Abstract
Growth differentiation factor 8 (GDF8)/Myostatin is a latent TGFβ family member that potently inhibits skeletal muscle growth. Here, we compared the conformation and dynamics of precursor, latent, and Tolloidcleaved GDF8 procomplexes to understand structural mechanisms underlying latency and activation of GDF8. Negative stain electron microscopy (EM) of precursor and latent procomplexes reveals a Vshaped conformation that is unaltered by furin cleavage and sharply contrasts with the ringlike, crossarmed conformation of latent TGFβ1. Surprisingly, Tolloidcleaved GDF8 does not immediately dissociate, but in EM exhibits structural heterogeneity consistent with partial dissociation. Hydrogen–deuterium exchange was not affected by furin cleavage. In contrast, Tolloid cleavage, in the absence of prodomain–growth factor dissociation, increased exchange in regions that correspond in pro-TGF-β1 to the α1-helix, latency lasso, and β1 strand in the prodomain and to the β6’–7’ strands in the growth factor. Thus, these regions are important in maintaining GDF8 latency. Our results show that Tolloid cleavage activates latent GDF8 by destabilizing specific prodomain–growth factor interfaces and primes the growth factor for release from the prodomain.
