Universität zu Köln

Daverkausen-Fischer, Lea Valeria (2024). ER-resident co-chaperones in mammalian cells A critical analysis of experimental data published on structure, localization, regulation and function. PhD thesis, Universität zu Köln.

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Abstract

The ER (Endoplasmic reticulum) of mammalian cells is an important intracellular calcium storage and the place where lipids are generated. Furthermore, the maturation and folding of transmembrane proteins as well as secretory proteins, which constitute around one third of all synthesized proteins within the cell, takes place within the ER. The Sec61 translocon constitutes the main channel for protein translocation across the ER membrane. Upon translocation into the ER lumen, proteins associate with chaperones that assist their substrate proteins in reaching their final conformation. Under conditions of nutrient deficiency, hypoxia or disturbed calcium homeostasis, protein folding and maturation can be impaired. ER-stress develops as a consequence of the increased burden of ER luminal unfolded or misfolded proteins. The cellular response to ER-stress is called the Unfolded Protein Response (UPR). During the UPR, general protein translation is downregulated and transcription and translation of chaperones is upregulated in order to maintain ER luminal proteostasis. Terminally misfolded proteins are retrotranslocated to the cytosol for proteasomal degradation. If these mechanisms are insufficient to restore ER homeostasis, apoptosis is initiated by the cell. An important ER-resident chaperone is the Hsp70 family member BiP/GRP78. HSP70 chaperones bind to hydrophobic domains of unfolded or misfolded proteins, promote folding and prevent aggregation of their substrates. In order to promote folding of substrate proteins, an interaction of Hsp70 chaperones with Hsp40 co-chaperones is necessary. Hsp40 co-chaperones possess a conserved J-domain. The J-domain binds to Hsp70 chaperones and stimulates the ATPase activity of the Hsp70 chaperones. Currently, eight members of the Hsp40 family (ERdj1 to ERdj8) are known to be localized within the ER of mammalian cells. ERdj1 to ERdj7 have been shown to bind to BiP and stimulate the ATPase activity of BiP. As ERdj8 has only recently been discovered in 2020 there is only little data on the protein and there is no information on a possible interaction with BiP so far. Apart from their function as co-chaperones of BiP, ERdj1, ERdj2 and ERdj6 were shown to regulate protein translation and in the case of ERdj2 it was shown that the co-chaperone also regulates protein translocation. For ERdj3, ERdj4, ERdj5 and ERdj6 a role in degradation of terminally misfolded proteins has been proposed. Furthermore, ERdj1, ERdj3, ERdj5 and ERdj6 are involved in the regulation of cellular calcium homeostasis. Due to their various central functions within the cell, dysfunction of the ERdj co-chaperones can result in different diseases. For example, mutations in the ERdj2 protein have been associated with Polycystic liver disease (PLD), while ERdj4 is a known biomarker for fibrillary glomerulonephritis (FGN). With regard to ERdj7 and ERdj8, only little data exists as compared with ERdj1 to ERdj6. For these co-chaperones there is a multitude of published experimental data on structure, localization, function and regulation. Also, there is a large amount on publications on diseases associated with the co-chaperones. Until now, there has not been a complete and critical summary and analysis of all published experimental data regarding ERdj1 to ERdj8. This was the motivation for this thesis. In order to create a complete and critical analysis of all published data on ERdj1 to ERdj8, a systematic literature search was done on PubMed. The published experimental results were carefully analysed with regard to their experimental design, the controls that were applied and their significance. Afterwards, the published experimental data was compared with special focus on topology and localization of the co-chaperones, their functions regarding protein translation, translocation, degradation and cellular calcium homeostasis. Most results of this thesis have already been published in four reviews and one original paper.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Daverkausen-Fischer, Lea Valeria UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-720996
Date:	2024
Language:	English
Faculty:	Faculty of Medicine
Divisions:	Faculty of Medicine > Anatomie > Institut II für Anatomie
Subjects:	Natural sciences and mathematics Life sciences Medical sciences Medicine
Uncontrolled Keywords:	Keywords Language Endoplasmic reticulum; co-chaperones; ER stress UNSPECIFIED
Date of oral exam:	6 December 2023
Referee:	Name Academic Title Pröls, Felicitas Professorin Dr.rer.nat. Trifunovic, Aleksandra Universitätsprofessorin Dr. rer. nat.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/72099