Structure of the Month: December 2008 [see all]

Structural insights into the role of mutations in amyloidogenesis

Elizabeth M. Baden^† , Edward G. Randles^† , Awo K. Aboagye^† , James R. Thompson^‡ , and Marina Ramirez-Alvarado^†
Departments of ^†Biochemistry and Molecular Biology and ^‡Physiology and Biomedical Engineering, 
College of Medicine, Mayo Clinic, Rochester, Minnesota

Light Chain Amyloidosis (AL) is a misfolding disease characterized by the deposition of monoclonal immunoglobulin light chains as amyloid fibrils affecting several organs, causing dysfunction. Understanding the relationship between the native protein structure and thermodynamic stability may unlock several mysteries of this disorder and unravel pathways via which amyloid fibrils can form.

We focus on several different AL proteins, which despite sharing at least 95% amino acid sequence identity have significantly different thermodynamic and biochemical properties. Our X-ray crystallographic studies show that AL proteins form dimers. One specific protein, AL-09, has an altered dimer interface that is rotated 90° from the interface observed in the κI O18/O8 germline protein¹. We believe the altered dimeric form could be important to the progression and pathology of the disease.

To assess the role of mutations in generating an amyloidogenic AL protein structure, we conducted a systematic restoration of AL-09 to its germline sequence by mutating the non-conservative somatic mutations located in the light chain dimer interface and studied this using X-ray crystallography². We also created mutants introducing the amyloidogenic residue into the germline protein. We were able to diffract the crystals of AL-09 H87Y, κI O18/O8 Y87H and κI O18/O8 N34I/Y87H to a high resolution, which revealed a small but significant structural deviations in our AL proteins. We found structural rearrangements in the conformation of residues in the loop regions, including the loss of key hydrogen bonds within loops and between monomers, which we believe may increase structural instability and could provide routes for protein misfolding to occur. We find that adding amyloidogenic mutations into the germline protein illustrates mutational cooperativity in promoting amyloidogenesis.

This work was performed in the laboratory of Dr. Marina Ramirez-Alvarado in collaboration with Dr. James R. Thompson at the College of Medicine, Mayo Clinic.

Data collection details

References

1. Baden, E. M., Owen, B. A., Peterson, F. C., Volkman, B. F., Ramirez-Alvarado, M. & Thompson, J. R. (2008). Altered dimer interface decreases stability in an amyloidogenic protein. J Biol Chem 283, 15853-60.
2. Baden, E. M., Randles, E. G., Aboagye, A. K., Thompson, J. R. & Ramirez-Alvarado, M. (2008). Structural insights into the role of mutations in amyloidogenesis. J Biol Chem 283, 30950-6.