Activity Relationship
  • Trans-Activators*


  • Basic Helix-Loop-Helix Leucine Zipper Transcription Factors
  • DNA-Binding Proteins
  • MXD1 protein, human
  • Receptors, Transforming Growth Factor beta
  • Repressor Proteins
  • SMAD2 protein, human
  • Smad2 Protein
  • Trans-Activators