Myostatin is a secreted growth factor of the transforming growth-factor $\beta$ (TGF$\beta$) superfamily, and a powerful negative regulator of muscle mass in vertebrates. As such, there is considerable interest in developing pharmacological agents which inhibit myostatin signalling in order to stimulate muscle growth in the context of pathological muscle wasting. Like other TGF$\beta$ family proteins, myostatin is biosynthesised as an inactive (latent) precursor protein which requires proteolytic processing to liberate the mature bioactive growth factor. To examine the molecular basis of pro-myostatin latency and the mechanism by which it is activated in the extracellular space, I have determined the crystal structure of unprocessed human pro-myostatin and studied the properties of the protein at various stages of activation.

Crystallographic analysis of pro-myostatin reveals a unique domain-swapped dimeric structure, with an open V-shaped conformation distinct from the prototypical family member, TGF$\beta$1. Following cleavage of the prodomains by furin, pro-myostatin persists as a stable non-covalent complex which is resistant to the natural inhibitor follistatin and exhibits significantly weaker bioactivity than the mature growth factor. A number of distinct structural features combine to stabilise the interaction between pro and mature domains and in doing so confer latency to the pro-complex. This facilitates a controlled, step-wise process of activation in the extracellular space and contributes to a complex network of regulatory control.

The results presented here provide a structural basis for understanding the effect of natural polymorphisms on myostatin function and a starting point for structure-guided development of next generation myostatin inhibitors. As a proof-of-concept, I present preliminary data on prodomain derived stapled peptides as inhibitors of myostatin signalling.