Institut fuer Allgemeine Genetik (240), Universitaet Hohenheim, D- 70593 Stuttgart, F.R.G. , § ROCHE AG, CH 4004 Basel, Switzerland,
The haplo-insufficient loss of function mutation Hairless is recessive larval lethal and causes the dominant loss of external sensory bristles and wing veination defects in adults. Genetically, Hairless antagonizes the activity of genes belonging to the Notch signalling pathway in Drosophila. Signals received by the Notch receptor appear to be transduced with the help of Su(H) which is thought to act as a transcriptional activator of target genes, e.g. bHLH genes of the E(spl) complex. The Hairless protein has been shown to directly bind to Su(H), thereby interfering with its transregulatory activity. Thus, Hairless could antagonize either the signal transduction and/or the signal implementation by blocking Su(H). Molecular analysis identified Hairless as a novel protein without any similarities to previously characterized structural motifs. The gene encodes a very basic protein, highly enriched in serine, alanine and proline residues with an apparent molecular weight of 150 kD. The protein is ubiquitously expressed throughout development at a rather low level, even though it appears to be quite stable. Hairless antigen is detected in the cytoplasm as well as in the nucleus where it appears to co-localize with chromatin. In polytene chromosomes of salivary glands, the protein concentrates on a limited number of loci. Nuclear Hairless protein appears to be masked since it is detected by antisera directed against its central portion in the nucleus only after ectopic expression. This masking component might include Su(H), a hypothesis we are currently investigating. In order to identify functionally important domains of the Hairless protein, a systematic structure- function analysis was initiated. We made use of the observation that transgenic flies carrying the Hairless gene under the control of a heatshock promoter produce specific 'anti- Hairless' phenotypes dependant on the time of heat shock application. These hypermorphic phenotypes mimick loss of function of Notch or Su(H) in the adult cuticle, predominantly affecting bristle development. More specific developmental defects can be caused by local ectopic expression of Hairless using the GAL4 system. Several heat inducible Hairless deletion constructs were tested in transgenic flies. These mutant derivatives allowed to separate two distinct neural domains required for sensory organ formation. One of them seems to correspond to the Su(H) binding site, a hypothesis we are currently trying to verify by investigating protein-protein interactions directly.