Two Distinct Regions of Latency-associated Peptide Coordinate Stability of the Latent Transforming Growth Factor-β1 Complex*

Transforming growth factor-β1 (TGF-β1) is secreted as part of an inactive complex consisting of the mature dimer, the TGF-β1 propeptide (latency-associated peptide (LAP)), and latent TGF-β-binding proteins. Using in vitro mutagenesis, we identified the regions of LAP that govern the cooperative assembly and stability of the latent TGF-β1 complex. Initially, hydrophobic LAP residues (Ile⁵³, Leu⁵⁴, Leu⁵⁷, and Leu⁵⁹), which form a contiguous epitope on one surface of an amphipathic α-helix, interact with mature TGF-β1 to form the small latent complex. TGF-β1 binding is predicted to alter LAP conformation, exposing ionic residues (Arg⁴⁵, Arg⁵⁰, Lys⁵⁶, and Arg⁵⁸) on the other side of the α-helix, which form the binding site for latent TGF-β-binding proteins. The stability of the resultant large latent complex is dependent upon covalent dimerization of LAP, which is facilitated by key residues (Phe¹⁹⁸, Asp¹⁹⁹, Val²⁰⁰, Leu²⁰⁸, Phe²¹⁷, and Leu²¹⁹) at the dimer interface. Significantly, genetic mutations in LAP (e.g. R218H) that cause the rare bone disorder Camurati-Engelmann disease disrupted dimerization and reduced the stability of the latent TGF-β1 complex.