University of California-San Francisco
Thursday, February 10, 2011
What are the driving forces for the aggregation of proteins into crystals or amyloid fibrils?
Protein solutions have a rich phase diagram that includes multiple solid and
liquid states. These phases have important applications in biotechnology,
medicine, and industry, yet little is known about how to navigate the phase
space in a rational manner. In this talk, I will show how solution conditions
can be manipulated to control the aggregation of two important states: amyloid
fibrils and protein crystals.
Amyloid fibrils are linear, unbranched protein aggregates that are associated with many diseases including Alzheimer's, Type II diabetes, and Mad Cow. Recent evidence has suggested that disease progression is driven, not by the fibrils, but by smaller oligomers that appear prior to fibril formation. I will present a simple theory that describes the equilibrium between fibrils, oligomers, and monomeric proteins. The theory describes how fibrils can be used to "soak up" excess protein from the solution, preventing the formation of toxic oligomers. It also quantitatively captures the effects of salt and pH on fibril stability, and resolves a conflict in the literature by showing that the urea denaturation pathway depends qualitatively on the protein concentration.
The growth of crystals is the major bottleneck in the determination of a protein's three-dimensional structure. This step is currently accomplished by trial-and-error, highlighting the poor understanding of the physics behind the stabilization and growth of crystals. I will show that the electrostatic contribution to crystal stability is dominated by the entropic cost of confining counterions within the crystal, and present a model that quantitatively describes the solubility of lysozyme crystals as a function of pH, temperature, and salt concentration. I will also show that the non-specific binding of proteins to the crystal surface imposes a "speed limit" to crystal growth leading to the counter-intuitive conclusion that crystal growth is accelerated by destabilizing the crystal.