The structure and function of SGTA, a key regulator of protein quality control

Proteins are essential to life, providing important building blocks for our cells and performing numerous complex roles that maintain animal and human health and combat disease. Proteins are rather complex macromolecules, and this in turn means that making them is a complicated process made up of several stages, each of which can potentially go wrong resulting in the production of faulty or defective proteins. There are many reasons why faulty proteins might be made, including different kinds of physiological stress and mutations to the genes that encode them, but importantly the cell has developed a mechanism for checking its newly made proteins to make sure they are made correctly. This process is normally referred to as "protein quality control" and when it is operating correctly it acts to recognise and remove faulty proteins. This normal cellular quality control process is vitally important, since if faulty proteins are not dealt with quickly they have a tendency to stick to each other and clump together to form aggregates. In many cases these aggregates are toxic and can prevent cells from working properly, or even cause them to die. The misfolding and aggregation of proteins lies at the root of the prion diseases that are suffered by many animal species, including BSE and scrapie. Likewise, protein aggregates are a feature of human prion diseases, such as Creutzfeldt-Jakob disease, and neurological disorders, most notably Alzheimer's disease and Parkinson's disease. It is also suggested that one of the consequences of ageing is a reduced capacity for cellular protein quality control, and that this in turn might impact on ageing related diseases.

Misfolded membrane proteins have a strong tendency to aggregate, and mammalian cells like ours seem to have developed a fast track system to recognise this class of proteins and deal with them quickly, thereby avoiding any potential problems they might cause. We have identified a protein called SGTA which acts as a quality control factor and plays a very important role in the cells ability to deal with mislocalised membrane proteins that are misfolded because they have ended up in the wrong location with a cell. There is also evidence that SGTA is linked to certain cancers and viral infections. However, although there is a strong case that SGTA is a critical component of cellular quality control pathways, we really know very little detail about how it works, and it is precisely this question that we will answer during the course of this project. In particular, we will find out how SGTA specifically recognises mislocalised membrane proteins, and establish what it does with these aberrant proteins once they are bound, focussing on its ability to inhibit their destruction and looking directly at its role in protein aggregation. The purpose of our research is to understand the normal workings of SGTA in a healthy cell that is capable of efficiently removing mislocalised membrane proteins and avoiding a build up of protein aggregates.

I am well placed to accede the overwhelming focus of protein targeting to the ER has been the fidelity and elegance of signal recognition particle dependent delivery. In contrast, efficiency was not a major concern, and hence the idea that a significant proportion of proteins destined for the ER can mislocalise to the mammalian cytosol is new. Nevertheless, the evidence for, and importance of, mislocalised proteins (MLPs) are now compelling, and a key component of the cellular pathway that deals with them is the focus of this project. MLPs are membrane and secretory protein precursors that fail to reach their intended destination and default to the cytosol. It is now apparent that MLPs are dealt with by a specific branch of the global cellular quality control network, and that SGTA plays a pivotal role in dictating their fate. SGTA acts in tandem with a second component, the BAG6 complex, forming a cycle: SGTA directs its MLP substrates towards deubiquitination and hence stabilisation; BAG6 directs them towards polyubiquitination and proteasomal degradation. Key to this pivotal role of SGTA is its capacity to recognise and bind to MLPs, yet we know little about the nature or extent of its substrate-binding region and have no detailed structural information whatsoever. Likewise the functional role of its novel N-terminal interaction motif is completely unexplored. Our major goals are therefore to obtain a complete molecular understanding of the interaction between SGTA and membrane proteins, and to define the importance of its UBL- and substrate- binding domains to the cellular quality control of MLPs. These studies will build on our recent successful studies of SGTA structure and function, and exploit a range of established tools, reagents and validated model MLPs, combined with robust in vitro and in vivo functional readouts.

The quality control and regulated degradation of misfolded and mislocalised membrane proteins is an intrinsic cellular process that is relevant to many areas of biology, both academic and industrial. As such, the immediate academic impact of this project will be with research scientists working in related areas of the life sciences, including productive protein folding, protein aggregation and the actions of molecular chaperones. Our work will provide new knowledge, insight and scientific advances that will be relevant to all those that are active in these areas, irrespective of whether their research is being carried out from a basic or a medical perspective. Our work is also of direct relevance to virologists, and we will engage this important group through our collaboration with the University Clinic in Heidelberg. Another major stakeholder with a fundamental interest in this field is the pharmaceutical industry, and components involved in cellular protein quality control and degradation are proven targets for specific drug-based therapies. We will engage the industrial members of the Manchester Chemical Biology Network, including AstraZeneca, GSK, Pfizer and Syngenta, to explore potential opportunities for interventions that target the quality control of aggregation prone precursors including mislocalised membrane proteins.

The three applicants are all strongly committed to engaging with the general public about the importance of Science and the specific goals of their research, actively participating in a variety of outreach activities. These range from Faculty and Institute open days to giving scientific presentations at English Literature conferences and informal spoken word events designed for a wide audience. We are also proactive in our interactions with local schools, and in recent years we have contributed to a number of science-based activities and provided mentoring. We strongly encourage our staff and students to take part in raising the profile and understanding of science in the wider society, and the PDRAs appointed to this project will participate in outreach activities throughout the course of the project. In addition, the two PDRAs that are appointed will each acquire a range of advanced skills, techniques and knowledge that will make them both well placed to pursue a long-term career in science, thereby contributing to the UK knowledge base and economy.