Evolution of a protein superfamily: Relationships between vertebrate lens crystallins and microorganism dormancy proteins

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Abstract
A search of sequence databases shows that spherulin 3a, an encystment-specific protein ofPhysarum polycephalum, is probably structurally related to the β- and γ-crystallins, vertebrate ocular lens proteins, and to Protein S, a sporulation-specific protein ofMyxococcus xanthus. The β- and γ-crystallins have two similar domains thought to have arisen by two successive gene duplication and fusion events. Molecular modeling confirms that spherulin 3a has all the characteristics required to adopt the tertiary structure of a single γ-crystallin domain. The structure of spherulin 3a thus illustrates an earlier stage in the evolution of this protein superfamily. The relationship of β- and γ-crystallins to spherulin 3a and Protein S suggests that the lens proteins were derived from an ancestor with a role in stressresponse, perhaps a response to osmotic stress.