The body wall muscles in the Drosophila larva arise from interactions between Dumbfounded/Kirre and Irregular chiasm C-roughest (IrreC-rst)-expressing founder myoblasts and Sticks-and-stones (Sns)-expressing fusion competent myoblasts in the embryo. Sns, a member of the immunoglobulin superfamily that is essential for myoblast fusion (Bour, 2000), mediates heterotypic adhesion of S2 cells with Duf/Kirre and IrreC-rst-expressing S2 cells, and colocalizes with these proteins at points of cell contact. These properties are independent of their transmembrane and cytoplasmic domains, and are observed quite readily with GPI-anchored forms of the ectodomains. Heterotypic interactions between Duf/Kirre and Sns-expressing S2 cells occur more rapidly and to a greater extent than homotypic interactions with other Duf/Kirre-expressing cells. In addition, Duf/Kirre and Sns are present in an immunoprecipitable complex from S2 cells. In the embryo, Duf/Kirre and Sns are present at points of contact between founder and fusion competent cells. Moreover, Sns clustering on the cell surface is dependent on Duf/Kirre and/or IrreC-rst. Finally, although the cytoplasmic and transmembrane domains of Sns are expendable for interactions in culture, they are essential for fusion of embryonic myoblasts (Galletta, 2004).

In most organisms, muscle fibers are composed of large multinucleate cells generated by the fusion of committed myoblasts. Prerequisites to this process include recognition by, migration toward, and adhesion to, other myoblasts. In Drosophila, formation of the larval body wall muscles involves two distinct myoblast populations -- founder cells and fusion competent cells, each of which is essential for formation of multinucleate fibers. Founder cells control the unique identities of each muscle fiber, and seed the fusion process. In contrast, the fusion competent myoblasts, which comprise a much larger population, recognize, migrate to, and adhere to a founder cell. Coincident with and subsequent to myoblast adhesion, intracellular events include the appearance of electron dense vesicles at sites of cell-cell contact, resolution of these vesicles into 'fusion plaques', and vesiculation of the cell membrane. The fusion competent myoblasts are then thought to take on the identity of the founder cell with which they fuse (Galletta, 2004 and references therein).

Implicit in the above mechanism is an apparent requirement that founder and fusion competent myoblasts recognize each other, and studies have revealed that members of the immunoglobulin superfamily (IgSF) serve this purpose. Sticks and stones (Sns), which is found exclusively on the surface of the fusion competent myoblasts, appears in cells just prior to fusion and decreases rapidly thereafter. It is essential for fusion of both somatic and visceral muscles (Bour, 2000 and Klapper, 2002), and the fusion competent myoblasts of sns mutant embryos do not migrate to or associate with the founder cells (reviewed in Abmayr, 2003). Sns shares significant homology with Hibris (Hbs), which appears to play a nonessential regulatory role in myoblast fusion, since both ectopic expression and loss of Hbs cause subtle defects in myoblast fusion that do not result in lethality. Two additional IgSF members, Dumbfounded/Kin of IrreC (Duf/Kirre) and Irregular chiasm C-roughest (IrreC-rst), serve redundant functions in the embryonic founder cells. Specifically, removal of both genes results in a complete absence of myoblast fusion, and targeted expression of either Duf/Kirre or IrreC-rst is sufficient to rescue the muscle loss. Fusion competent cells fail to migrate toward their founder cell partners in embryos lacking both duf/kirre and irreC-rst, and both can serve as attractants for the fusion competent myoblasts when ectopically expressed. While Duf/Kirre expression is limited to the founder cells, IrreC-rst is likely expressed in some fusion competent cells as well as the founder cells (Galletta, 2004 and references therein).

The differential expression of Sns in the fusion competent myoblasts and either Duf/Kirre or IrreC-rst in the founder cells, along with their respective loss-of-function phenotypes, supports a model in which these molecules mediate recognition and adhesion between founder and fusion competent cells. Consistent with this suggestion, Schneider line 2 (S2) cells transiently transfected with Sns adhere to cells that express Duf/Kirre (Dworak, 2001). While the in vivo relevance remains unclear, studies have also demonstrated that Duf/Kirre and IrreC-rst can direct homotypic aggregation when expressed in S2 cells, and accumulate at points of cell–cell contact (Galletta, 2004).

