Oligomerization and phase transitions in aqueous solutions of native and truncated human βB1-crystallin

Human βB1-crystallin is a major eye-lens protein that undergoes in vivo truncation at the N-terminus with aging. By studying native βB1 and truncated βB1ΔN41, which mimics an age-related in vivo truncation, we have determined quantitatively the effect of truncation on the oligomerization and phase transition properties of βB1 aqueous solutions. The oligomerization studies show that the energy of attraction between the βB1ΔN41 proteins is about 10% greater than that of the βB1 proteins. We have found that βB1ΔN41 aqueous solutions undergo two distinct types of phase transitions. The first phase transition involves an initial formation of thin rodlike assemblies, which then evolve to form crystals. The induction time for the formation of rodlike assemblies is sensitive to oligomerization. The second phase transition can be described as liquid-liquid phase separation (LLPS) accompanied by gelation within the protein-rich phase. We refer to this process as heterogeneous gelation. These two phase transitions are not observed in the case of βB1 aqueous solutions. However, upon the addition of poly(ethylene glycol) (PEG), we observe heterogeneous gelation also for βB1. Our PEG experiments allow us to estimate the difference in phase separation temperatures between βB1 and βB1ΔN41. This difference is consistent with the increase in energy of attraction found in our oligomerization studies. Our work suggests that truncation is a cataractogenic modification since it favors protein condensation and the consequent formation of light scattering elements, and highlights the importance of the N-terminus of βB1 in maintaining lens transparency.