PROSITE documentation PDOC00197 [for PROSITE entry PS50915]

Crystallins beta and gamma 'Greek key' motif profile

Crystallins are the dominant structural components of the eye lens. Among the different type of crystallins, the β and γ crystallins form a family of related proteins [1,2]. Structurally, β and γ crystallins are composed of two similar domains which, in turn, are each composed of two similar motifs with the two domains connected by a short connecting peptide. Each motif, which is about forty amino acid residues long, is folded in a distinctive 'Greek key' motif, composed of four antiparallel β-strands: a, b, c, and d (see <PDB:1A45>).

Apart from the different types of β and γ crystallins, this family also includes the following proteins:

• Two related proteins from the sporulating bacterium Myxococcus xanthus: protein S, a calcium-binding protein that forms a major part of the spore coat, and a close homolog of protein S.
• Spherulin 3a from the slime mold Physarum polycephalum. Spherulin 3a is a development specific protein synthesized in response to various kinds of stress leading to encystment and dormancy. The sequence of Spherulin 3a consists of two 'Greek key' motifs [3].
• Epidermis differenciation-specific protein (EDSP or ep37) of the amphibian Cynops pyrrhogaster.
• Mammalian absent in melanoma 1 protein (AIM1). It contains 12 'Greek key' motifs.

β/γ 'Greek key' motifs may be further classified as A- or B-type. Vertebrate members of the β/γ superfamily conform to an ABAB motif pattern. B-type motifs are the most highly conserved and form most of the contacts between domains. In protein S, the order of motifs is reversed to BABA, suggesting a separate history of duplication events [4].

The profile we developed for this family of proteins covers the entire 'Greek key' motif.