Latent Transforming Growth Factor-β Binding Protein Domains Involved in Activation and Transglutaminase-dependent Cross-Linking of Latent Transforming Growth Factor-β

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Abstract
Transforming growth factor-β (TGF-β) is secreted by many cell types as part of a large latent complex composed of three subunits: TGF-β, the TGF-β propeptide, and the latent TGF-β binding protein (LTBP). To interact with its cell surface receptors, TGF-β must be released from the latent complex by disrupting noncovalent interactions between mature TGF-β and its propeptide. Previously, we identified LTBP-1 and transglutaminase, a cross-linking enzyme, as reactants involved in the formation of TGF-β. In this study, we demonstrate that LTBP-1 and large latent complex are substrates for transglutaminase. Furthermore, we show that the covalent association between LTBP-1 and the extracellular matrix is transglutaminase dependent, as little LTBP-1 is recovered from matrix digests prepared from cultures treated with transglutaminase inhibitors. Three polyclonal antisera to glutathione S–transferase fusion proteins containing amino, middle, or carboxyl regions of LTBP-1S were used to identify domains of LTBP-1 involved in crosslinking and formation of TGF-β by transglutaminase. Antibodies to the amino and carboxyl regions of LTBP-1S abrogate TGF-β generation by vascular cell cocultures or macrophages. However, only antibodies to the amino-terminal region of LTBP-1 block transglutaminase-dependent cross-linking of large latent complex or LTBP-1. To further identify transglutaminase-reactive domains within the amino-terminal region of LTBP-1S, mutants of LTBP-1S with deletions of either the amino-terminal 293 (ΔN293) or 441 (ΔN441) amino acids were expressed transiently in CHO cells. Analysis of the LTBP-1S content in matrices of transfected CHO cultures revealed that ΔN293 LTBP-1S was matrix associated via a transglutaminasedependent reaction, whereas ΔN441 LTBP-1S was not. This suggests that residues 294–441 are critical to the transglutaminase reactivity of LTBP-1S.

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