Eukaryotic cells contain a diverse array of internal organelles that are specialised for specific functions, such as protein synthesis and folding, post-translational modification, energy production or degradation. These organelles require specific sets of proteins to carry out these specialised tasks. In many cases, proteins are localised to organelles by signal sequences, which are recognised by specific intracellular receptors, which then regulate the trafficking of these proteins to their intended destinations.
Within the endoplasmic reticulum (ER), millimolar levels of chaperones required for protein folding are selectively retained within this organelle. In contrast, newly synthesized secretory and membrane proteins pass through the ER on their way to the Golgi apparatus. Luminal chaperones and ER-resident membrane proteins carry a retrieval signal at their C-terminus, that binds an integral membrane protein in the Golgi apparatus, which then returns these proteins back to the ER. In mammalian cells the retrieval signal has the sequence Lys-Asp-Glu-Leu (KDEL), which was first discovered for protein chaperones, such as BiP and Hsp70. Variants of this signal are found in all eukaryotes. In budding yeast for example, the signal ER retrieval signal is HDEL, whereas in K. lactis it is DDEL.
When luminal proteins escape to the Golgi, they are recognized by the KDEL receptor (KDELR). Binding of the KDEL receptor to the KDEL signal peptide triggers incorporation of the receptor-protein complex into a COPI coated vesicles, which returns the complex back to the ER. Once in the ER, the receptor-protein complex dissociates, in a process regulated by the pH difference between ER (pH neutral) and Golgi (pH slightly acidic). The empty KDEL receptor exits the ER in COPII vesicles, returning to the Golgi for further rounds of retrieval.
KDEL receptor controls protein trafficking between the Golgi and ER.
In our recent paper, published in Science, we describe the first crystal structures of the KDEL receptor in both the apo and peptide bound states.
Comparisons of these two states identified the conformational switch that exposes the COPI ER retrieval motif. We further recapitulated the in vivo binding and release cycle of the receptor using purified components, demonstrating the receptor is the minimal component required to bind KDEL ligands in the Golgi. Using a combination of cellular and biochemical assays, we showed that the ability of the receptor to traffic protein between the Golgi and ER relies on the formation of a short hydrogen bond, an extremely rare intramolecular interaction, that may be stablised through local pH changes within the organelles.
(A) Crystal structure of the KDEL receptor showing overlay between the apo and peptide bound states (PDB: 6I6B & 6I6H). (B) Zoomed in view of the peptide binding site, showing the location of the short hydrogen bond and location of key side chains and helices.
Several neurodegenerative diseases are caused by problems associated with protein folding. The role of protein chaperones in regulating and controlling protein folding and misfolding is still poorly understood in the context of cellular physiology and neuronal function. A key aim of this research project is to develop a mechanistic understanding of protein trafficking in the early secretory pathway, which will enable a more complete understanding of chaperone distribution within cells.