Characterization of a 150 kDa accessory receptor for TGF-beta 1 on keratinocytes: Direct evidence for a GPI anchor and ligand binding of the releasedform

Transforming growth factor-beta (TGF-beta) is a key modulator of epidermal development and homeostasis, and has been shown to potently regulate kerati nocyte migration and function during wound repair. There are three cloned T GF-beta receptors termed type I, type II, and type III that are found on mo st cell types. The types I and II are the signaling receptors, while the ty pe III is believed to facilitate TGF-beta binding to the types I and II rec eptors. Recently, we reported that in addition to these receptors, human ke ratinocytes express a 150 kDa TGF-beta1 binding protein (r150) which forms a heteromeric complex with the TGF-beta signaling receptors. This accessory receptor was described as glycosyl phosphatidylinositol-specific anchored based on its sensitivity to phosphatidylinositol phospholipase C (PIPLC). I n the present study, we demonstrate that the GPI-anchor is contained in r15 0 itself and not on a tightly associated protein and that it binds TGF-beta 1 with an affinity similar to those of the types I and II TGF-beta signalin g receptors. Furthermore, the PIPLC released (soluble) form of this protein is capable of binding TGF-beta1 independently from the signaling receptors . In addition, we provide evidence that r150 is released from the cell surf ace by an endogenous phospholipase C. Our observation that r150 interacts w ith the TGF-P signaling receptors, together with the finding that the solub le r150 binds TGF-beta1 suggest that r150 in either its membrane anchored o r soluble form may potentiate or antagonize TGF-beta signaling. Elucidating the mechanism by which r150 functions as an accessory molecule in TGF-beta signaling may be critical to understanding the molecular mechanisms underl ying the regulation of TGF-beta action in keratinocytes. J. Cell. Biochem. 83: 494-507, 2001. (C) 2001 Wiley-Liss, Inc.