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.