To better understand the roles of Duf/Kirre, IrreC-rst, and Sns, their behavior was examined in both cultured S2 cells and intact embryos. Consistent with a direct role in cell adhesion, it was demonstrated that all three proteins are enriched and colocalize at points of cell-cell contact within S2 cell clusters. Moreover, Duf/Kirre and Sns colocalize at points of contact between fusion competent and founder-cells in the embryo, and are present in a complex that can be immunoprecipitated from heterotypic S2 cell clusters. In both S2 cells and in embryos, localization of Sns is dependent on the presence of Duf/Kirre and/or IrreC-rst-expressing cells, and requires only the extracellular domain. Although the cytoplasmic and transmembrane domains are absolutely essential for myoblast fusion, neither aggregation nor colocalization is dependent on their presence. Thus, these data support a model in which Sns mediates cell adhesion through its extracellular domain and plays a role in other aspects of myoblast fusion through its cytoplasmic domain and/or transmembrane region (Galletta, 2004).

Previous studies have demonstrated that Duf/Kirre and IrreC-rst, but not Sns, can mediate homotypic aggregation of S2 cells. The behavior of Duf/Kirre and IrreC-rst is similar to that of their vertebrate ortholog Neph1. Unlike Sns, however, its vertebrate ortholog nephrin interacts homotypically (Khoshnoodi, 2003). In any case, Sns can clearly direct aggregation with cells that express Duf/Kirre (Dworak, 2001) or IrreC-rst. These data, in combination with the Sns, Duf/Kirre and IrreC-rst loss of function phenotypes and embryonic patterns of expression, suggest that these molecules mediate interaction between founder and fusion competent myoblasts (Galletta, 2004).

Using indirect immunofluorescence to identify the cells expressing Duf/Kirre, IrreC-rst and Sns, as well as the specific location of these proteins within the cell, it has been established that cells expressing any one of these proteins do not adhere to untransfected S2 cells. Thus, cell adhesion requires the presence of these proteins on opposing cells, and cell surface molecules endogenous to S2 cells are not sufficient to direct aggregation with Duf/Kirre, IrreC-rst or Sns-expressing cells. In addition, examination of subcellular localization revealed enrichment of IrreC-rst, Duf/Kirre and Sns at points of contact between S2 cells in an adherent pair. Such sites of co-localization are observed in embryonic cells in a segmentally repeated pattern reminiscent of the spatial distribution of founder cells. Most convincingly, Duf/Kirre and Sns co-localization is observed at points of contact between elongated founder cells and associated fusion competent myoblasts. Thus, these molecules are present at the appropriate time and subcellular location to mediate a direct cell-cell interaction. Finally, Sns is closely associated with Duf/Kirre, as evidenced by their co-immunoprecipitation from S2 cell aggregates (Galletta, 2004).

While these data strongly support the direct association of Sns with Duf/Kirre, they do not eliminate the possibility that either protein requires interaction in cis with a molecule endogenous to S2 cells to interact in trans. However, two additional observations argue against such a mechanism. These findings relate to the behavior of Sns and Duf/Kirre ectodomains when localized to the cell membrane with a GPI-anchor. These truncated molecules are able to direct cell adhesion quite effectively. Thus neither the transmembrane nor the cytoplasmic domains, or any proteins that associate in cis with these domains, are necessary for cell adhesion. Moreover, GPI-linked Sns does not function as a dominant inhibitor when overexpressed in the embryonic musculature, as one might anticipate if the extracellular domain normally mediates an essential interaction in cis. Though indirect, and not definitive, these data strongly support a direct interaction between Sns and Duf/Kirre (Galletta, 2004).

Although Duf/Kirre and IrreC-rst are capable of directing homotypic adhesion when expressed in S2 cells, the biological relevance of these interactions in the embryonic musculature is unclear. Numerous studies suggest that Duf/Kirre, IrreC-rst-expressing founder cells seed the fusion process, and pattern the resulting muscle fibers through information transferred to the naïve fusion competent cells as fusion progresses. Moreover, founder cells do not appear to interact with each other in the embryo. The data demonstrates that Duf/Kirre-expressing S2 cells have a greater affinity for cells expressing Sns than they do for other cells expressing Duf/Kirre. This observation may reflect a difference that also occurs in vivo, such that founders have a higher affinity for fusion competent cells than for other founder cells. This difference in affinity, coupled with the fact that fusion competent myoblasts outnumber founder cells by at least 10 to 1, may underlie preferential founder:fusion competent cell association (Galletta, 2004).

