Abstract
Systematic databank-based sequence comparisons with the cDNA sequence for a lens-specific rat protein localized between lens fiber cells disclosed its similarity to galectins. In mammals, two sequence changes occur within the seven positions of amino acids commonly engaged in contacts to the beta-galactoside core of glycans. Taking the compilation of GRIFIN genes to the level of diverse vertebrates revealed an exceptional variability: mammals shared alteration at two sites, birds and reptiles at only one site and amphibians and fish presented complete reconstitution. The homodimeric (proto-type) GRIFINs of chicken and zebrafish thus are active lectins. Crystallographical information for chicken GRIFIN illustrates the contact profile to lactose without one otherwise conserved site due to the Arg-to-Val substitution. That GRIFIN is enormously stable in vertebrate lenses, can act like a glue (or a bridge) due to its structure and interacts with alpha-crystallin (shown for murine GRIFIN) suggests a role in well-ordered packing of lens proteins. Referring to a likely analogy in plants, oligomeric leguminous lectins, beta-sandwich proteins as galectins, are also assumed to participate in depositing and spatially organizing cell contents, here storage proteins in protein bodies and their contact to the membrane in carbohydrate-dependent and-independent manners, a likely case of structural and functional convergence.
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