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The neurofibromatosis type 1 (NF1) tumor suppressor protein is thought to restrict cell proliferation by functioning as a Ras-specific guanosine triphosphatase-activating protein. However, Drosophila homozygous for null mutations of an NF1 homolog showed no obvious signs of perturbed Ras1-mediated signaling. Loss of NFl resulted in a reduction in size of larvae, pupae, and adults. This size defect was not modified by manipulating Ras1 signaling but was restored by expression of activated adenosine 3',5'-monophosphate-dependent protein kinase (PKA). Thus, NF1 and PKA appear to interact in a pathway that controls the overall growth of Drosophila.
The gene responsible for human NF1 encodes a large protein that contains a central domain related to Ras-specific guanosine triphosphatase-activating proteins (RasGAPs) (1). Although loss of NFl expression correlates with increased Ras activity in several mammalian tumor cell types (2), it is not known which pathways are altered to produce the diverse symptoms observed in NF1 patients, which in addition to frequent benign and infrequent malignant tumors also include short stature and learning disabilities (1).
We identified a conserved Drosophila NFl homolog (3). Comparison of 13,295 base pairs (bp) of genomic and 9750 bp of cDNA sequence showed that Drosophila NFl consists of 17 constitutive and 2 alternatively spliced COOH-terminal exons. The two cDNAs predict proteins of 2764 and 2802 amino acids that are 60% identical to the human NF1 protein, neurofibromin (Fig. 1). Sequence similarity is observed over the entire length of the proteins, including regions outside of the catalytic GAP-related domain (GRD) or the more extensive segment related to yeast inhibitor of RAS activity (IRA) proteins (4). No related sequences were identified during screens of several cDNA and genomic libraries, indicating that the identified gene may be the only Drosophila NFl homolog. RNA in situ hybridization and staining of embryos and imaginal discs with monoclonal antibodies to the Drosophila protein (5) indicated that NFI is widely expressed in low amounts during all developmental stages (6). The Drosophila NFI gene was mapped to cytogenetic interval 96F and subsequently localized to a 30-kb DNA segment between the bride of sevenless gene and the Enhancer of split [E(spl)] complex (7).
To isolate mutant alleles at the NFl locus, we mobilized a P-element transposon located within the E(spl) complex, about 15 kb downstream...