LDL receptor

Familial hypercholesterolemia (FH) is caused by a mutation in the LDL receptor. In many patients, mutant LDL receptor has lost its activity because of a folding and maturation problem. To understand the defects, we need to understand the folding of wild-type LDL receptor in molecular detail. The LDL receptor contains 30 disulfide bonds. Still, it is a very efficiently folding molecule. We found that this protein does not fold along the expected lines of its domain structure. Instead of folding sequentially from N- to C-terminus, the LDL receptor first collapses into a compact form, which is followed by a second stage of folding, in which many disulfide bonds are corrected by isomerization. Only upon extensive isomerization of disulfide bonds the LDL receptor reaches a native state that can leave the ER. Because of the extensive isomerization of disulfide bonds, we can safely assume that protein disulfide isomerases must be important helpers during the folding of this protein.

Jansens A, van Duijn E & Braakman I Coordinated nonvectorial folding in a newly synthesized multidomain protein. Science 2002;298:2401-3. full text pdf supplementary data
Gent J & Braakman I Low-density lipoprotein receptor structure and folding. Cell Mol Life Sci 2004;61:2461-70. pdf

We are currently further characterizing the folding pathway of the LDL receptor in intact cells. At the same time we are in the process of identifying factors, both proteinaceous and of low molecular weight, that influence folding of the LDL receptor in any way. We use biochemical and cell biological assays, most of them routine in our laboratory, including radioactive pulse-chase labeling, immunoprecipitation, indirect immunofluorescence, SDS-PAGE, and biotinylation.

Our goal is to find the weak points of the LDL receptor in terms of folding, and the assistance it is getting from cellular factors, rangeing from ATP and calcium to chaperones and disulfide isomerases. We hope to find factors that improve folding, such that we can use this information for the design of drugs that improve folding of wild-type protein, and hopefully that of mutants as well.

• The LDL receptor crew
  Ineke Braakman
  Jürgen Gent (alumnus)
  Sander Engels (master student)
  Florentina Pena (alumna)
  
• Helena Zabranska (alumna)
  Annemieke Jansens (alumna)