Abstract
Purpose: :
To examine the structure-function relationship of the lens protein αA-crystallin by comparing the amino acid sequences and functional characteristics of this protein from six fish species differing in physiological temperature.
Methods: :
αA-crystallin cDNA was cloned from six fish species ranging in physiological temperature from -2 to 42 deg C and used to produce recombinant proteins. The chaperone-like activity of each recombinant protein was quantified by measuring its ability to prevent the chemically induced aggregation of lactalbumin and insulin. Structures of the α-crystallin domains of the six cloned and three additional fish αA-crystallins were modeled using the 3D-PSSM and PHYRE servers and refined using Look v.3.5. The surface accessible areas and hydrophobic folding energy were calculated for each protein structure.
Results: :
The six cloned fish αA-crystallin amino acid sequences ranged in length from 173 to 176 residues and had identities between 77.3% and 98.9%. αA-crystallins from species with lower average body temperatures showed stronger chaperone-like activity when proteins were assayed at the same lower temperature. This correlation between physiological temperature and the ability to prevent aggregation occurred when either lactalbumin or insulin was used as a target protein. While no specific amino acid changes correlated with the change in optimal temperature, calculations of hydrophobic folding energies for the six cloned and three additional fish αA-crystallins demonstrated a linear decrease in energy values from -63 kcal/mol to -69 kcal/mol as the corresponding average physiological body temperatures increased from 1 to 28 deg C.
Conclusions: :
Chaperone-like activity assays and measurements of hydrophobic folding energy indicate that fish αA-crystallins have adapted evolutionarily to a broad range of physiological temperatures. Our data suggest that αA-crystallin requires a more stable protein fold at higher physiological temperatures and that hydrophobic stabilization might play a role in adapting chaperone properties to different thermal environments. The comparative approach used in this study highlights the power of using evolutionary variation to examine relationships between α-crystallin structure and function.
Keywords: crystallins • protein structure/function • chaperones