Gene name - nubbin
Synonyms - dOct1, dPOU-19, twain, POU domain protein 1
Cytological map position - 33-F1
Function - transcription factor
Symbol - nub
Genetic map position - 2 -
Classification - homeodomain and POU domain
Cellular location - nuclear
pdm-1 and pdm-2 are closely linked genes. They are coordinately regulated, with overlapping functions. Their expression is mostly ectodermal and both are essential for proper neuron development. The effects of mutation show that pdm-2 is quantitatively more important in neuroblast differentiation than pdm-1 (Yang, 1995). There is significant independently regulated endodermal expression for pdm-1 as well as expression of pdm-1in the wing.
pdm-1, more properly termed nubbin is expressed in both the anterior and posterior midgut primordia and in the developing endoderm. However, pdm-1 is not expressed in the central domain of the gut. This lack of central domain expression is thought to be due to both Ubx and dpp expression in the visceral mesoderm. Two gaps in pdm-1 expression are found: one in the anterior domain, underlying a region of dpp expression, and the second in parasegment 7, underlying a region of Ubx induced dpp expression. No developmental defects are known to result from pdm-1 expression in endoderm (Affolter, 1995).
Expression of pdm-1 in the wing disc is ubiquitous. In spite of this, it appears to be required locally in the wing hinge. Removal of pdm-1 activity from the hinge region results in a severe wing phenotype, while removal from more distal regions results in a less severe disruption. It is hypothesized that the effects in the hinge region are due to a disruption in the proximal-distal axis (Ng, 1995). pdm-1 expression in the wing disc is regulated by wingless which has a primary role in specifying the proximal-distal axis of the wing (Ng, 1996).
Cell interactions mediated by Notch-family receptors have been implicated in the specification of tissue boundaries in vertebrate andinsect development. Although Notch ligands are often widely expressed, tightly localized activation of Notch is critical for the formationof sharp boundaries. Evidence is presented that the POU domain protein Nubbin contributes to the formation of a sharpdorsoventral (DV) boundary in the Drosophila wing. Nubbin represses Notch-dependent target genes and sets a threshold for Notch activitythat defines the spatial domain of boundary-specific gene expression (Neumann, 1998).
Certain features of the abnormal wings in flies mutant for nubbin suggest a possible role for Nubbin protein in spatially limiting Notch activity at the DV boundary of the wing. The row of sensory bristles that makes up the wing margin is disorganized in nubbin wing mutants, suggesting a defect in Wingless or Notch activity. In preparations where the wing margin is viewed edge on, this disorganization reflects a broadening of the region where bristles form. Margin bristles are normally specified in cells very close to the DV boundary, reflecting a requirement for high levels of Wingless signaling activity. The broadening of the margin suggests that Wingless might be ectopically expressed in nubbin mutant wing discs. Wingless is normally expressed in a stripe of two to three cells straddling the DV boundary. In nubbin mutant discs, this stripe is widened considerably. Expression of the Notch targets vestigial and cut are similarly expanded at the DV boundary in nubbin mutants (Neumann, 1998).
To determine whether the effect on bristle specification is a direct consequence of removing nubbin activity, clones of nubbin mutant cells were generated in a wild-type background. Ectopic wing margin bristles are found in nubbin mutant clones located near the endogenous wing margins. The nubbin mutant clones show ectopic expression of neuralized, a molecular marker for precursors of the sensory neurons that innervate the bristles. The nubbin mutant clones misexpress wingless and vestigial. The largely autonomous effect of nubbin mutant clones on bristle specification may be due to the relatively low levels of Wg protein expressed in nubbin mutant clones. Together with the results on cut expressioon, these observations suggest that Notch target genes are transcriptionally up-regulated in nubbin mutant cells near the DV boundary (Neumann, 1998).
To test whether ectopic activation of these genes in nubbin mutant clones directly depends on Notch signaling activity, clones of cells were generated that were simultaneously mutant for nubbin and Suppressor of Hairless [Su(H)]. Su(H) encodes a DNA binding protein that mediates transcriptional activity of Notch target genes. Su(H) is autonomously required for the expression of wingless, vestigial, and cut at the DV boundary and binds directly to the vestigial DV boundary enhancer. Clones of cells mutant for both nubbin and Su(H) do not ectopically activate wingless, demonstrating that ectopic expression of wingless in nubbin mutant cells depends on activity of the Notch pathway. To confirm that Nubbin acts downstream of Notch, a test was performed to see whether overexpression of Nubbin could suppress the effects of a ligand-independent form of Notch. When Nubbin is coexpressed with such a constitutively active Notch, ectopic Wingless expression is strongly reduced. Together, these observations suggest that Nubbin may act as a direct repressor of Notch-dependent target gene expression. These findings argue that the effects of Nubbin are unlikely to be mediated by indirect effects on the expression of Notch ligands (Neumann, 1998).
Exons - four
Bases in 3' UTR - 485
PDM-1 has a homeodomain (the POU homeodomain) and a POU domain (Lloyd, 1991).
The 75 amino acid POU-specific (POUs) domain and a 60 amino acid carboxy-terminal homeo (POUh) domain are joined by a hypervariable linker segment that can vary from 15 to 56 amino acids in length in different POU domain proteins. Thus the POU domain is not a single structural domain; indeed, the POUs and POUh segments form separate structurally independent domains. The POUs and POUh domains are, however, always found together and have therefore coevolved. Both POUs and POUh domains contain helix-turn-helix motifs. The POUs-domain structure is very similar to that of lambda and 434 bacteriophage proteins, but there are significant differences in the length of the first alpha helix, and the "turn" connecting the two HTH alpha helices is also longer. Both POUs and POUh bind DNA, and the length of the linker regulates the efficacy of binding various DNA sequence motifs, especially because POUs and POUh DNA binding sites have different spacings in different promoter elements (Herr, 1995).
date revised: 3 MAR 97
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.