Identification and characterization of an eight-cysteine repeat of the latent transforming growth factor-β binding protein-1 that mediates bonding to the latent transforming growth factor-β1

Abstract

Most cultured cell types secrete small latent transforming growth factor-β (TGF-β) as a disulfide-bonded complex with a member of the latent TGF-β binding protein (LTBP) family. Using the baculovirus expression system, we have mapped the domain of LTBP-1 mediating covalent association with small latent TGF-β1. Coexpression in Sf9 cells of small latent TGF-β1 with deletion mutants of LTBP-1 showed that the third eight-cysteine repeat of LTBP-1 is necessary and sufficient for covalent interaction with small latent TGF-β1. Analysis by mass spectrometry of this eight-cysteine repeat, produced as a recombinant peptide in Sf9 cells, confirmed that it was N- glycosylated, as expected from the primary sequence. No other post- translational modifications of this domain were detected. Alkylation of the recombinant peptide with vinyl pyridine failed to reveal any free cysteines, indicating that, in the absence of small latent TGF-β, the eight cysteines of this domain are engaged in intramolecular bonds. These data demonstrate that the third LTBP-1 eight-cysteine repeat recognizes and associates covalently with small latent TGF-β1 through a mechanism that does not require any specific post-translational modification of this domain. They also suggest that this domain adopts different conformations depending on whether it is free or bound to small latent TGF-β.