BIOLOGICAL OVERVIEW

In a first phase of its developmentally crucial expression, decapentaplegic is responsible for dorsal/ventral polarity in the fly. In a second phase of its activity, as segments appear, dpp functions in the definition of boundaries between segmental compartments. As part of this process, dpp, along with wingless and hedgehog, defines the position of future limbs, including wings, legs and antenna. Dpp also has an independent role in the structuring of the mesoderm. Later, during the final process of appendage development, and acting downstream of engrailed and hedgehog, dpp defines boundaries between appendage compartments assuring correct anterior/posterior polarity. dpp has an analogous function in the development of the eye, where it is primarily responsible for the progression of the morphogenetic furrow, the induction site of the Drosophila retina.

In the first process, the structuring of dorsal/ventral polarity, dpp is repressed in the ventral portion of the trunk by the dorsal gene product. Receptors for the secreted Dpp protein, Saxophone, Thick veins and Punt, are found in both the ectoderm and the underlying mesoderm. They mediate the transduction of the dpp signal to the interior of the cell through a phosphorylation cascade activating gene transcription (for review, see Raftery, 1999). Phosphorylation, the major mechanism of the cell for transduction of signals from protein to protein, involves the attachment of phosphate residues to appropriate target molecules.

The effects on the dorsal-most region of the fly are regulated by Saxophone in conjuction with Thick veins and Punt. Without Dpp signals through Saxophone, the amnioserosa, the most dorsal ectodermal tissue, does not develop properly and dorsal closure, the sealing of a dorsal "hole" in the developing embryo does not take place. The dorsal region is ventralized, and it develops characteristics of the ventral neuroectoderm. In a sense, the Saxophone-defective fly develops upside down, and the dorsal region starts to resemble ventral tissue.

The initial effects of Dpp on the heart and on visceral mesoderm (gut muscles) are mediated by Thick veins and Punt. An early and important event in the subdivision of the mesoderm is the restriction of tinman expression to dorsal mesodermal cells, the precursors of heart cells.

Whereas initial Dpp signals eminate from the ectoderm, later Dpp takes on a life of its own in the visceral mesoderm. It is activated by Ultrabithorax and repressed by abdominal-A. In the mesodermal midgut, the cluster of homeotic genes Sex combs reduced, Antennapedia, and abd-A are expressed in non-overlapping anterior/posterior domains. They are responsible for drawing string-like constrictions in three parts of the midgut and the outgrowth of pockets, or as they are termed, caeca. Dpp with its ability to define boundaries has a role in this process. Dpp secreted by the mesoderm also leads to the induction of labial in the endoderm. Thus Dpp also induces local differentiation in the endoderm (Manak, 1995). The actions of dpp on gene activation are not always positive. dpp actively suppresses the development of the proventriculus, confining it to the foregut.

The importance of Dpp expression cannot be overstated, both early in allocation of cells to appendages and later in compartment subdivision of these appendages. Secreted in an anterior to posterior stripe in the trunk, Dpp intersects wingless expressing cells under the control of hedgehog in segmentally repeated dorsal/ventral stripes. All three proteins are needed to allocate cells for the formation of imaginal discs, which will ultimately develop into appendages. This process involves induction of distal-less and aristaless, both of which are needed to specify the tips of appendages (Campbell, 1993 and Diaz-Benjumea, 1994).

The imaginal disk expression of Dpp in a narrow stripe of cells along the anterior-posterior compartment boundary is essential for proper growth and patterning of the Drosophila appendages. Dpp receptor function was examined in the formation of the wing to understand how this localized Dpp expression produces its global effects in appendage development. This work depicts in a very clear fashion how Dpp functions as a morphogen. Clones of saxophone (sax) or thick veins (tkv) mutant cells, defective in one of the two type I receptors for DPP, show shifts in cell fate along the anterior-posterior axis. In the adult wing, clones that are homozygous for a null allele of sax or a hypomorphic allele of tkv show shifts to more anterior fates when the clone is in the anterior compartment and to more posterior fates when the clone is in the posterior compartment. The effect of these clones on the expression pattern of the downstream gene spalt-major also correlates with these specific shifts in cell fate. The shift in cell fate is explained by assuming that the cells in mutant clones act as though they see a lower than normal Dpp concentration. Thus cell fate along the A/P axis is directly related to the perceived Dpp level. It is concluded that cell fate is directly related to the distance of cells from the source of Dpp at the A/P axis and that Dpp is responsible for patterning of the entire wing blade in direct response to the long-range Dpp signal. The similar effects of sax null and tkv hypomorphic clones indicate that the primary difference in the function of these two receptors during wing patterning is that Tkv transmits more of the Dpp signal than does Sax. These results are consistent with a model in which a gradient of Dpp reaches all cells in the developing wing blade to direct anterior-posterior pattern. While current evidence suggests that Tkv is absolutely required for Dpp signaling, there appears to be no such absolute requirement for Sax. Thus Dpp receptor complexes that lack a Tkv subunit cannot transmit a sufficient level of Dpp signal to trigger a biological response in the receiving cell. In contrast, receptor complexes lacking Sax subunits are still capable of significant signal reception and downstream signaling (Singer, 1997).

