Structural Homology of Lens Crystallins

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Abstract
The X‐ray crystallographic structure of bovine γ‐crystallin shows four similar folding motifs each composed of about 42 residues arranged as four topologically sequential, anti‐parallel β‐strands. Since the β and γ‐crystallin sequences show good homology, proposals for a four‐motif β‐crystallin model have been made. The other bovine eyelens protein species, α‐crystallins, are not homologous to β or γ‐crystallin in primary structure. In the present work, smoothed plots of amino acid sequence number versus a residue characteristic (e.g. hydrophobicity) were calculated for the various crystallins. Cross‐correlation coefficients were then determined between pairs of crystallin plots for various registers of the curves. The correlation plots were then combined for several characteristics and for pairwise comparisons between β or γ‐crystallin and the α‐crystallins. The resulting plots showed four peaks separated by about 42 residues for the α‐crystallins, suggesting that they also possess a four‐motif β‐barrel structure. The physical parameter comparison technique appears generally applicable in suggesting a structural and functional relationship amongst proteins that show no primary sequence homology.