[Show abstract][Hide abstract] ABSTRACT: Myc proto-oncoproteins are important regulators of growth and proliferation in development. Their functions have been evolutionarily conserved from insects to vertebrates, although the sequence conservation is limited to a few short domains. Here, we analyze the requirement for the most highly conserved domains, called Myc boxes 2 and 3 (MB2 and MB3), and for the weakly conserved N-terminus for the biological activity of the single Drosophila Myc protein in the animal in vivo. We find that a Myc mutant lacking the N-terminus retains very little activity, whereas Myc transgenes carrying a deletion of MB3 have a moderately increased ability to promote growth and apoptosis; mutation of MB2 reduces transcriptional output and the biological activities of Myc. Surprisingly though, Myc without MB2 retains enough activity to partially rescue the lethality of a Myc null mutation. Thus, although MB2 and MB3 are highly conserved in evolution, loss of either domain has comparatively mild consequences on Myc activity in vivo.
[Show abstract][Hide abstract] ABSTRACT: Myc proteins are powerful proto-oncoproteins and important promoters of growth and proliferation during normal development. They are thought to exercise their effects upon binding to their partner protein Max, and their activities are largely antagonized by complexes of Max with Mnt or an Mxd family protein. Although the biological functions of Myc, Mxd and Mnt have been intensively studied, comparatively little is known about the in vivo role of Max. Here we generate Max loss-of-function and reduction-of-function mutations in Drosophila melanogaster to address the contribution of Max to Myc-dependent growth control. We find that many biological activities of Myc do not, or only partly, require the association with Max--for example, the control of endoreplication and cell competition-and that a Myc mutant that does not interact with Max retains substantial biological activity. We further show that Myc can control RNA polymerase III independently of Max, which explains some of Myc's observed biological activities. These studies show the ability of Myc to function independently of Max in vivo and thus change the current model of Max network function.
[Show abstract][Hide abstract] ABSTRACT: Dorso-ventral patterning results in the establishment of the two germ layers in the Drosophila embryo, mesoderm and mesectoderm, that are separated by a strip of cells giving rise to the mesectoderm and eventually to the ventral midline. The mesectoderm is specified by the expression of single-minded (sim) which is activated through the concerted action of Dorsal and Twist in addition to a Notch signal. In the mesoderm, sim is repressed by Snail together with the co-repressor C-terminal binding protein (CtBP). Here, we address the involvement of the two co-repressors CtBP and Groucho (Gro) in repression of sim in the neuroectoderm. It was shown earlier that sim is restricted in the neuroectoderm with help of Suppressor of Hairless [Su(H)] and Hairless. Using the female sterile technique, we generated germ line clones deficient for Gro, CtBP or Hairless and assayed sim mRNA relative to snail mRNA expression. We show that sim repression requires both co-repressors Gro and CtBP to be fully repressed in the neuroectoderm, suggesting that a repression complex is assembled including Su(H) and Hairless as was shown for other Notch target genes before. Moreover, our work implies that Gro is important for the repression of sim specifically within the mesoderm anlagen, indicating that Snail and CtBP are insufficient to entirely silence sim in this germ layer.