Dpp function in the wing imaginal disc

Cell proliferation and patterning must be coordinated for the development of properly proportioned organs. If the same molecules were to control both processes, such coordination would be ensured. This possibility has been investigated in the Drosophila wing using the Dpp signaling pathway. Previous studies have shown that Dpp forms a gradient along the AP axis that patterns the wing, that Dpp receptors are autonomously required for wing cell proliferation, and that ectopic expression of either Dpp or an activated Dpp receptor, Tkv^Q253D, causes overgrowth. These findings are extended with a detailed analysis of the effects of Dpp signaling on wing cell growth and proliferation. Increasing Dpp signaling by expressing Tkv^Q253D accelerates wing cell growth and cell cycle progression in a coordinate and cell-autonomous manner. Conversely, autonomously inhibiting Dpp signaling using a pathway specific inhibitor, Dad, or a mutation in tkv, slows wing cell growth and division, also in a coordinate fashion. Stimulation of cell cycle progression by Tkv^Q253D is blocked by the cell cycle inhibitor RBF, and requires normal activity of the growth effector, PI3K. Among the known Dpp targets, vestigial was the only one tested that was required for Tkv^Q253D-induced growth. The growth response to altering Dpp signaling varies regionally and temporally in the wing disc, indicating that other patterned factors modify the response (Martín-Castellanos, 2002).

To address the cell autonomous effects of the Dpp signaling pathway, the Flp/Gal4 method was used to activate or suppress Dpp signaling in clones of cells marked with GFP. First, a mutant version of the Dpp type I receptor Thick veins, Tkv^Q253D, containing a point mutation in the glycine/serine rich domain (GS) was expressed. This mutation mimics the receptor phosphorylation that occurs upon ligand binding, and therefore renders the receptor constitutively active and ligand independent. Tkv^Q253D expression strongly activates the Dpp signaling pathway, inducing high levels of the phospho-Mad transducer and expression of two Dpp targets, omb and spalt. Initially, clones of cells that expressed Tkv^Q253D were induced in early second instar larvae (at 48 hours AED) and the cells were allowed to proliferate until the end of larval development (120 hours AED). Wing cell clones expressing Tkv^Q253D showed smooth borders compared with control clones, which showed jagged borders, and were also larger than control clones. This phenotype is stronger in lateral areas of the disc, far from the endogenous Dpp source. Approximately half of the lateral clones were completely round and bulged out of the disc epithelium, which generated extra folds around the clones. This phenotype was not seen when Tkv^Q253D was expressed throughout the disc, indicating that the round bulging clonal phenotype is a consequence of abnormal heterotypic interactions between Tkv^Q253D-expressing cells and wild-type cells (Martín-Castellanos, 2002).

Induction of clones by heat shock allowed the age of the clones to be controlled, and also allowed the inference of cell proliferation rates from the number of cells per clone. Since cell death was observed by Acridine Orange staining in Tkv^Q253D-expressing clones, the apoptotic inhibitor p35 was expressed to block cell death. This was necessary to obtain accurate proliferation rate measurements, which are confounded by cell death. Clones were induced and allowed to proliferate for a short time in the period of larval development when imaginal wing cells proliferate exponentially. Because of the regional phenotype described above, the number of cells per clone was counted in lateral and medial areas, as well as in the entire presumptive wing region. Cells over-expressing the activated Dpp receptor proliferate faster than control cells. This phenotype is stronger in lateral areas, where Tkv^Q253D-expressing cells proliferate 20% faster than controls. Tkv^Q253D-expressing cells proliferate 10% faster than controls in the medial region. This regional phenotype reflects the graded activity of endogenous Dpp signaling; lateral areas normally low in Dpp are more sensitive to signaling activation (Martín-Castellanos, 2002).

