Award details

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

Reference	BB/L006952/1
Principal Investigator / Supervisor	Dr Rivka Isaacson
Co-Investigators / Co-Supervisors
Institution	King's College London
Department	Chemistry
Funding type	Research
Value (£)	333,534
Status	Completed
Type	Research Grant
Start date	01/03/2014
End date	28/02/2017
Duration	36 months

Abstract

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.

Summary

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 asa 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.

Impact Summary

Protein secretion is a fundamental cellular process that is relevant in a number of spheres including biology, industry and medicine. The early stages of secretory protein production are intimately linked to the function of the endoplasmic reticulum, and our goal is to define the molecular basis of a novel pathway for the entry of proteins into the mammalian secretory pathway. The most direct economic impact of this work will be on the use of cultured mammalian cells for the production of high-value therapeutic proteins or biopharmaceuticals. The biological complexity of such components, combined with the physiological requirements of their targets, makes the use of mammalian cells the most viable platform for their production. The high cost and modest yields of such systems means that companies like Novo Nordisk have a substantial interest in trying to exploit current and emerging basic knowledge in order to enhance and improve output. Such approaches include strategies to enhance the early stages of secretory protein biogenesis at the endoplasmic reticulum by the rational engineering of cell line, and our work is directly relevant to these aspirations. In addition, there is anecdotal evidence that "premature" quality control may also reduce the yields of selected biopharmaceuticals, and our work addresses this issue directly. Pharmaceutical companies, including Novartis, have previously developed small molecules that directly inhibit the translocation of selected secretory proteins into the endoplasmic reticulum and explored their use as potential therapeutics. The outcomes of this project will provide the basis for complementary approaches that selectively inhibit the production of short secretory proteins. Our membership of the research council funded Manchester Chemical Biology Network gives us informal access to pharmaceutical companies (including AstraZeneca, GSK, Pfizer and Syngenta), providing a direct opportunity to explore and refine this strategy. Both applicants are strongly committed to engaging with the general public about the importance of Science and the specific goals of their research. We actively participate in University and Faculty open days, and are proactive in our interactions with local schools. Hence, we have contributed to a variety of science-based activities in schools and provided mentoring to a nearby six-form college. We strongly encourage our staff and students to take part in raising the profile and understanding of science in the wider society, and in recent years our groups have participated in public events hosted by the University (Darwin Day at the Manchester Museum) and the Faculty of Life Sciences (Science Stars Days for Secondary School pupils).

Committee	Research Committee D (Molecules, cells and industrial biotechnology)
Research Topics	Structural Biology
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

Associated awards:

BB/L006510/1 The structure and function of SGTA, a key regulator of protein quality control

I accept the terms and conditions of use (opens in new window)

export PDF file

back to list new search