BIOLOGICAL OVERVIEW

Border cells are a small group of specialized follicle cells in the ovary that undergo a dramatic cell migration during stage 9 of oogenesis. During stage 9 there is a posteriorward movement of the outer layer of follicle cells; eventually 95% of the follicle cells stack up in the posteior half of the egg chamber, in contact with the oocyte. The remaining cells stretch to cover the nurse cell cluster, anterior of the oocyte.

During this rearrangement of cells, six to ten follicle cells, referred to as border cells, remain rounded at the anterior tip of the egg chamber. One or two border cells extend cytoplasmic processes in between the anterior nurse cells, initiating their own unique migratory journey. Over the course of several hours, they move posteriorly, traversing a distance of over 15 cell diameters, passing several nurse cell junctions, making correct choices at each juncture along the way. The migration terminates once they reach the oocyte-nurse cell boundary. At this time, the anteriormost oocyte-associated follice cells begin moving inward to cover the anterior end of the oocyte, eventually joining the border cells at the anterior end.

The slbo gene was discovered in a brute force experiment. 7800 lines of flies, made mutant by the random insertion of a P element vector in the fly genome, were examined for border cell defects. One strain was found, and as luck would have it, the P element, designed to drive ß-galactosidase expression when inserted into an expressed gene, was capable of driving ß-galactosidase in border cells, beginning just prior to their migration. The P element was used to clone the gene into which it had inserted. In this manner the slbo gene was characterized. Slow border cells also express the novel protein torso-like, a maternal gene involved in determination of terminal pattern elements (Montell, 1992).

SLBO is the Drosophila homolog of vertebrate C/EBP, the CCAAT/enhancer-binding protein, a transcriptional activator of a group of adipose-specific genes. There is evidence that a basic-leucine zipper protein has a similar function in Drosophila: a bZIP site is involved in the regulation of yolk protein (Yp) genes at a transcriptional enhancer that regulates sex- and tissue-specific transcription. Such regulation is complex, involving interaction between a bZIP protein and Doublesex, a sex specific transcription factor. Refusing any neat or simple characterization, SLBO does not appear to be the bZIP protein that activates Yp genes (An, 1995).

However, SLBO does play a role in regulation of transcriptional control of breathless, coding for an FGF receptor homolog involved in the developmental control of cell migration. During embryonic development btl expression registers in several locations: in cells of the tracheal system, in a subset of glial cells and in salivary duct cells. Mutations in the btl locus cause defects in the migration in a pair of midline glia and in the migration of tracheal cells. SLBO binds to eight sites in the btl regulatory sequence, suggesting that its regulation of btl is direct. Most likely additional SLBO target genes contribute to efficient and complete border cell migration: a null allele in btl does not eliminate border cell migration, as does the loss or even a reduction in SLBO expression (Murphy, 1995).

SLBO is unlikely to regulate breathless expression during embryogenesis because SLBO expression in the tracheal system does not begin until long after breathless expression (Rorth, 1992). Recently however, a POU domain transcription factor, Drifter, has been described which may enhance btl expression in tracheal cells. Drifter protein is expressed in tracheal cells near the time that btl expression initiates: the dfr mutant phenotype is similar to btl; and dfr expression is not altered in btl mutants (Anderson, 1995). Thus it is possible, even likely, that dfr regulates btl expression. Preliminary experments suggest that dfr is not expressed in the border cells. One interpretation then, is that DFR may regulate btl in the embryo in much the same way that SLBO does in the ovary (Murphy, 1995).

GENE STRUCTURE

There are two major transcription start sites of SLBO mRNA: P1 (distal) and P2 (proximal) (Rorth, 1992).

Exons - 1

PROTEIN STRUCTURE

Amino Acids - 444

Structural Domains

The C-terminal region of SLBO contains a basic region followed by a leucine zipper domain. The leucine zipper domains of the mammalians C/EBPs are less conserved than are the basic regions. 19 of 21 amino acids of the basic region are identical. The leucine zipper domain shows only 26% identity to mammalian C/EBP alpha. The leucine zipper functions to stabilize C/EBP dimerization. The only N-terminal region with an identifiable sequence is a stretch of 23 glutamines encoded for by an opa (CAG/A) repeat (Montell, 1992).

date revised: 5 Dec 97

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