To further analyze the cellular phenotype, flow cytometry (FACS) was performed using co-expressed GFP to identify Tkv^Q253D-expressing cells. The GFP-negative cell population from the same discs was used as an internal control. Tkv^Q253D overexpression shifts the distribution of cells in the different phases of the cell cycle. A smaller proportion of the Tkv^Q253D-expressing cells are in the G1 phase and greater proportion in G2. These data, together with the shorter doubling time of these cells, suggests that Tkv^Q253D preferentially promotes G1/S progression. This cell cycle phenotype is more severe if the activated receptor is expressed for a longer period of time (Martín-Castellanos, 2002).

To address more carefully the autonomy of the effects of Tkv^Q253D, the expression patterns of String and Cyclin E protein were analyzed in discs containing Tkv^Q253D-expressing clones. String and Cyclin E limit progression of the imaginal disc cell cycle through G2/M and G1/S transitions, respectively. S-phase progression in Tkv^Q253D-expressing clones was also assessed using BrdU incorporation, and mitosis by phospho-Histone H3 detection. The BrdU incorporation assay yielded a result consistent with increased proliferation within Tkv^Q253D-expressing clones in lateral regions of the discs: these clones show a uniform increase in BrdU uptake. Increased BrdU incorporation is limited to within the Tkv^Q253D-expressing clones, and no non-autonomous effects were detected. This result implies that Tkv^Q253D stimulates cell proliferation cell-autonomously. No changes were detected in Cyclin E, String or phospho-Histone H3 expression levels in Tkv^Q253D-expressing clones or surrounding cells (Martín-Castellanos, 2002).

Although raising the levels of Dpp signaling increases rates of cell proliferation, it does not appear to bypass the developmentally programmed proliferation arrest that occurs at the end of larval development. Tkv^Q253D-expressing clones induced late in larval development (96 hours AED) contain the same number of cells as control clones. The same result was obtained when p35 was co-expressed. In addition, Tkv^Q253D-expressing clones induced early (48 hours AED) and analyzed in pupae (168 hours AED) do not contain mitotic cells. This suggests that a dominant, developmentally programmed signal prevents Tkv^Q253D-expressing cells from continuing to divide beyond the normal proliferation stage (Martín-Castellanos, 2002).

Induction of cell proliferation does not necessarily indicate increased growth. To more directly assess the ability of Tkv^Q253D to induce growth, areas of the disc epithelium encompassed by Tkv^Q253D-expressing clones were measured. Clones were induced early in larval development and analyzed at the end of the larval period. The average area of Tkv^Q253D-expressing clones was 2.5 times larger than that of control clones, indicating that Tkv^Q253D-expressing cells grow faster than wild-type cells. This phenotype depends on the position of the clone in the anterior-posterior axis. Clones in the lateral areas, far from the source of endogenous Dpp, showed the strongest phenotype. Fifty percent of these lateral clones were larger than the largest control clone. On average, lateral clones expressing Tkv^Q253D were 3.7 times larger than lateral control clones (Martín-Castellanos, 2002).

The cellular growth effects of Tkv^Q253D were further assessed using FACS analysis to measure cell size. The ratio of the mean forward light scatter (FSC) of GFP⁺ cells versus GFP^- cells was measured as a cell size indicator. GFP expression did not cause a significant change in cell size. Tkv^Q253D-expressing cells analyzed by FACS generally showed a size that was not significantly different from wild-type cells. In some experiments, however, these cells were slightly larger than controls. The fact that Tkv^Q253D-expressing clones are much larger than controls, but consist of cells of roughly normal size, confirms that Tkv^Q253D accelerates cell cycle progression. Taking the in situ and FACS analyses together, it is concluded that activation of Dpp signaling coordinately increases both rates of cell proliferation and cell growth (Martín-Castellanos, 2002).

To complement these experiments, the effects of autonomously inhibiting Dpp signaling were analyzed by overexpressing the pathway-specific inhibitor Dad, or by generating cell clones mutant for tkv. Dad is an inhibitory Smad protein that, when overexpressed, blocks omb expression and the adult wing phenotypes induced by ectopic Dpp signaling. It is normally activated by Dpp signaling and expressed in a broad domain centered on the AP axis. When Dad was overexpressed using the Flp/Gal4 method, clones were not recovered in the dorsomedial area of wing blade. However, Dad-expressing clones were recovered in medial areas when the apoptotic inhibitor p35 was co-expressed. These clones contained fewer cells than controls, indicating that Dad overexpression impairs proliferation of cells at medial positions. The cell doubling time of Dad overexpressing medial cells was more than 3 hours (22%) longer than the control doubling time. Slow-growing cells are eliminated by a mechanism known as cell competition when normal growing cells surround them. Because Dad overexpressing cells proliferate slowly, this may explain why they are not recovered unless the apoptotic inhibitor p35 is co-expressed (Martín-Castellanos, 2002).

