Homodimers of the 60 kDa Phosphatidylinositol-Anchored Transforming Growth Factor-β2 Binding Proteins in FBHEC and MG-63 Cells

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Abstract
Phophatidylinositol (PI) anchored TGF-β2 binding proteins of approximately 60 and 140 kDa were recently identified in MG-63 human osteogenic sarcoma and fetal bovine heart endothelial cells (FBHEC) (Cheifetz, S., Massagué, J. (1991) J. Biol. Chem. 266, 20767-20772). The relationship between these two PI-anchored TGF-β2 binding proteins was investigated. Specifically labeled bands of approximately 60 and 110 kDa were observed when 125I-TGF-β2 labeled FBHEC and MG-63 cells were separated by SDS-PAGE under non-reducing conditions. Partial proteolysis of the affinity labeled 60 and 110 kDa species yielded similar peptides. The integrity of the 110 kDa species under denaturing conditions is dependent on both disulfide bonds and chemical cross-linking: reduction of the 110 kDa species yielded an affinity labeled species co-electrophoresing with the 60 kDa band; cleavage of chemical cross-links in the 110 kDa complex yielded a labeled 60 kDa component. These results indicate that the 110 kDa affinity labeled species in FBHEC and MG-63 cells is a homodimer. Within this complex the binding protein monomers can be chemically cross-linked to opposite arms of the disulfide-linked TGF-β2 homodimer.