The expression of two phenotypically-contrasting LDL receptor mutations was characterized in cultured fibroblasts from the genetically-homozygous Afrikaner subjects, FH1a and lb, and FH3a and 3b, respectively. Surface receptor expression and functional activity were studied by ligand (¹²⁵I-LDL) and monoclonal antibody (¹²⁵I-IgG-C7) binding, and c35s]-methionine pulse-chase experiments were used to analyze biosynthesis, processing and degradation of IgG-C7- immunoprecipitable mutant receptors. Cells from the "receptor-negative" subjects, FH3a and 3b exhibited reduced, but significant (40-60% of normal) LDL receptor synthesis rates. Newly-synthesized precursors were processed slowly (t½ 1.5 hours versus normal t½ of approximately 15 minutes) to mature receptors which reached the cell-surface, but were rapidly degraded thereafter with a half-life of approximately 1.7 hours (normal value 12.6 hours) thus representing a new type of LDL receptor defect. Lysosomotropic weak bases such as ammonium chloride partially inhibited rapid degradation of the mutant receptors, suggesting the involvement of proteolysis in acidic compartments such as lysosomes or endosomes. Fibroblasts from FH1a and lb exhibited normal synthesis rates of LDL receptor precursors that were processed at a severely reduced rate (t½ approximately 5 hours) to functionally heterogeneous mature surface receptors. Onethird of the receptors (20% of normal levels) bound ¹²⁵I-LDL with normal affinity at 4°C and 37°C, whereas the majority were able to recognize only ¹²⁵I-IgG-C7, and apparently showed defective internalisation and subsequent degradation of the bound IgG-C7 at 37°C. The existence of the two receptor populations was further supported by selective intracellular trapping and degradation of only the active, LDL-binding population, in the presence of ammonium chloride and LOL. The abnormal form predominated even in newly-synthesized receptors and reached a maximum of 50-70% of normal levels after 48 hours of upregulation. Upregulation kinetics and degradation rates (t½ = 10-11 hours) of both functionally-active and abnormal receptor populations were similar to normal. A progressive increase in apparent molecular weight of the slowly-processed precursor receptors suggested a possible role for abnormal glycosylation in the formation of both "normal" and abnormal conformations of the same receptor molecule.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/27174 |
| Date | January 1989 |
| Creators | Fourie, Anne Madeleine |
| Contributors | Van der Westhuyzen, Deneys R |
| Publisher | University of Cape Town, Faculty of Health Sciences, Division of Medical Biochemistry and Structural Biology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, Doctoral, PhD |
| Format | application/pdf |
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