Structure and function on inhibins and the role of pro-domains in their regulation.

Activins and related growth factors in the TGF-beta family require two different receptors for signalling. The type I receptors are responsible for the specific signal transduction inside the cells, and they interact with the mature growth factor by inserting a short into a groove in the so-called wrist epitope of the growth factor. With over 30 members in this super family but only seven different type I receptors, there is significant promiscuity between ligands and receptors, and regulation of signalling through a particular receptor is likely to affect signalling by many ligands, The PhD project will build on the results of the rotation project, with main aim of determing the three dimentional structure of the hetero-dimeric pro-inhibin A and B complexes. This work will reveal is the precise molecular mechanism by which inhibins achieve their function, whether they interact with receptors at all, and how the inhibition of activin signalling is achieved. The structure will also reveal how the inhibin beta chains (which exist also in homodimeric form) can participate in heterodimerisation with the alpha chains, and how this heterodimerisation is specific to only these proteins. Activins (the homo and heterodimers of inhibin betas) are key regulators of embryonic development and used extensively in stem cell research to drive the cells to differentiate to defined cellular lineages and differentiated tissues. Inhibins have the potential to act as modulators of this process, and availability of large quantities of pure and homogenous inhibins and activin isoforms will allow evaluation of their potential for modulation of stem cell differentiation.
making it difficult to study physiological effects of individual growth factors.

Several crystal structures in the recent years have revealed that the binding site for the type I receptors is used by many other interacting proteins, all of which insert a helical epitope into the receptor binding groove. These interactions raise an intriguing idea that we could develop specific peptide-based inhibitors for these growth factors by mimicking and modifying the known binding epitopes and thus create unique tools to study these proteins in vitro and in vivo. Also, TGF-beta family proteins are increasingly important therapeutically and in regenerative medicine, and the tools created in this project can be used as starting points for the development of small molecule modulators of the growth factors.

We propose to analyse the contribution of isolated helices to the interaction with the growth factor and to create high affinity variants by combining features of different helical epitopes into a single non-natural sequence. We will also employ technologies developed in the group of Prof Spring in the Department of Chemistry to add chemical staples to the peptides stabilise the helical conformation, to increase the affinities for the growth factor, to enable specific labelling of the peptides and to make them resistant to proteolysis. Given the dimeric nature of these growth factors, monomeric peptide binders can be potentiated by chemical dimerisation from one end, increasing affinity and specificity by co-operative binding to targets.

We will use activin A as the model system for the development of these peptide-based tools. We can make the protein easily, we have significant amount of structural information available to help to guide the design of optimal helices and highly robust luciferase assays to evaluate the effect of the peptides on cellular signalling. Once we understand how specific inhibitors can be developed for activin A we will replicate the work on other related proteins and build a toolkit of peptidic inhibitors for key members of this family.