Ectodomain cleavage and shedding of the type III transforming growth factor-beta receptor in lung membranes - Effect of temperature, ligand binding and membrane solubilization

Previous studies from our laboratory [Philip, A. & O'Connor-McCourt, M. D. (1991) J. Biol. Chem. 266, 22290-22296] have shown that the lung exhibited the highest uptake of circulating [I-125]-transforming growth factor-beta 1 (TGF-beta 1) on a per gram basis. This observation, together with the lack of information on TGF-beta receptor expression in the lung, prompted us to attempt to characterize TGF-beta receptors in this tissue. In the present report. we show that the type III TGF-beta receptor is the most abundant TG F-beta binding protein in rat lung membranes and that it exhibits a 10-fold higher affinity for TGF-beta 2 than for TGF-beta 1. We observed that the m ajority of the type III receptor population in lung membranes is cleaved at a site in the central portion of the ectodomain, the resulting two fragmen ts (95 kDa and 58 kDa) being held together by disulfide bonds. Furthermore, we demonstrate that a soluble form of the ectodomain of the type III recep tor is shed from rat lung membranes in an efficient manner, with protease c leavage occurring at a site close to the transmembrane domain. This sheddin g is controllable by temperature, thus providing a system to study the mech anism of ectodomain release. Using this system, we show that the shedding i s inhibited by prior ligand binding and by membrane solubilization. The ide ntification of a membrane preparation which exhibits controllable and quant itative release of the type III receptor ectodomain provides a unique cell- free system for further studies of the mechanism of shedding of the type II I TGF-beta receptor ectodomain.