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Cleaved beta 2-microglobulin partially attains a conformation that has amyloidogenic features
Niels H H Heegaard1, Peter Roepstorff, Steen G Melberg
1Department of Autoimmunology, Statens Serum Institut, DK-2300 Copenhagen S, Denmark. nhe@ssi.dk
Abstract:
beta(2)-Microglobulin, a small protein localized in serum and on cell surfaces, can adopt specific aggregating conformations that generate amyloid in tissues and joints as a complication to long-term hemodialysis. We characterize a proteolytic variant of beta(2)-microglobulin (cleaved after Lys(58)) that as a trimmed form (Lys(58) is removed) can be demonstrated in the circulation in patients with chronic disease. An unexpected electrophoretic heterogeneity of these two cleaved variants was demonstrated by capillary electrophoresis under physiological conditions. Each separated into a fast and a slow component while appearing homogeneous, except for a fraction of oxidized species detected by other techniques. The two components had different binding affinities for heparin and for the amyloid-specific dye Congo red, and the equilibrium between the two forms was dependent on solvent conditions. Together with analysis of the differences in circular dichroism, the results suggest that beta(2)-microglobulin cleaved after Lys(58) readily adopts two equilibrium conformations under native conditions. In the cleaved and trimmed beta(2)-microglobulin that appears in vivo, the less populated conformation is characterized by an increased affinity for Congo red. These observations may help elucidate why beta(2)-microglobulin polymerizes as amyloid in chronic hemodialysis and facilitate the search for means to inhibit this process.
Insights
beta(2)-microglobulin amyloidosis in hemodialysis patients is linked to a cleaved protein variant. This variant exists in two conformations, with one showing higher affinity for Congo red, potentially explaining amyloid formation.
Area of Science:
- Biochemistry
- Protein Chemistry
- Medical Research
Background:
- beta(2)-microglobulin (β2M) aggregates into amyloid in patients undergoing long-term hemodialysis.
- This amyloid deposition causes complications in tissues and joints.
Purpose of the Study:
- To characterize a specific proteolytic variant of β2M found in the circulation of chronic disease patients.
- To investigate the conformational properties of this cleaved β2M variant and their relation to amyloid formation.
Main Methods:
- Capillary electrophoresis to assess electrophoretic heterogeneity.
- Binding affinity studies using heparin and Congo red.
- Circular dichroism analysis to study protein structure.
Main Results:
- Two cleaved β2M variants (cleaved after Lys58 and trimmed) exhibited unexpected electrophoretic heterogeneity, separating into fast and slow components.
- These components displayed differential binding affinities for heparin and Congo red.
- Solvent conditions influenced the equilibrium between the two conformations, suggesting distinct structural states.
Conclusions:
- Cleaved β2M readily adopts two equilibrium conformations under native conditions.
- The less populated conformation in vivo demonstrates increased affinity for Congo red, a key factor in β2M polymerization.
- These findings may elucidate the mechanism of β2M amyloidosis and aid in developing inhibition strategies.