The multifunctional cytokine TGF-b is produced in a latent form (L-TGF-b) where a RGD containing homodimeric prodomain forms a "ring" encircling mature TGF-b, shielding it from its receptors. Thus L-TGF-b must be activated to function, a process driven by dynamic allostery resulting from integrin binding the L-TGF-b RGD motif. Here we provide critical evidence that defines a domain-swapped architecture of L-TGF-b, an essential component in the dynamic allostery mechanism of L-TGF-b activation.