December 19, 2017

PBIO seminar series: Rachel Martin

What can we learn about protein solubility and aggregation from a cold fish?
Rachel Martin
Professor Departments of Chemistry and Molecular Biology & Biochemistry UC Irvine
host: Sharona Gordon
seminar abstract: The βγ-crystallin fold that is ubiquitous in the structural proteins of the vertebrate eye lens is an ancient structural motif found in diverse organisms from all three domains of life. In organisms without eyes, e.g. archaea, bacteria, tunicates, and sponges, βγ-crystallins serve as calcium-binding proteins.  In vertebrates, they are primarily found in the eye lens, where they play an important role in controlling the refractive index gradient of this specialized tissue.  The ubiquitous βγ-crystallins of the vertebrate lens are believed to have descended from an ancestral single-domain Ca²⁺-binding crystallin by a process that included gene duplication resulting in two copies of the double Greek key domain per chain, as well as selection for high refractive index. Because the lens has negligible protein turnover, the crystallins must remain stable and soluble for the lifetime of the organism despite their extremely high concentrations. In particular, we are interested in the resistance to phase separation of the cold-adapted crystallins of the Antarctic toothfish, Dissostichus mawsoni. The eye lens of D. mawsoni is evolutionarily adapted to function in the permanently sub-freezing waters of the Southern Ocean.  This is in contrast to temperate and tropical fishes, and endothermic mammals, the lenses of which undergo liquid-liquid phase separation at low temperatures.  Mammalian lenses phase separate at temperatures between 10 °C and 20 °C – well above the Antarctic’s sub-zero marine environment.  The ability of the toothfish lens to maintain transparency in this frigid environment is particularly remarkable given that fish lenses have a high concentration of constituent proteins ≥1000 mg * mL^-1). Recent work in my group focuses on testing the hypothesis that γ-crystallin isoform heterogeneity coupled with cold selective evolutionary pressures contribute to the clarity of the toothfish lens.  We have measured the thermal stabilities and phase diagrams of seven key γ-crystallin lens proteins, and we are able to control the onset of liquid-liquid phase separation by introducing a small number of surface mutations. The implications of our findings with respect to the roles of frustration, ionic interactions, and protein flexibility liquid-liquid phase separation will be discussed.