Thermal and acid denaturation of bovine lens alpha-crystallin

Publication Type:

Journal Article

Source:

Proteins, Volume 79, Issue 6, pp. 1747-1758 (2011)

ISBN:

1097-0134 (Electronic)08

DOI Name (links to online publication)

10.1002/prot.22998

Abstract:

The chaperone-like protein alpha-crystallin is a approximately 35 subunit hetero-oligomer consisting of alphaA and alphaB subunits in a 3:1 molar ratio and has the function of maintaining eye lens transparency. We studied the thermal denaturation of alpha-crystallin by differential scanning calorimetry (DSC), circular dichroism (CD), and dynamic light scattering (DLS) as a function of pH. Our results show that between pH 7 and 10 the protein undergoes a reversible thermal transition. However, the thermodynamic parameters obtained by DSC are inconsistent with the complete denaturation of an oligomeric protein of the size of alpha-crystallin. Accordingly, the CD data suggest the presence of extensive residual secondary structure above the transition temperature. Within the pH range from 4 to 7 the increased aggregation propensity around the isoelectric point (pI approximately 6) precludes observation of a thermal transition. As pH decreases below 4 the protein undergoes a substantial unfolding. The secondary structure content of the acid-denatured state shows little sensitivity to heating. We propose that the thermal transition above pH 7 and the acid-induced transition at ambient temperature result in predominant denaturation of the alphaB subunit. Although the extent of denaturation of the alphaA subunit cannot be estimated from the current data, the existence of a native-like conformation is suggested by the preserved association of the subunits and the chaperone-like activity. A key difference between the thermal and the acid denaturation is that the latter is accompanied by dissociation of alphaB subunits from the remaining alphaA-oligomer, as supported by DLS studies. Proteins 2011; (c) 2011 Wiley-Liss, Inc.

30/05/2011