title: Calcium as a crucial cofactor for low density lipoprotein receptor folding in the endoplasmic reticulum.
abstract: The family of low density lipoprotein (LDL) receptors mediate uptake of a plethora of ligands from the circulation and couple this to signaling, thereby performing a crucial role in physiological processes including embryonic development, cancer development, homeostasis of lipoproteins, viral infection, and neuronal plasticity. Structural integrity of individual ectodomain modules in these receptors depends on calcium, and we showed before that the LDL receptor folds its modules late after synthesis via intermediates with abundant non-native disulfide bonds and structure. Using a radioactive pulse-chase approach, we here show that for proper LDL receptor folding, calcium had to be present from the very early start of folding, which suggests at least some native, essential coordination of calcium ions at the still largely non-native folding phase. As long as the protein was in the endoplasmic reticulum (ER), its folding was reversible, which changed only upon both proper incorporation of calcium and exit from the ER. Coevolution of protein folding with the high calcium concentration in the ER may be the basis for the need for this cation throughout the folding process even though calcium is only stably integrated in native repeats at a later stage.
authors.affiliation: Cellular Protein Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, 3584 CH Utrecht, The Netherlands.
authors.forename: Florentina
authors.initials: F
authors.lastname: Pena
authors.forename: Annemieke
authors.initials: A
authors.lastname: Jansens
authors.forename: Guus
authors.initials: G
authors.lastname: van Zadelhoff
authors.forename: Ineke
authors.initials: I
authors.lastname: Braakman
mesh_terms: D002118:Calcium / Q000378:metabolism*; D002412:Cations; D004247:DNA / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry; D004229:Dithiothreitol / Q000737:chemistry; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D000939:Epitopes / Q000737:chemistry; D006031:Glycosylation; D006056:Golgi Apparatus / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D047468:Immunoprecipitation; D017510:Protein Folding; D011973:Receptors, LDL / Q000737:chemistry*; D013997:Time Factors
chemical_list: D002412:Cations; D004220:Disulfides; D000939:Epitopes; D011973:Receptors, LDL; D004247:DNA; D002118:Calcium; D004229:Dithiothreitol