The absence of the transmembrane and cytoplasmic domains of Duf/Kirre has a significant effect on formation and/or stability of homotypic cell interactions. By contrast, the cytoplasmic/transmembrane domains of Duf/Kirre and Sns play little, if any, role in their ability to direct heterotypic cell adhesion. One potential explanation for this effect is that the apparently weaker homotypic interaction of Duf/Kirre relies more heavily on stabilization provided by association with the cytoskeleton, and is therefore more sensitive to removal of the cytoplasmic domain. Interestingly, the cytoplasmic domain of Duf/Kirre can interact with Anti-social/Rolling pebbles (Ants/Rols), which recruits the cytoskeletal protein D-titin to distinct points between founders and fusion competent cells. The Duf/Kirre interaction with Sns, which appears to form more efficiently or stably, may be less sensitive to the presence of the corresponding cytoplasmic domains (Galletta, 2004).

Examination of cell adhesion driven by other IgSF members has revealed examples in which the cytodomain is important, such as PECAM, and examples in which it is dispensable, such as L1-CAM and neuroglian. Interestingly, several IgSF proteins associate with components of the cytoskeleton, including L1-CAM, ICAM-1, ICAM-2, and CEACAM1-L, possibly to stabilize interactions with other proteins (Galletta, 2004).

Although not necessary to promote adhesion of S2 cells, the transmembrane and/or cytoplasmic domains of Sns are absolutely essential in the embryo. Since Sns is necessary for migration of fusion competent cells toward the founder myoblasts (Abmayr, 2003), and this behavior can be rescued by full length, but not GPI-anchored Sns, the cytodomain/transmembrane requirement likely reflects a role for Sns in directing migration of the fusion competent cells (Galletta, 2004).

There is already ample evidence implicating other IgSF proteins in cell signaling through their cytoplasmic tails. For example, migratory responses to axon guidance cues are controlled by the cytoplasmic tails of Roundabout (Robo) and Frazzled. Moreover, the Robo cytoplasmic domain interacts with such signaling molecules as Enabled and Abelson. It remains to be determined whether the Sns domain also functions in events subsequent to cell migration and adhesion. Intracellular events critical to myoblast fusion include the recruitment of organelles to points of contact between myoblasts. Binding to the cytoplasmic domains of Duf/Kirre or Sns may provide a mechanism by which proteins or vesicles can be recruited to sites of membrane fusion. For example Ants/Rols, which interacts with the Duf/Kirre cytodomain, can also interact physically with MBC, a molecule implicated in the Rac1 pathway. Ants/Rols, MBC, and the small GTPases Rac1 and Rac2 are all required for myoblast fusion, suggesting that the Duf/Kirre cytodomain may recruit molecules essential to fusion. In addition, Loner, an ARF guanine nucleotide exchange factor that is essential for myoblast fusion, also co-localizes with Duf/Kirre in the founder myoblasts. Whether its cytodomain is associated solely with migration or with both migration and fusion, Sns appears to act as a signaling molecule that likely mediates its effects through interaction with intracellular components (Galletta, 2004).

Sns and Kirre, the Drosophila orthologs of Nephrin and Neph1, direct adhesion, fusion and formation of a slit diaphragm-like structure in insect nephrocytes

The Immunoglobulin superfamily (IgSF) proteins Neph1 and Nephrin are co-expressed within podocytes in the kidney glomerulus, where they localize to the slit diaphragm (SD) and contribute to filtration between blood and urine. Their Drosophila orthologs Kirre (Duf) and Sns are co-expressed within binucleate garland cell nephrocytes (GCNs) that contribute to detoxification of the insect hemolymph by uptake of molecules through an SD-like nephrocyte diaphragm (ND) into labyrinthine channels that are active sites of endocytosis. The functions of Kirre and Sns in the embryonic musculature, to mediate adhesion and fusion between myoblasts to form multinucleate muscle fibers, have been conserved in the GCNs, where they contribute to adhesion of GCNs in the 'garland' and to their fusion into binucleate cells. Sns and Kirre proteins localize to the ND at the entry point into the labyrinthine channels and, like their vertebrate counterparts, are essential for its formation. Knockdown of Kirre or Sns drastically reduces the number of NDs at the cell surface. These defects are associated with a decrease in uptake of large proteins, suggesting that the ND distinguishes molecules of different sizes and controls access to the channels. Moreover, mutations in the Sns fibronectin-binding or immunoglobulin domains lead to morphologically abnormal NDs and to reduced passage of proteins into the labyrinthine channels for uptake by endocytosis, suggesting a crucial and direct role for Sns in ND formation and function. These data reveal significant similarities between the insect ND and the SD in mammalian podocytes at the level of structure and function (Zhuang, 2009).

