GRIFIN, a Novel Lens-specific Protein Related to the Galectin Family

Abstract

The vertebrate lens is a relatively simple cellular structure that has evolved to refract light. The ability of the lens to focus light on the retina derives from a number of properties including the expression at high levels of a selection of soluble proteins referred to as the crystallins. In the present study, we have used differential cDNA display techniques to identify a novel, highly abundant and soluble lens protein. Though related to the family of soluble lectins called galectins, it does not bind ␤-galactoside sugars and has atypical sequences at normally conserved regions of the carbohydrate-binding domain. Like some galectin family members, it can form a stable dimer. It is expressed only in the lens and is located at the interface between lens fiber cells despite the apparent lack of any membrane-targeting motifs. This protein is designated GRIFIN (galectin-related inter-fiber protein) to reflect its exclusion from the galectin family given the lack of affinity for ␤-galactosides. Although the abundance, solubility, and lens-specific expression of GRIFIN would argue that it represents a new crystallin, its location at the fiber cell interface might suggest that its primary function is executed at the membrane.

Cell Cycle Regulation by Galectin-12, a New Member of the Galectin Superfamily

“…Proteins containing sequence similarities with galectins but without demonstrable lectin activity also exist. For example, a novel protein structurally related to galectins, but lacking ␤-galactoside binding activity, has been identified in the lens and designated as GRIFIN (galectin-related interfiber protein) (7).…”

Members of the Galectin Network with Deviations from the Canonical Sequence Signature. 1. <i>G</i>alectin-<i>R</i>elated <i>I</i>nter-<i>F</i>iber Prote<i>in</i> (GRIFIN)

“…Ensuing northern blot analysis detected the corresponding mRNA "at high level in the eye but not elsewhere" (36). The respective cDNA sequence was not unique.…”

“…The quest for genes differentially expressed during postnatal stages of lens development in rat tracked down a PCR signal, whose intensity increased within the period of days 29.5 to 50 post-conception (36). Ensuing northern blot analysis detected the corresponding mRNA "at high level in the eye but not elsewhere" (36).…”

“…These changes let a loss of carbohydrate-binding activity become expectable, and an assay with lactose-bearing beads bore out this expectation (36). Intriguingly, the reconstitution of the sequence signature in steps and its combination with a Val47Phe change to neutralize another difference to active galectins, establishing a set of rat GRIFIN mutants, did not re-establish activity to bind lactose (36). The resulting triple mutant of rat GRIFIN thus is an inactive lectin, too, despite the formal presence of the sequence signature, an indication for significance of further structural aspects.…”

“…However, the galectin family includes members with deviations from the sequence signature, as documented for one case in Fig. 1 (a second case will be described in the accompanying review (35) (Article for special issue on Galectins) SE2 (GRIFIN) (36). The ensuing search for homologues beyond mammals uncovered i) presence in vertebrates and ii) a unique variability concerning lectin activity, that is a species-dependent switch from inactive (mammals) to fully active (fish, birds).…”