Early endosomal regulation of Smad-dependent signaling in endothelial cells.
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.
Panopoulou
Ekaterini
E
Laboratory of Biological Chemistry, University of Ioannina Medical School, 45110 Ioannina, Greece.
Gillooly
David J
DJ
Wrana
Jeffrey L
JL
Zerial
Marino
M
Stenmark
Harald
H
Murphy
Carol
C
Fotsis
Theodore
T
D000818:Animals; D002352:Carrier Proteins / Q000502:physiology; D002417:Cattle; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000502:physiology*; D011992:Endosomes / Q000502:physiology*; D004730:Endothelium, Vascular / Q000502:physiology*; D007700:Kinetics; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D018125:Receptors, Transforming Growth Factor beta / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D051785:Smad Proteins; D015534:Trans-Activators / Q000502:physiology*; D016335:Zinc Fingers / Q000502:physiology; D020696:rab5 GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism
D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D018125:Receptors, Transforming Growth Factor beta; D051785:Smad Proteins; D015534:Trans-Activators; D020696:rab5 GTP-Binding Proteins