Tyrosine residues mediate fibril formation in a dynamic light chain dimer interface

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Misfolding Diseases

Background:

• Light chain amyloidosis is a fatal protein misfolding disease caused by misfolded immunoglobulin light chains forming amyloid fibrils.
• Amyloidogenic light chains AL-09 and AL-103 were previously found not to form fibrils at pH 10.
• Tyrosine residues at the dimer interface may ionize at pH 10, inhibiting fibril formation through repulsion.

Purpose of the Study:

• To investigate the role of specific tyrosine residues in the fibril formation of light chain amyloidosis proteins AL-09 and AL-103.
• To characterize the impact of single and double tyrosine-to-phenylalanine mutations on fibril formation at pH 10.

Main Methods:

• Site-directed mutagenesis to create tyrosine-to-phenylalanine mutants in AL-09 and AL-103.
• Fibril formation assays at pH 10.
• Nuclear Magnetic Resonance (NMR) spectroscopy to study protein conformation and interactions.

Main Results:

• All AL-09 tyrosine-to-phenylalanine mutants formed fibrils at pH 10, indicating tyrosine's inhibitory role in this variant.
• None of the AL-103 tyrosine-to-phenylalanine mutants formed fibrils at pH 10, suggesting other factors are involved.
• NMR studies indicated that AL-09 favors specific dimer conformations where tyrosines are critical for amyloid formation, while AL-103 exhibits different conformational dynamics.

Conclusions:

• Tyrosine residues play a critical, conformation-dependent role in inhibiting fibril formation in light chain amyloidosis.
• The differential behavior of AL-09 and AL-103 mutants highlights the structural heterogeneity influencing amyloidogenesis.
• Targeting these tyrosine interactions could offer therapeutic strategies for light chain amyloidosis.