To better understand the basis of this proliferative defect, tkv^- clones were generated by mitotic recombination. A recessive lethal allele, tkv⁷, was used that carries a point mutation in a conserved glutamate residue in the kinase domain and results in loss of expression of Dpp targets. In the medial wing pouch, tkv⁷ clones survive for 36 hours but are lost within 48 hours of induction (in the 72-120 hours AED interval). In lateral areas, tkv⁷ mutant clone survival is greater, however mutant clones are still small compared with wild-type twin spots, and show round morphology. This lateral-medial survival phenotype reflects the lower requirement for Dpp signaling in lateral areas of the wing imaginal disc (Martín-Castellanos, 2002).

Flow cytometry was used to analyze tkv⁷ cells. To counteract cell competition and enrich the population of mutant cells, a cell lethal Minute mutation, M(2)32^A1, was used that carries a lesion in ribosomal protein S13, and slows growth when heterozygous. Since M^-/- cells are not viable, only M^+/+ cells were recovered after mitotic recombination. These M^+/+ cells were tkv⁷ homozygous. In the Minute background, tkv⁷ cells survive at least 4 days and colonize more tissue than in a wild-type background. However, they are still growth impaired relative to wild-type cells growing in the same Minute^+/- background, and they still appear mainly in lateral areas. Approximately 30% of the tkv⁷ discs showed an aberrant morphology, probably caused by abnormal adhesive interactions between mutant and wild-type cells. tkv⁷ cells show a cell cycle profile consistent with a proliferation defect; the S phase fraction is extremely reduced and the G1 fraction is increased. This phenotype is opposite that of cells overexpressing Tkv^Q253D, which has a shortened G1. FACS analysis also showed that tkv⁷ cells are not detectably different in size from control cells. Previous studies indicate that when cell cycle progression is specifically delayed, cell size increases since cells continue to grow at the normal rates. Since tkv⁷ cells proliferate very slowly while maintaining a normal cell size, evidently they are impaired for growth as well as cell cycle progression (Martín-Castellanos, 2002).

Interestingly, M(2)32^A1/+ cells are larger than wild-type cells. This suggests that these cells divide more slowly than they grow, and thus that the growth defect caused by the Minute mutation affects cell cycle progression preferentially. In fact, in both budding and fission yeast cell cycle control genes are sensitive to translational conditions. Studies using another Minute mutation that encodes a ribosomal protein, M(3)95A, detected no size alteration in M/+ cells, and thus this effect may be gene specific (Martín-Castellanos, 2002).

Using a third approach to avoid the effects of cell competition, Dad was induced ubiquitously throughout the wing disc using the A9-Gal4 driver. This causes a reduction of disc size. This size reduction is especially pronounced along the AP axis and thus is opposite that of the phenotype resulting from Tkv^Q253D expression using the same driver, which enlarges the wing disc preferentially along the AP axis. These results show that inhibition of Dpp signaling reduces growth and impairs proliferation, whereas activation of Dpp signaling increases growth and accelerates proliferation (Martín-Castellanos, 2002).

If growth and cell cycle progression are independently regulated by Tkv, one would expect to detect the proliferative effect of Tkv^Q253D even in growth-impaired cells. Alternatively, if Tkv^Q253D were to promote cell cycle progression indirectly via stimulating cellular growth, the proliferative effect of Tkv^Q253D should be inhibited when cell growth is impaired (Martín-Castellanos, 2002).

