title: Endoglin structure and function: Determinants of endoglin phosphorylation by transforming growth factor-beta receptors.
abstract: Determination of the functional relationship between the transforming growth factor-beta (TGFbeta) receptor proteins endoglin and ALK1 is essential to the understanding of the human vascular disease, hereditary hemorrhagic telangiectasia. TGFbeta1 caused recruitment of ALK1 into a complex with endoglin in human umbilical vein endothelial cells (HUVECs). Therefore, we examined TGFbeta receptor-dependent phosphorylation of endoglin by the constitutively active forms of the TGFbeta type I receptors ALK1, ALK5, and the TGFbeta type II receptor, TbetaRII. Of these receptors, TbetaRII preferentially phosphorylated endoglin on cytosolic domain serine residues Ser(634) and Ser(635). Removal of the carboxyl-terminal tripeptide of endoglin, which comprises a putative PDZ-liganding motif, dramatically increased endoglin serine phosphorylation by all three receptors, suggesting that the PDZ-liganding motif is important for the regulation of endoglin phosphorylation. Constitutively active (ca)ALK1, but not caALK5, phosphorylated endoglin on cytosolic domain threonine residues. caALK1-mediated threonine phosphorylation required prior serine phosphorylation, suggesting a sequential mechanism of endoglin phosphorylation. Wild-type, but not a threonine phosphorylation-defective endoglin mutant blocked cell detachment and the antiproliferative effects of caALK1 expressed in HUVECs. These results suggest that ALK1 is a preferred TGFbeta receptor kinase for endoglin threonine phosphorylation in HUVECs and indicate a role for endoglin phosphorylation in the regulation of endothelial cell adhesion and growth by ALK1.
authors.affiliation: Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, Maine 04074, USA.
authors.forename: Rositsa I
authors.initials: RI
authors.lastname: Koleva
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Conley
authors.forename: Diana
authors.initials: D
authors.lastname: Romero
authors.forename: Kristin S
authors.initials: KS
authors.lastname: Riley
authors.forename: Jarrod A
authors.initials: JA
authors.lastname: Marto
authors.forename: Andreas
authors.initials: A
authors.lastname: Lux
authors.forename: Calvin P H
authors.initials: CP
authors.lastname: Vary
mesh_terms: D030301:Activin Receptors, Type II / Q000378:metabolism; D000595:Amino Acid Sequence; D015703:Antigens, CD / Q000737:chemistry* / Q000502:physiology; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D000071063:Endoglin; D004730:Endothelium, Vascular / Q000166:cytology; D006801:Humans; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000737:chemistry* / Q000502:physiology; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D014471:Umbilical Veins / Q000166:cytology
chemical_list: D015703:Antigens, CD; C064206:ENG protein, human; D000071063:Endoglin; D011956:Receptors, Cell Surface; D018125:Receptors, Transforming Growth Factor beta; C425364:ACVRL1 protein, human; D030301:Activin Receptors, Type II