A possible structure for alpha-crystallin.

alpha-Crystallin, the major protein of the mammalian eye lens, is found in vivo as a multimeric aggregate composed of two closely related subunits whose molar ratio is widely variable from species to species. Attempts to determine the arrangement of the subunits within the aggregate, or even to determine the size of the aggregate and the number of subunits composing it, have not resulted in general agreement. Because of the variability in alpha-crystallin particle size, the apparent dependence of this parameter on certain environmental factors (e.g. temperature), the absence of a specific requirement for either alpha-crystallin isoform in aggregation, and the sharp division in the amino acid sequence between a strong hydrophobic region and a sharply hydrophilic one, it is suggested that the alpha-crystallin aggregate has the properties of a protein micelle. This hypothesis is consistent with what is known of the alpha-crystallin molecule and aggregate, and can be tested experimentally. If this hypothesis is shown to be true, then alpha-crystallin will be the first example of a naturally occurring protein micelle.