Molecular interactions that confer latency to transforming growth factor-β

A major point of regulation of transforming growth factor-beta (TGF-beta) function is through control of activation of the latent TGF-beta complex, which consists of the latency associated peptide (LAP) secreted in non-covalent association with mature TGF-beta. Activation involves proteolysis, dissociation, or altered binding of LAP. However, the mechanism by which LAP confers latency to TGF-beta is poorly understood. Previously, we identified a conserved sequence near the N terminus of LAP as a site of thrombospondin-1 (TSP1) binding to the latent complex. Now we show that expression of the TGF-beta1-latent complex deleted in the TSP1 binding site ((LSKL)-L-54) of LAP (DeltaLSKL LAP) results in secretion of LAP, but not of mature TGF- beta. DeltaLSKL LAP also fails to bind soluble or immobilized TGF-beta1. Consistent with an inability to bind the mature domain, DeltaLSKL LAP is unable to confer latency to TGF-beta, suggesting that the LSKL sequence is important, not only for TSP1 binding and activation, but also for binding to the mature domain. We identified the sequence (RKPK)-R-94 in the receptor-binding region of mature TGF-beta1 as the binding site for LAP. Peptides of the RKPK sequence bind LAP and inhibit LAP/TGF-beta association. RKPK peptides also activate latent TGF-beta, presumably by disrupting LAP-mature TGF-beta interactions. These studies provide a molecular basis for both latency and activation by TSP1 through the LSKL sequence of LAP binding to the RKPK sequence of mature TGF-beta.