ASSOCIATION OF THE SMALL LATENT TRANSFORMING GROWTH-FACTOR-BETA WITH AN 8-CYSTEINE REPEAT OF ITS BINDING-PROTEIN LTBP-1

Transforming growth factor-beta s (TGF-beta s) are produced by most ce lls in large latent complexes of TGF-beta and its propeptide (LAP) ass ociated with a binding protein. The latent TGF-beta binding proteins ( LTBPs-1, -2 and -3) mediate the secretion and, subsequently, the assoc iation of latent TGF-beta complexes with the extracellular matrix (ECM ), The association of beta 1-LAP with LTBP-1 was characterized at the molecular level with an expression system in mammalian cells, where TG F-beta 1 and various fragments of LTBP-1 were co-expressed and secrete d with the aid of a signal peptide synthesized to the LTBP-1 construct s. Immunoblotting of the fusion protein complexes indicated that the t hird 8-Cys repeat of LTBP-1 bound covalently to the LAP region of TGF- beta 1 The cysteine required for the association between LTBP-1 and be ta 1-LAP was mapped to Cys33 of beta 1-LAP. The N-terminal region of L TBP-1 consisting of the first 400 amino acids was found to associate c ovalently with the ECM, The data indicate that an 8-Cys repeat of LTBP is capable of covalent and specific protein-protein interactions, The se interactions are mediated by exchanging cysteine disulfide bonds be tween the core 8-Cys repeat and an optionally associated protein durin g the secretion. This is, to our knowledge, the first demonstration of an extracellular protein module that is able to exchange cysteine dis ulfide bonds with heterologous ligand proteins.