In the absence of type III receptor, the transforming growth factor (TGF)-beta type II-B receptor requires the type I receptor to bind TGF-beta2.
Transforming growth factor beta (TGF-beta) ligands exert their biological effects through type II (TbetaRII) and type I receptors (TbetaRI). Unlike TGF-beta1 and -beta3, TGF-beta2 appears to require the co-receptor betaglycan (type III receptor, TbetaRIII) for high affinity binding and signaling. Recently, the TbetaRIII null mouse was generated and revealed significant non-overlapping phenotypes with the TGF-beta2 null mouse, implying the existence of TbetaRIII independent mechanisms for TGF-beta2 signaling. Because a variant of the type II receptor, the type II-B receptor (TbetaRII-B), has been suggested to mediate TGF-beta2 signaling in the absence of TbetaRIII, we directly tested the ability of TbetaRII-B to bind TGF-beta2. Here we show that the soluble extracellular domain of the type II-B receptor (sTbetaRII-B.Fc) bound TGF-beta1 and TGF-beta3 with high affinity (K(d) values = 31.7 +/- 22.8 and 74.6 +/- 15.8 pm, respectively), but TGF-beta2 binding was undetectable at corresponding doses. Similar results were obtained for the soluble type II receptor (sTbetaRII.Fc). However, sTbetaRII.Fc or sTbetaRII-B.Fc in combination with soluble type I receptor (sTbetaRI.Fc) formed a high affinity complex that bound TGF-beta2, and this complex inhibited TGF-beta2 in a biological inhibition assay. These results show that TGF-beta2 has the potential to signal in the absence of TbetaRIII when sufficient TGF-beta2, TbetaRI, and TbetaRII or TbetaRII-B are present. Our data also support a cooperative model for receptor-ligand interactions, as has been suggested by crystallization studies of TGF-beta receptors and ligands. Our cell-free binding assay system will allow for testing of models of receptor-ligand complexes prior to actual solution of crystal structures.
del Re
Elisabetta
E
Department of Medicine, Massachusetts General Hospital, Harvard Medical School, Charleston, Massachusetts 02129, USA.
Babitt
Jodie L
JL
Pirani
Alnoor
A
Schneyer
Alan L
AL
Lin
Herbert Y
HY
D030201:Activin Receptors, Type I / Q000378:metabolism*; D000818:Animals; D015153:Blotting, Western; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002474:Cell-Free System; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D017930:Genes, Reporter; D006031:Glycosylation; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008156:Luciferases / Q000378:metabolism; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D010641:Phenotype; D011485:Protein Binding; D020033:Protein Isoforms; D017346:Protein Serine-Threonine Kinases; D017434:Protein Structure, Tertiary; D011509:Proteoglycans / Q000378:metabolism*; D000077293:Receptor, Transforming Growth Factor-beta Type I; D000077294:Receptor, Transforming Growth Factor-beta Type II; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism* / Q000502:physiology*; D015398:Signal Transduction
D018076:DNA, Complementary; D008024:Ligands; D020033:Protein Isoforms; D011509:Proteoglycans; D018125:Receptors, Transforming Growth Factor beta; C071736:betaglycan; D008156:Luciferases; D017346:Protein Serine-Threonine Kinases; D030201:Activin Receptors, Type I; D000077293:Receptor, Transforming Growth Factor-beta Type I; D000077294:Receptor, Transforming Growth Factor-beta Type II