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J. Biol. Chem., Vol. 266, Issue 30, 20079-20084, 10, 1991

Micellar subunit assembly in a three-layer model of oligomeric alpha- crystallin

MT Walsh, AC Sen and B Chakrabarti
Department of Biophysics, Boston University School of Medicine, Massachusetts 02118.

Differential scanning calorimetry was performed to monitor the heat- induced changes that occur in the structural domain of lens alpha- crystallin. Circular dichroism and fluorescence also were used to resolve the controversial issue of the quaternary structure of alpha- crystallin. Based on the thermal behavior as monitored by these techniques, a model is proposed that can account for all previous data as well as the currently reported thermal data. The proposed model of native alpha-crystallin has a three-layer structure in which the inner layer (core) is a micelle containing 12 subunits arranged in cuboctahedral symmetry. The apolar region is directed inward constituting a hydrophobic core similar to a micelle and adding structural stability. A second layer of six subunits has a similar but not identical structure to the first layer, directing its apolar face toward the hydrophobic core. Thus, these two layers constitute a micelle-like structure with octahedral symmetry. The third layer adds more subunits for a total of not more than 24. Differential scanning calorimetry, circular dichroism, and fluorescence studies indicated that the inner two-layer structure of molecular mass 360 kDa is highly stable and is most likely of the alpha m form. The three-layer structure of the native protein, however, is rather unstable. At 35-45 degrees C the outer layer dissociates from the inner two layers, and at higher temperatures rapidly reassociates to a slightly modified two- layer structure with a stability similar to that of alpha m. The proposed model does not require any specific assembly of the alpha A and alpha B subunits in each layer, but the fluorescence results suggest that the native inner two layers probably contain mostly alpha A.
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