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-cry stallin were studied by Fourier-transform infrared spectroscopy, circu lar dichroism, and differential scanning calorimetry. Infrared spectro scopic data provide evidence which corroborates the view that the seco ndary structure of alpha-crystallin is highly ordered and consists pre dominantly of beta-sheets. However, the present results fail to suppor t the widespread notion of an extremely high thermal stability of the protein. All three experimental approaches used in this study show tha t alpha-crystallin undergoes a major thermotropic transition with a mi dpoint at 60-62 degrees C. Furthermore, Fourier-transform infrared spe ctra provide evidence that this conformational transition is associate d with a massive loss of the native beta-sheet structure. These result s shed new light on structural properties of alpha-crystallin and have important implications for understanding the mechanism of the chapero ne-like action of this protein.