In Drosophila, the Immunoglobulin superfamily (IgSF) proteins encoded by kin of irre [kirre; also known as dumbfounded (duf)], roughest (rst), sticks and stones (sns) and hibris (hbs) function as ligand-receptor pairs on the surface of founder cells and fusion competent myoblasts. These proteins mediate recognition, adhesion and fusion to form multinucleate syncitia through direct interaction at sites of myoblast contact. However, neither their action nor their expression is exclusive to the musculature, and previous studies noted their role in cell recognition and adhesion in the Drosophila eye. Moreover, multiple studies have confirmed the presence of the kirre transcript and sns transcript in the binucleate garland cell nephrocytes (GCNs). These nephrocytes possess a structure visible by transmission electron microscopy (TEM) reminiscent of the slit diaphragm (SD) in the vertebrate kidney, and process waste products from the hemolymph. It is therefore compelling that the fly detoxification machinery may have similarities to that in mammals, and that Sns and Kirre play roles similar to those of their vertebrate counterparts (Zhuang, 2009).

Removal of waste products from the closed circulatory system of vertebrates takes place in the kidney glomerulus. Podocytes, kidney epithelial cells that surround the capillary blood vessels, extend foot processes that contact the surface of these vessels. Filtration then occurs as molecules flow out of the bloodstream through slits between adjacent foot processes into the urine. Neph1, vertebrate orthologs of the above Drosophila IgSF proteins, localize to this filter and appear to be an important determinant of glomerular permeability (Hamano, 2002; Liu, 2003). Mutations in nephrin and neph1 are associated with congenital nephrotic syndrome as a consequence of defects in this filtration diaphragm. Lack of either nephrin or neph1 leads to podocyte foot process effacement and detachment of podocytes from the glomerular basement membrane, loss of SDs, and proteinuria (Donoviel, 2001; Putaala, 2001; Zhuang, 2009 and references therein).

In addition to their high degree of homology, Nephrin and Neph1 have other features in common with Sns and Kirre. Heterophilic interactions occur in trans between the extracellular domains of Nephrin and Neph1, and Sns and Kirre. Studies have suggested that, in addition to serving as a scaffold onto which other proteins in the SD assemble, Nephrin and Neph1 function as signaling molecules to direct downstream cytoplasmic events (Benzing, 2004). They cooperate to transduce a signal that directs actin polymerization (Garg, 2007), and activation of this pathway occurs through interaction of phosphorylated tyrosines in the cytoplasmic domains of Nephrin and Neph1 to adaptor proteins (Jones, 2006; Verma, 2006). These adaptor proteins recruit components of the actin polymerization machinery that include N-WASp and Arp2/3. Similar phosphotyrosine modifications are important for Sns function and studies have shown that the WASp and Arp2/3 actin polymerization machinery functions in Drosophila myoblast fusion, probably downstream of Sns and Kirre (Zhuang, 2009).

The pericardial cells and garland cells comprise two subpopulations of Drosophila nephrocytes that, along with Malpighian tubules, form the excretory system. Approximately 25-30 tightly associated binucleate GCNs encircle the anterior end of the proventriculus in a 'garland' at its junction with the esophagus. The cortical region of the cytoplasm includes elaborate channels that are generated by invagination of the plasma membrane during embryogenesis and early larval instar stages. The initial invagination is associated with formation of a junction between two sites on the plasma membrane that are visible by TEM. Through a mechanism that is not entirely clear, this initial invagination expands into an extensive array of labyrinthine channels by the third-instar larval stage. The GCNs are very active in endocytosis via coated vesicles at sites deep within these labyrinthine channels. Thus, molecules to be eliminated must gain access to the endocytic machinery deep in these channels. These studies also identified a thin bridge spanning the channel opening that is visually similar to the vertebrate SD. The presence of Sns and Kirre and a slit diaphragm-like structure in these binucleate cells raised the possibility that these IgSF proteins might function in GCN fusion and/or in formation of this structure (Zhuang, 2009).

