Latent TGF-β structure and activation

Abstract

Transforming growth factor (TGF)-β is stored in the extracellular matrix as a latent complex with its prodomain. Activation of TGF-β1 requires the binding of α(v) integrin to an RGD sequence in the prodomain and exertion of force on this domain, which is held in the extracellular matrix by latent TGF-β binding proteins. Crystals of dimeric porcine proTGF-β1 reveal a ring-shaped complex, a novel fold for the prodomain, and show how the prodomain shields the growth factor from recognition by receptors and alters its conformation. Complex formation between α(v)β(6) integrin and the prodomain is insufficient for TGF-β1 release. Force-dependent activation requires unfastening of a 'straitjacket' that encircles each growth-factor monomer at a position that can be locked by a disulphide bond. Sequences of all 33 TGF-β family members indicate a similar prodomain fold. The structure provides insights into the regulation of a family of growth and differentiation factors of fundamental importance in morphogenesis and homeostasis.

Figure 1. Architecture of proTGF-β1

Arm, straitjacket and TGF-β1 monomer segments are coloured differently. a, b, Overall structure. Spheres mark the last residue visible in density in the prodomain and the first residue of the growth factor. Disordered segments are dashed. Red arrows show the directions of forces during activation by integrins. Key side chains are shown in stick representation, including Asp of the RGD motif in cyan and CED mutations in white. Disulphide bonds and the Cys 4 mutation to Ser are in yellow. c, Schematic of the structure and activation mechanism. SS, disulphide bonds. d, Hydrophobic residues near Asp 217 of the RGD motif. e, Arm domain. Side chains for the hydrophobic core are shown in gold (also marked in Fig. 2 and Supplementary Fig. 2), conserved α2-helix residues that interact with the growth factor are in pink, fastener residues are in silver and bowtie residues are in light green for aliphatics and yellow for Cys. f–h, Straitjacket and fastener details.

Figure 2. The TGF-β family

a, Sequence alignment of five representative prodomains. Orange circles mark the core hydrophobic residues shown in Fig. 1e. Inh-α, inhibin-α; Myost, myostatin. Black dots over TGF-β1 sequence mark decadal residues. Vertical dashed lines mark cleavage sites. b, Phylogenetic tree of the TGF-β family, based on the alignment in Supplementary Fig. 2.

Figure 3. Shielding from receptor binding

ProTGF-β1 and TGF-β1 in complex with its receptors (R type I and II) (ref. 15) were superimposed on the TGF-β dimers. For clarity, only one monomer of each is shown. The receptors are shown as transparent molecular surfaces. Elements of the prodomain that clash with the receptors are labelled.

Figure 4. ProTGF-β1 complexes with LTBP and and α_vβ₆ integrin, and activation of TGF-β

a–d, Representative negative-stain electron microscopy class averages of proTGF-β (a), the complex of proTGF-β with a fragment of LTBP1 containing TGF-βinding domain 3–EGF–EGF–TGF-βinding domain 4 (b) and complexes of proTGF-β1 with α_vβ₆ integrin, prepared with an excess of proTGF-β1 (c) or an excess of α_vβ₆ integrin (d). Scale bars, 100Å. e, Non-reducing SDS–PAGE of the complex peak from S200 gel filtration used for electron microscopy in d (lane 1), α_vβ₆ integrin (lane 2) and proTGF-β (lane 3). LAP, latency-associated protein (prodomain). f, Activation of TGF-β1. 293T cells stably transfected with α_vβ₆ integrin or a mock control were additionally transfected with the indicated wild-type (WT) or mutant proTGF-β1 constructs, or with empty vector (mock), and co-cultured with TGF-β indicator cells. g, Material made by the indicated mutants in 293T cells was heated at 80 °C and assayed with indicator cells. Error bars in f and g show the s.e.m. of 3–9 samples from 1–3 representative experiments. h, i, Western blots of proTGF-β1 secreted by the indicated transfectants, using an antibody to the prodomain (h) or streptavidin to detect biotinylated cysteines (i).