Characterization of a 60-kDa cell surface-associated transforming growth factor-β binding protein that can interfere with transforming growth factor-β receptor binding

We have characterized a 60‐kDa transforming growth factor‐β (TGF‐β) binding protein that was originally identified on LNCaP adenocarcinoma prostate cells by affinity cross‐linking of cell surface proteins by using 125I‐TGF‐β1. Binding of 125I‐TGF‐β1 to the 60‐kDa protein was competed by an excess of unlabeled TGF‐β1 but not by TGF‐β2, TGF‐β3, activin, or osteogenic protein‐1 (OP‐1), also termed bone morphogenetic protein‐7 (BMP‐7). In addition, no binding of 125I‐TGF‐β2 and 125I‐TGF‐β3 to the 60‐kDa binding protein on LNCaP cells could be demonstrated by using affinity labeling techniques. The 60‐kDa TGF‐β binding protein showed no immunoreactivity with antibodies against the known type I and type II receptors for members of the TGF‐β superfamily. Treatment of LNCaP cells with 0.25 M NaCl, 1 μg/ml heparin, or 10% glycerol caused a release of the 60‐kDa protein from the cell surface. In addition, we found that the previously described TGF‐β type IV receptor on GH3 cells, which does not form a heteromeric complex with TGF‐β receptors, could be released from the cell surface by these same treatments. This suggests that the 60‐kDa protein and the similarly sized TGF‐β type IV receptor are related proteins. The eluted 60‐kDa LNCaP protein was shown to interfere with the binding of TGF‐β to the TGF‐β receptors. Thus, the cell surface‐associated 60‐kDa TGF‐β binding protein may play a role in regulating TGF‐β binding to TGF‐β receptors. J. Cell. Physiol. 173:447–459, 1997. © 1997 Wiley‐Liss, Inc.