This study, along with that of Weavers (2009) demonstrates that Sns and Kirre are present in, and crucial for, the nephrocyte diaphragm (ND). Knockdown of Kirre or Sns results in a severely diminished number of NDs and smoothening of ND-associated furrows on the GCN surface, implicating Sns and Kirre in their formation. Mutations in the extracellular domain of Sns cause major perturbations in the ND, establishing that Sns also dictates fundamental aspects of its structure. Similar smoothening of the GCN surface occurs upon knockdown of Polychaetoid (Pyd), the Drosophila ortholog of the zonula occludens (ZO-1) tight junction protein that interacts with Neph1, providing strong support for functional conservation of these molecules. The ND controls access of molecules to the labyrinthine channels for uptake by endocytosis, and can discriminate between molecules of different sizes in a rate-dependent manner. Finally, in contrast to that reported by Weavers (2009) and reminiscent of their action in the embryonic musculature, Sns and Kirre contribute to the adhesion of the GCNs into an organized garland and their fusion into binucleate cells (Zhuang, 2009).

These data those of Weavers (2009) demonstrate that the GCNs have significant structural and functional similarities to podocytes in the mammalian kidney. Sns and Kirre are instrumental in directing and/or stabilizing interactions at sites of membrane invagination that become the NDs. These proteins parallel the role of their mammalian orthologs Nephrin and Neph1 in the SD that forms between podocyte foot processes in the kidney glomerulus. In addition, Sns and Kirre mediate tight adhesion between GCNs in the embryo, and, in contrast to Weavers this study notes that these proteins also direct GCN fusion. Both proteins are expressed during larval life and significant cell death occurs in their absence. Sns clearly plays a specific structural role in the ND that is perturbed by mutations in its extracellular domain. Finally, the SD and ND both mediate the flow of molecules between the circulatory system and the excretory system, and appear to discriminate between molecules on the basis of size and rate of passage (Zhuang, 2009).

The GCNs are thought to process waste material and detoxify the insect hemolymph, its open circulatory system, through a process of endocytosis and degradation. Endocytosis occurs from sites deep within labyrinthine channels that form by invagination of the plasma membrane, and proteins associated with endocytosis localize to the cortical region of the cytoplasm in membranes associated within these channels. The channels and associated membranes expand in mutants that block endocytosis, and compounds such as horseradish peroxidase, dye-conjugated BSA or avidin, and various dextrans, readily pass through the plasma membrane into these channels. Access appears to occur through a structure that is reminiscent of the SD in vertebrates. This study has shown that this nephrocyte diaphragm is dependent on the presence of Sns and Kirre, and that perturbation of the Sns extracellular domain causes obvious defects in the ND. Thus, IgSF homologs appear to be a structural component of this access point in both insects and vertebrates (Zhuang, 2009).

The number of NDs decreases significantly upon knockdown of Sns or Kirre, but a small number still remain. The uptake of large molecular tracers is severely diminished under these conditions, suggesting that the NDs are a major route of access to the endocytic machinery within the labryinthine channels. Perhaps more revealing relative to the initial findings of Weavers, it was found that the uptake of small molecules is slower under conditions of Sns or Kirre knockdown but ultimately achieves normal levels. Thus, like the SD, the ND appears to be more permeable to small molecules. Interestingly, studies in vertebrates have addressed the relative contributions of the podocyte basement membrane and the slit diaphragm to glomerular permeability, and Nephrin and Neph1 were found to be crucial. Moreover, electron tomography has identified Nephrin as a decisive determinant for filtration of molecules larger than BSA (Zhuang, 2009).

Nephrin and Neph1 are capable of forming both homodimers and heterodimers, and these abilities could reflect interactions that occur in vivo in cis and/or in trans. The diameter of the vertebrate SD is consistent with a model in which this distance could be spanned by homophilic interaction of Nephrin or heterophilic interaction between Neph1 and Nephrin in trans. The similar diameter of the Drosophila ND therefore supports a model in which interactions between the Kirre and Sns ectodomains determine this distance. The exact molecular interactions remain to be determined, however, and may differ in vertebrates and Drosophila. For example, Nephrin is capable of homophilic interactions in trans, a property that Sns does not appear to have. Thus, it seems unlikely that Sns spans this distance, as suggested for Nephrin. Homophilic interactions of Kirre, which can occur, could serve this purpose. One might then predict the spacing to be decreased from the observed 30-35 nm due to the shorter extracellular domain of Kirre. Of note, kinetic studies in Drosophila S2 cells indicate a strong preference for interaction with Sns. Moreover significant levels of Sns or Kirre remain in GCNs from second instar larvae upon knockdown of the corresponding partner, yet the number of NDs is diminished. Localization of each protein by immunoEM analysis under these conditions may prove to be illustrative in this regard. Given the above interaction studies and fact that both proteins are continuously present in the GCN, a model is favoed in which heterotypic interactions are preferred as in the embryonic musculature. One fundamental difference between Sns and Kirre in the embryonic musculature and the GCNs is that they are expressed in different myoblast cell types but co-expressed within individual garland cells. However, their co-expression in GCNs is another feature in common with Nephrin and Neph1 in vertebrate podocytes (Zhuang, 2009).

