Early endosomal regulation of Smad-dependent signaling in endothelial cells

J Biol Chem. 2002 May 17;277(20):18046-52. doi: 10.1074/jbc.M107983200. Epub 2002 Mar 4.


Transforming growth factor beta (TGFbeta) receptors require SARA for phosphorylation of the downstream transducing Smad proteins. SARA, a FYVE finger protein, binds to membrane lipids suggesting that activated receptors may interact with downstream signaling molecules at discrete endocytic locations. In the present study, we reveal a critical role for the early endocytic compartment in regulating Smad-dependent signaling. Not only is SARA localized on early endosomes, but also its minimal FYVE finger sequence is sufficient for early endosomal targeting. Expression of a SARA mutant protein lacking the FYVE finger inhibits downstream activin A signaling in endothelial cells. Moreover, a dominant-negative mutant of Rab5, a crucial protein for early endosome dynamics, causes phosphorylation and nuclear translocation of Smads leading to constitutive (i.e. ligand independent) transcriptional activation of a Smad-dependent promoter in endothelial cells. As inhibition of endocytosis using the K44A negative mutant of dynamin and RN-tre did not lead to activation of Smad-dependent transcription, the effects of the dominant-negative Rab5 are likely to be a consequence of altered membrane trafficking of constitutively formed TGFbeta/activin type I/II receptor complexes at the level of early endosomes. The results suggest an important interconnection between early endosomal dynamics and TGFbeta/activin signal transduction pathways.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Carrier Proteins / physiology
  • Cattle
  • Cells, Cultured
  • DNA-Binding Proteins / physiology*
  • Endosomes / physiology*
  • Endothelium, Vascular / physiology*
  • Kinetics
  • Phosphorylation
  • Promoter Regions, Genetic
  • Receptors, Transforming Growth Factor beta / physiology
  • Signal Transduction / physiology*
  • Smad Proteins
  • Trans-Activators / physiology*
  • Zinc Fingers / physiology
  • rab5 GTP-Binding Proteins / genetics
  • rab5 GTP-Binding Proteins / metabolism


  • Carrier Proteins
  • DNA-Binding Proteins
  • Receptors, Transforming Growth Factor beta
  • Smad Proteins
  • Trans-Activators
  • rab5 GTP-Binding Proteins