To suppress cell growth a truncated version of p60, Deltap60, was expressed. This is an adaptor molecule for the class I Phosphoinositide 3-Kinase (PI3K/Dp110 in Drosophila. Dp110 signaling is a potent growth inducer. Adaptor molecules, such as p60, bind to the Dp110 kinase and recruit it to the Insulin Receptor, allowing full activation of the enzyme. Deltap60 binds the Insulin Receptor but cannot bind Dp110, and thus inhibits Dp110 signaling in a dominant-negative manner. When expressed in wing cells, Deltap60 reduces cell size and strongly delays G1 progression. Flp/Gal4 clones expressing Deltap60 contain very few cells compared with controls. Overexpressed Deltap60 also dominantly blocks the growth and proliferation effects of Tkv^Q253D. Clones of cells that co-express Deltap60 and Tkv^Q253D contain as few cells as those expressing Deltap60 alone, and these cells are just slightly larger than those expressing Deltap60 alone. Thus, loss of growth resulting from loss of PI3K activity cannot be rescued by hyperactivating Dpp signaling, and cell proliferation induced by Dpp probably requires Dp110 activity. These results are consistent with the model in which Dpp-driven cell growth indirectly promotes cell cycle progression (Martín-Castellanos, 2002).

Although clonal growth is blocked by co-expressing Deltap60 and Tkv^Q253D, cells that co-express Deltap60 and Tkv^Q253D do not show the G1 delay characteristic of cells expressing Deltap60 alone. Thus, Tkv^Q253D appears to be able to promote G1/S progression even in the presence of Deltap60. This suggests that some aspects of cell cycle progression induced by Tkv^Q253D may be Dp110 independent. However, the slight increase in size observed in cells co-expressing Deltap60 and Tkv^Q253D makes it difficult to rule out the possibility that this effect on G1/S progression also occurs indirectly, as a consequence of increased growth (Martín-Castellanos, 2002).

In the wing imaginal disc, omb, spalt and vestigial (vg) have been reported to respond to Dpp signaling. It was of interest to know which if any of these genes was involved in controlling tissue growth effected by Tkv^Q253D. spalt is probably not required, since Spalt protein is not induced by Tkv^Q253D expression in the lateral areas of the wing disc, where the strongest overgrowth effects are observed. In the case of omb and vg, null alleles were used as a genetic background in which the expression of the activated Dpp receptor was induced. Tkv^Q253D can promote growth in the absence of Omb (Martín-Castellanos, 2002).

By contrast, Tkv^Q253D is not able to promote tissue growth in a null vg^83b27R background. This result points to Vg as a possible effector of growth induced by Dpp signaling. Consistently, ectopic Vg expression induces wing-like outgrowths in imaginal discs. However, it was surprising to find that clones expressing Tkv^Q253D do not show increased levels of Vg protein, regardless of their position in the disc. Some lateral clones express Vg, but these most probably originate in the Vg expression domain. In fact, clones in lateral positions where Vg is expressed over-grow better than in other regions. These results suggest that activation of Dpp signaling is not sufficient to induce Vg expression, but that Tkv^Q253D and Vg might synergize to effect tissue growth (Martín-Castellanos, 2002).

Thus cell growth and cell cycle progression are coordinately regulated. These findings extend earlier studies that indicated a role for Dpp signaling in tissue growth. The 'balanced' effects on cell growth and cell proliferation caused by Tkv^Q253D differ markedly from results obtained when other growth stimulatory factors are manipulated in the developing wing. Ras, Myc and PI3K stimulate wing cell growth. Growth mediated by ectopic expression of these factors leads to a truncated G1 phase, which in the case of Ras and Myc has been attributed to post-transcriptional upregulation of the G1/S regulator Cyclin E. However, hyperactivation of Ras, Myc or PI3K signaling does not increase overall rates of wing cell proliferation, apparently because of a failure to stimulate G2/M progression. Consequently, these factors drive 'unbalanced' growth characterized by substantial increases in cell size. By contrast, ectopic Tkv^Q253D causes an increase in overall rates of cell division. Thus, Tkv^Q253D must induce G2/M as well as G1/S progression. Although no changes in Cyclin E or String levels have been detected by immunofluorescence, it is possible that small differences not detectable by antibody staining are responsible for G1/S and G2/M promotion (Martín-Castellanos, 2002).

Although early studies of wing development suggested that gradients of signaling might be the driving force that promotes cell growth in the wing, recent work has suggesting that Dpp signaling need not be employed in a gradient to stimulate growth. Dpp signaling in Tkv^Q253D-expressing clones is intense and homogenous, as assayed by anti-phospho-Mad staining, even in lateral areas. This suggests that gradients of Dpp signaling within these clones have been obliterated. Nevertheless, a variety of assays indicate that cell proliferation is promoted uniformly and autonomously throughout the clones, rather than at their edges, where sharp differentials of signaling intensity occur. Gradient models also predict non-autonomous effects on growth in regions bordering Tkv^Q253D-expressing clones. Although cell growth rates were not directly analyzed in these regions, inspection of markers for cell cycle progression did not detect major non-autonomous effects on cell proliferation. Thus, all these observations suggest that absolute intracellular levels of Dpp signaling, rather than gradients, are important for growth (Martín-Castellanos, 2002).

