Misfolded glycoprotein recognition in the ER: it takes one to know one!

28 July 2022

Biologists can struggle for years to come up with a model that explains their observations. Once they succeed, and a model is arrived at that successfully explains the available data, the authors herald it, the rest of the field accepts and uses it, and everyone move on to the next set of questions. This is all very well – except that very few biological models are complete down to the molecular detail: biologists are not always interested in pushing their understanding to the molecular level, so once they prove that two molecules interact, they often won’t try to discover how they do it. A situation therefore sometimes arises in which research in a particular field of biology moves on, and yet nobody has really understood the molecular facts underpinning of the biology explained by the current model.

This is exactly what happened in the field of glycoprotein folding quality control and degradation. A glycoprotein is a protein that carries sugars, which typically are a great asset when a protein needs to function in the extracellular environment. Baby glycoproteins begin life in an organelle called endoplasmic reticulum (ER), which is like a gym in which they can flex their muscles and acquire their shape, and progress towards secretion only once they are matured. About a quarter of the human genome encodes secreted glycoproteins and unsurprisingly glycoprotein folding quality control is central to medical fields such as cancer, hereditary rare disease and virology. Ari Helenius and co-workers proposed in 1994 that an enzyme called UGGT is able to recognise defective glycoproteins and flag them for retention in the Endoplasmic Reticulum. Jeffrey Brodsky and co-workers in 1996 proposed that glycoprotein that are terminally misfolded are dispatched to a cytosolic proteasome for degradation. In 2001, Kazuhiro Nagata and co-workers discovered that an enzyme called EDEM is responsible for the recognition of terminal misfold and their dispatching to degradation. Yet – to this date – nobody really understands several important aspects of ER glycoprotein quality control (folding and degradation) – and in particular we lack the detail needed for the synthesis of drugs to modulate UGGT and EDEM activity, to be used as antivirals, or chemotherapy or for the treatment of patients suffering from inherited rare diseases. For example, what exactly does it mean that a glycoprotein is misfolded and how does UGGT recognise it as such? How does EDEM recognise terminally misfolded glycoproteins? Do UGGT and EDEM compete for their substrates? In which case, how does a misfolded glycoprotein choose the path at the fork in the road between ER retention and degradation? And so on.

Ten years ago we began to work towards elucidation of these and similar questions, at the Universities of Oxford (2013-2018) and Leicester (2018-2021). In 2017 we determined the first crystal structures of a UGGT. Our UGGT structures suggest that conformation mobility is important for misfold recognition. We also serendipitously discovered that UGGT acts on itself – which seems to imply UGGT itself evolved never to fold completely! If this is true, UGGT is like fly paper for sticky misfolded glycoproteins. Perhaps when it comes to misfolded glycoproteins – it takes one to know one!  Since 2021 we moved to CNR IBBA, where we are in the process of tracing the first Cryo-EM structure of an EDEM. UGGT and EDEM are important for agricultural biology because they contribute to plant and animal responses to heat and drought/thirst. Our structure of EDEM suggests that this is a redox enzyme. We are in the process of testing if these properties of UGGT and EDEM are shared by both: does UGGT have redox activity? Does EDEM act on itself? If so, we will have unravelled a combination of redox chemistry and misfold-misfold pairing as the molecular determinants of ER misfold glycoprotein recognition. The results of our work – beyond the understanding of basic biology mechanisms – have the potential to lay the foundation for the synthesis of antivirals, anti-cancer drugs and drugs for the treatment of patients suffering from rare hereditary diseases, as well as help towards the generation of plants that resist drought and heat.

Author: Pietro Roversi


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