Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 100, Issue 3, Pages 970-974Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.242746499
Keywords
protein; cataract; polyethylene glycol; phase transition; partitioning
Categories
Funding
- NEI NIH HHS [EY05217] Funding Source: Medline
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gammaS-crystallin (gammaS) is an important human and bovine eye lens protein involved in maintaining the transparency of the eye. By adding small amounts of polyethylene glycol (PEG) to the binary aqueous bovine gammaS solutions, we have observed liquid-liquid phase separation (LLPS) at -8degreesC and revealed that, in the binary gammaS-water system, this phase transition would occur at -28degreesC. We have measured both the effect of PEG concentration on the LLPS temperature and protein/PEG partitioning between the two liquid coexisting phases. We use our measurements of protein/PEG partitioning to determine the nature and the magnitude of the gammaS-PEG interactions and to quantitatively assess the effectiveness of PEG as a crystallizing agent for gammaS. We use our measurements of LLPS temperature as a function of protein and PEG concentration to successfully determine the location of the critical point for the binary gammaS-water system. This phase transition cannot be observed in the absence of PEG because it is inaccessible due to the freezing of the system. Our findings indicate that the effective interactions between gammaS molecules in the binary gammaS-water system are attractive. We compare the magnitude of the attraction found for gammaS with the results obtained for the other gamma-crystallins for which the critical temperature is located above the freezing point of the system. This work suggests that PEG can be used to reveal the existence of LLPS for a much wider range of binary protein-water systems than known previously.
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