BIOLOGICAL OVERVIEW

furrowed (fw) is named after a characteristic furrowing observed in the eyes of furrowed mutants. Eyes are reduced in size, especially in the ventral margin, and show many deep penetrating furrows or crevices, suggestive of an effect on the structural integrity of the retina. The furrows penetrate the entire depth of the retina, reaching the basement membrane separating the retina from the first optic lobe. The ommatidial pattern is severely disorganized and the ommatidia show altered morphology -- many lose their typical hexagonal shape. The interommatidial bristles often show altered morphology and spacing, and are occasionally duplicated. The disturbances in the patterning and morphology of ommatidia suggest that in the developing retina furrowed mutations may affect the recuitment of cells into the ommatidia. The basement membrane separating the retina from the lamina layer of the optic lobe is not clearly defined in furrowed mutants. This membrane, which provides openings for photoreceptor axons to pass, is made up of the feet of cone and pigment cells. The furrowed mutation may affect the development of these epidermal cells (Leshko-Lindsay, 1997).

Selectins, the family of proteins to which Furrowed belongs, are integral transmembrane proteins composed of three extracellular domains: an amino-terminal carbohydrate recognition or lectin domain, an epidermal growth factor (EGF)-like domain (absent in Furrowed), and a variable number of consensus repeats found in complement binding proteins, followed by a transmembrane domain and a short cytoplasmic tail at the carboxy terminus. To date, three mammalian family members have been identified that differ in the number of complement binding repeats: L-selectin has two repeats, E-selectin has six repeats and P-selectin has nine repeats. The lectin domain is homologous to the family of C-type lectins, which are calcium dependent carbohydrate binding proteins (Lesko-Lindsay, 1997 and references). The lectin domain has been demonstrated to mediate protein-carbohydrate interaction necessary for cell adhesion via recognition of complex carbohydrate determinants found in several glycoproteins and glycolipids (Varki, 1994 and Rosen, 1994). The function and role of the EGF domain and complement binding repeats seems to be (respectively), ligand recognition and protein binding (Watson, 1991 and Kansas, 1994). The cytoplasmic domain is also suspected to have a role in regulating cell adhesion by controlling cytoskeletal interactions and/or receptor avidity (Kansas, 1993).

The finding of a Drosophila selectin is the first demonstration that these proteins function outside the immune system of mammals. Another carbohydrate binding protein, Gliolectin has been characterized in Drosophila. The furrowed mutation that causes defects in patterning and in cell determination of the ommatidia and bristles is an indication that the selectin is either directly or indirectly involved in neural cell determination. The defects in morphology of the compound eye and bristles, as well as the defects in bristle polarity, indicate that FW might mediate the cell-cell adhesion interaction necessary for proper morphogenesis of these structures. This could take place by directly providing mechanical cell-cell adherence and/or directing cell-cell communication via interactions with the cytoskeleton (Leshko-Lindsay, 1997).

GENE STRUCTURE

There are two transcripts of 4.5 and 4.0 kb. Both transcripts are present in pupae, where they accumulate at similar levels, whereas only the larger one can be detected in wild-type adults. The genomic region proximal to the 5' end of the composite cDNA contains promoter elements needed for the start of transcription including a CAAT box and TATA box, suggesting that the 3.8 kb composite transcript should represent the full length 4.0 kb mRNA. The 4.5 kb transcript differs from the 4.0 kb RNA by the presence of additional exons in the 5' region. The 4.5 kb transcript appears to have two unique exons at the 5' end of the mRNA, suggesting that the 4.5 kb transcript is expressed from an alternative promoter. The transcription start of the alternative promoter contains a CCAAT box and TATA box (Leshko-Lindsay, 1997).

Exons - 14

PROTEIN STRUCTURE

Amino Acids - 978

Structural Domains

The FW protein is most similar to P-selectin, which has nine complement repeats. FW lacks the EGF-like domain found in all other selectin family members. The lectin domain, consisting of 122 amino acids, is located in the amino-terminal region. The three exons encoding the lectin are present in both transcripts. Conserved amino acids include the carbohydrate and calcium binding regions. FW lectin domain is 70% identical to the consensus lectin domain sequence. Only one Ca2+ binding site is present. The putatitive FW lectin domain is most similar to the lectin domain of selectins, since both domains have only one of the two calcium binding sites. However, the carbohydrate recognition region of the FW lectin domain (QPN) is a hybrid between the recognition sequence found in mannose-binding proteins (EPN) that is specific for mannose, and the recognition sequence found in other lectin domains that are specific for galactose (Leshko-Lindsay, 1997, citing personal communication from M Quesenberry and Y. C. Lee) There are 10 complement-like repeats, each repeat comprising 58-60 residues, immediately adjacent to one another. The seventh repeat has 86 amino acids. The ten repeats each have conserved invariant cysteines. The transmembrane domain comprises 24 highly hydrophobic resides. The cytoplasmic domain consists of 130 amino acids (Leshko-Lindsay, 1997).

date revised: 18 MAR 97 

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