Cytokines of the TGF beta superfamily regulate many aspects of cellular function by activating receptor complexes consisting of two distantly related serine/threonine kinases. Previous studies have indicated that Drosophila dpp uses similar signaling complexes and strictly requires the punt and thick veins receptors to transduce the signal across the membrane. Here, we show that the schnurri (shn) gene is required for many aspects of dpp signaling. Genetic epistasis experiments indicate that shn functions downstream of the dpp signal and its receptors. The shn gene encodes a large protein similar to a family of mammalian zinc finger transcription factors. The shn protein might therefore act as a nuclear target in the dpp signaling pathway directly regulating the expression of dpp-responsive genes.

In insects, neurotransmitter catabolism, melatonin precursor formation, and sclerotization involve arylalkylamine N-acetyltransferase (aaNAT, EC 2.3.1.87) activity. It is not known if one or multiple aaNAT enzymes are responsible for these activities. We recently have purified an aaNAT from Drosophila melanogaster. Here, we report the cloning of the corresponding aaNAT cDNA (aaNAT1) that upon COS cell expression acetylates dopamine, tryptamine, and the immediate melatonin precursor serotonin. aaNAT1 represents a novel gene family unrelated to known acetyltransferases, except in two weakly conserved amino acid motifs. In situ hybridization studies of aaNAT1 mRNA in embryos reveal hybridization signals in the brain, the ventral cord, the gut, and probably in oenocytes, indicating a broad tissue distribution of aaNAT1 transcripts. Moreover, in day/ night studies we demonstrate a diurnal rhythm of melatonin concentration without a clear-cut change in aaNATI mRNA levels. The data suggest that tissue-specific regulation of aaNAT1 may be associated with different enzymatic functions and do not exclude the possibility of additional aaNAT genes.Amine acetylation in insects by the enzyme arylalkylamine N-acetyltransferase (aaNAT, EC 2.3.1.87) is involved in at least three different physiological functions: (i) It appears to be the major route of neurotransmitter catabolism (1-4), inactivating biogenic amines including tryptamine, tyramine, octopamine, serotonin, and norepinephrine. (ii) An acetylated amine, acetyldopamine, is subsequently oxidized to the respective quinone, which cross-links different proteins and/or chitin, resulting in stabilization and hardening (sclerotization) of the insect cuticle (5). (iii) Acetylation of serotonin plays an important role in the regulation of photoperiodically influenced physiological and behavioral processes (6). The major regulatory hormone is melatonin (7, 8), which is synthesized by the methylation of acetylserotonin. In vertebrates, melatonin is secreted periodically with high concentration at night, caused by a 10-to 100-fold nocturnal increase in aaNAT activity (8). The final steps of aaNAT activation are still unclear and presumably involve new RNA and protein synthesis (9-12). All living organisms are influenced by environmental factors such as light and dark and thus need mechanisms to coordinate their physiological processes in response to seasonal changes. Serotonin, aaNAT activity, and melatonin have indeed been found in several organs (eye, optic lobe, and brain) of various invertebrate species in which melatonin concentration also shows a rhythmic behavior (13). However, it is not known if aaNAT activity and melatonin are subject to diurnal variation in Drosophila melanogaster. aaNAT activity has been described in several insect species (14-18), including D. melanogaster (2, 19), but none of these enzymes could be purified to derive antibodies or sufficient internal sequence information for cloning. We have purified an aaNAT from D. melanogaster using tryptam...

Differentiation of the Drosophila oocyte takes place in a cyst of 16 interconnected germ cells and is dependent on a network of microtubules that becomes polarized as differentiation progresses (polarization). We have investigated how the microtubule network polarizes using a GFP-tubulin construct that allows germ-cell microtubules to be visualized with greater sensitivity than in previous studies. Unexpectedly, microtubules are seen to associate with the fusome, an asymmetric germline-specific organelle, which elaborates as cysts form and undergoes complex changes during cyst polarization. This fusome-microtubule association occurs periodically during late interphases of cyst divisions and then continuously in 16-cell cysts that have entered meiotic prophase. As meiotic cysts move through the germarium, microtubule minus ends progressively focus towards the center of the fusome, as visualized using a NOD-lacZ marker. During this same period, discrete foci rich in gamma tubulin that very probably correspond to migrating cystocyte centrosomes also associate with the fusome, first on the fusome arms and then in its center, subsequently moving into the differentiating oocyte. The fusome is required for this complex process, because microtubule network organization and polarization are disrupted in hts(1) mutant cysts, which lack fusomes. Our results suggest that the fusome, a specialized membrane-skeletal structure, which arises in early germ cells, plays a crucial role in polarizing 16-cell cysts, at least in part by interacting with microtubules and centrosomes.