Chaperone-like activity and temperature-induced structural changes of alpha-crystallin.
alpha-Crystallin is known to exhibit chaperone-like activity. We have studied its chaperone-like activity toward the aggregation of betaL-crystallin upon refolding of this protein from its unfolded state in guanidinium chloride. The chaperone-like activity of alpha-crystallin is less pronounced below 30 degrees C and is enhanced above this temperature. The plot of percentage protection as a function of temperature shows two transitions; one at 30 degrees C and another at around 55 degrees C. We have performed steady state fluorescence, fluorescence polarization, fluorescence quenching, circular dichroism, sedimentation analysis, and gel filtration chromatography to probe the temperature-induced structural changes of alpha-crystallin. Our results show that at above 50 degrees C, alpha-crystallin undergoes a transition to a multimeric molten globule-like state. Above 30 degrees C, a minor but detectable perturbation in its tertiary structure occurs that might lead to the observed exposure of its hydrophobic surfaces. These results support our earlier hypothesis that alpha-crystallin prevents the aggregation of other proteins by providing appropriately placed hydrophobic surfaces; a structural transition above 30 degrees C involving enhanced or reorganized hydrophobic surfaces of alpha-crystallin is important for its chaperone-like activity. It is possible that a structural alteration induced by temperature forms a part of the general mechanism of chaperone function, because they are required to function more effectively at nonpermissible temperatures.
Raman
B
B
Centre for Cellular and Molecular Biology, Hyderabad 500 007, India.
Rao
C M
CM
D018833:Chaperonins / Q000378:metabolism*; D002942:Circular Dichroism; D003459:Crystallins / Q000737:chemistry*; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature
D003459:Crystallins; D018833:Chaperonins