LOCALIZATION OF TRANSFORMING GROWTH-FACTOR-BETA BINDING-SITE IN BETAGLYCAN - COMPARISON WITH SMALL EXTRACELLULAR-MATRIX PROTEOGLYCANS

The most abundant binding molecule for transforming growth factor beta (TGF-beta) on many cell types is betaglycan, a transmembrane proteogl ycan. To localize the binding site for TGF-beta in betaglycan, parts o f the betaglycan extracellular portion were expressed as recombinant f usion proteins in bacteria and tested for their ability to compete for the binding of TGF-beta1 to Hep G2 cells. One fragment encompassing 2 26 residues near the transmembrane domain (amino acids 543-769) (Lopez -Casillas, F., Cheifetz, S., Doody, J., Andres, J. L., Lane, W. S., an d Massague, J. (1991) Cell 67, 785-795) was active, whereas fusion pro teins representing the other parts of the betaglycan ectodomain were i nactive. Affinity measurements revealed two classes of binding sites ( K(d) = 3.9 nM and K(d) = 145 nm) for the active fusion protein. The bi nding of the betaglycan fusion protein to immobilized TGF-beta-1 was i nhibited by fusion proteins representing the core proteins of the smal l interstitial proteoglycans decorin, biglycan and fibromodulin, each also known to bind TGF-beta. The effective concentrations of TGF-beta for binding to these other proteoglycans were similar to those require d for binding to betaglycan, indicating similar affinities for the bin ding of proteoglycans and betaglycan. Affinity cross-linking showed th at, at low concentrations, the betaglycan fragment enhanced the bindin g of TGF-beta to the type II receptor and to endogenous betaglycan but had no effect on the binding to type I receptor. At high concentratio ns, the TGF-beta binding fragment inhibited the binding of TGF-beta to all these receptors. The fragment enhanced the activity of TGF-beta i n mink lung cell bioassay at all active concentrations. The results in dicate that betaglycan and the decorin type proteoglycans all bind to the same or closely spaced sites in TGF-beta and compete with one anot her for the binding. In addition, betaglycan may cooperate with the ty pe II receptor on TGF-beta binding.