It is unclear whether Sns and Kirre function through interactions with signal transduction components that parallel those of Nephrin and Neph1 in the GCNs. Signaling molecules thought to be downstream of Sns and/or Kirre in the musculature, and known to be downstream of Nephrin, include N-WASp and components of the Arp2/3 pathway. One other functional parallel between the SD and ND is that of the tight junction protein Pyd, which contributes to formation of ND-associated furrows on the surface of the GCN. Although Pyd interacts biochemically with two different forms of Kirre, it remains to be shown whether this interaction occurs through postsynaptic density-95/disks large/zonula occludens-1 (PDZ)-binding sites in Kirre, as observed for binding of its vertebrate counterpart ZO-1 to Neph1 (Zhuang, 2009).

GCNs become binucleate before or immediately after their assimilation into the garland of cells that surrounds the esophagus at its junction with the proventriculus. This binucleate nature seems almost invariant, with cells rarely remaining mononucleate or having more than two nuclei. Although an explanation for this invariance is not apparent, the cell appears to accommodate multiple processes to ensure it. Quantitation of cells and nuclei over time, the absence of dying GCNs, and time-lapse imaging suggest that cell fusion is the primary mechanism utilized by wild-type GCNs, and that the IgSF proteins contribute to this process. Some mutant cells are still binucleate, but the possibility cannot be eliminate that other molecules contribute to GCN fusion or that these IgSF proteins function in yet more redundant ways to drive this fusion. Perhaps a drive to become binucleate has forced the cell to compensate for defects in fusion in other ways, such as cell division without cytokinesis. Although all efforts to address such a mechanism have yielded negative results, behavior of this type may be another common property between insect garland cell nephrocytes and mammalian podocytes (Zhuang, 2009).

GENE STRUCTURE

cDNA clone length - 8035 bp

Bases in 5' UTR - 548

Exons - 30

Bases in 3' UTR - 3038

PROTEIN STRUCTURE

Amino Acids - 1482

Structural Domains

The sns cDNA sequence, determined from overlapping clones, includes an apparent open reading frame (ORF) of 1483 amino acids, and untranslated regions of 549 and 3480 nucleotides on the 5' and 3' ends, respectively. The predicted size of the Sns protein without modification is 162 kD. The amino acid sequence strongly suggests that sns encodes a cell-adhesion molecule in the immunoglobulin superfamily. Sns contains eight putative Ig domains in the amino-terminal region, a single fibronectin type III domain and a putative transmembrane domain. A potential signal anchor sequence characteristic of membrane bound proteins is present in the amino terminus. This sequence, centered around a glutamine-rich stretch, is composed of 13 contiguous uncharged polar residues flanked by basic residues. A similar sequence of unknown function is present in the COOH terminal portion of Sns. BLAST database searches for proteins with structural homology to Sns have identified the human Nephrin protein (Kestila, 1998; Lenkkeri, 1999), which is expressed specifically in the kidney glomerulus and is associated with Congenital Nephrotic Syndrome. Sns has homology to Nephrin throughout the eight immunoglobulin domains and the fibronectin type III repeat. In Sns, this extracellular region includes 14 NXS and NXT consensus triplets for N-glycosylation. Like Nephrin, Sns also contains several SG doublets that are potential attachment sites for heparin sulfate. Homology to Nephrin continues through the putative transmembrane domain and into the cytosolic carboxy-terminal region. Although Sns has a longer cytoplasmic domain, the Nephrin-related region of Sns includes several tyrosine residues that may act as sites for phosphorylation. In addition to Nephrin, other genes with significant homology to Sns include the recently reported hibris gene of Drosophila (Bour, 2000).

Of note, the available Drosophila genomic sequence has revealed the presence of at least 20 introns, and a transcription unit that covers a minimum of 50 kb. These preliminary findings suggest that the genomic region that corresponds to the sns locus is quite large, not unlike that of the human Nephrin gene, which includes 29 introns spanning a total of 26 kb (Bour, 2000).

EVOLUTIONARY HOMOLOGS

For information on Sns homologs, inclucing mammalian Nephrin and Drosophila Hibris, see hibris.

date revised: 5 July 2005

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