Two distinct regions of latency-associated peptide coordinate stability of the latent transforming growth factor-beta1 complex

Transforming growth factor-beta1 (TGF-beta1) is secreted as part of an inactive complex consisting of the mature dimer, the TGF-beta1 propeptide (latency associated peptide; LAP) and latent TGF-beta binding proteins (LTBPs). Using in vitro mutagenesis, we identified the regions of LAP that govern the cooperative assembly and stability of the latent TGF-beta1 complex. Initially, hydrophobic LAP residues (Ile53, Leu54, Leu57 and Leu59), which form a contiguous epitope on one surface of an amphipathic alpha-helix, interact with mature TGF-beta1 to form the small latent complex. TGF-beta1 binding is predicted to alter LAP conformation exposing ionic residues (Arg45, Arg50, Lys56 and Arg58) on the other side of the alpha-helix, which form the binding site for LTBPs. The stability of the resultant large latent complex is dependent upon covalent dimerisation 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 dimerisation and reduced the stability of the latent TGF-beta1 complex.