Survival of tkv-cells is better in regions of the wing that experience low level Dpp signaling. However, even in lateral regions far from the Dpp source, tkv^- cells have a growth and proliferation defect. This suggests that all cells in the wing disc, including lateral cells, receive and require at least low levels of a Tkv ligand for normal growth. This led to the suggestion that some of the Dpp targets that mediate its growth effects might not have regionalized, nested expression patterns like two well-characterized Dpp targets, spalt and omb (which appear not to be mediators of Tkv^Q253D-induced growth). Instead, it seems plausible that some of the Dpp targets that mediate cell growth and proliferation are more uniformly expressed in regions where Dpp is required (Martín-Castellanos, 2002).

How might Dpp, expressed in a gradient, drive expression of growth regulatory targets more uniformly? It has been proposed that induction of target genes in cells receiving low levels of Dpp must overcome the activity of the transcriptional repressor, Brinker. brinker mutant clones in lateral areas of the wing disc exhibit a round morphology and over-growth phenotypes that are similar to Tkv^Q253D-expressing clones. brinker mutant discs also exhibit a dramatic over-growth phenotype along the AP axis similar to discs that overexpress Tkv^Q253D ubiquitously. Thus, it seems plausible that all wing cells require a threshold level of Dpp activity to grow, and that in lateral regions this threshold is equal to the amount of signaling activity needed to overcome repression of Dpp growth targets by Brinker. When Brinker is lost or Tkv^Q253D is expressed in lateral regions, this threshold level of signaling may be greatly surpassed, causing increased expression of growth regulators and acceleration of cell growth rates beyond normal levels (Martín-Castellanos, 2002).

The growth response of a cell to altered Dpp signaling varies according to its location in the disc. Ectopic Tkv^Q253D causes the strongest over-growth phenotypes in lateral regions, far from the source of endogenous Dpp, whereas inhibition of Dpp signaling has the strongest phenotypes in medial areas of the disc, where Dpp levels are normally high. Similar region-specific responses have been observed in experiments in which Notch or Wingless signaling is activated ectopically using cell autonomous effectors, or ligands. What is the significance of these region-specific responses? Without knowing the pertinent growth regulatory targets of these signaling systems, it is only possible to speculate. Perhaps the differential responses reflect cooperation between several regionally expressed signals that affect tissue growth, both positively and negatively, in a combinatorial fashion. Observations relating to vg seem consistent with this possibility. vg is required by Tkv^Q253D to promote tissue growth, yet Vg protein is not up-regulated by ectopic Tkv^Q253D, and Tkv^Q253D is capable of promoting overgrowth in wing regions where Vg is not detectable. The complex growth responses of cells to Dpp signaling illustrate how much is unknown about mechanisms of growth control. New, more global, approaches to studies of growth modulation will be required before its regulation by patterning signals can be understood. Important tasks for future studies include identifying the Dpp targets that stimulate cellular metabolism to effect growth, and determining how these targets integrate input from other patterning signals such as Wingless, Notch, Hedgehog and the Egfr ligands (Martín-Castellanos, 2002).

GENE STRUCTURE

cDNA clone length - There are 3 major transcripts ( 3.3, 3.8 and 4.3 kb). Transcript A is present early and reaches a peak at between 4 and 12 hours. Transcript B is detected from 8 to 12 hours and remains high throughout development. Transcript C accumulates early and remains at high levels. Minor transcripts are detected as well. Major transcripts differ in the first exon, which makes up most of the 3'UTR. Thus each of these different transcripts have different proximal promoters and each shares a second and third exon. The common second and third exons code for the single protein species of Dpp (St. Johnston, 1990).

Bases in 5' UTR - variable

Exons - three

Bases in 3' UTR - 1041 and variable

PROTEIN STRUCTURE

Amino Acids - 588

Structural Domains

The carboxy-terminal 100 amino acids have 25-40% homology to human and porcine TGFbeta, Inhibin A and Inhibin B. The homologous region is preceded by three arginine dimers that each function as proleolytic cleavage sites (Padget, 1987).

date revised: 26 Dec 96 

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