On the thermal stability of alpha-crystallin: a new insight from infrared spectroscopy.

alpha-Crystallin is a major structural protein of the vertebrate lens which shows structural and functional similarities to small heat shock proteins. The structure and the thermal stability of bovine alpha-crystallin were studied by Fourier-transform infrared spectroscopy, circular dichroism, and differential scanning calorimetry. Infrared spectroscopic data provide evidence which corroborates the view that the secondary structure of alpha-crystallin is highly ordered and consists predominantly of beta-sheets. However, the present results fail to support the widespread notion of an extremely high thermal stability of the protein. All three experimental approaches used in this study show that alpha-crystallin undergoes a major thermotropic transition with a midpoint at 60-62 degrees C. Furthermore, Fourier-transform infrared spectra provide evidence that this conformational transition is associated with a massive loss of the native beta-sheet structure. These results shed new light on structural properties of alpha-crystallin and have important implications for understanding the mechanism of the chaperone-like action of this protein.