Inactivation of both alleles of the fruit¯y D. melanogaster brain tumor (brat) gene results in the production of a tumor-like neoplasm in the larval brain, and lethality in the larval third instar and pupal stages. We cloned the brat gene from a transposon-tagged allele and identi®ed its gene product. brat encodes for an 1037 amino acid protein with an N-terminal B-box1 zinc ®nger followed by a B-box2 zinc ®nger, a coiled-coil domain, and a C-terminal b-propeller domain with six blades. All these motifs are known to mediate protein ± protein interactions. Sequence analysis of four brat alleles revealed that all of them are mutated at the bpropeller domain. The clustering of mutations in this domain strongly suggests that it has a crucial role in the normal function of Brat, and de®nes a novel protein motif involved in tumor suppression activity. The brat gene is expressed in the embryonic central and peripheral nervous systems including the embryonic brain. In third instar larva brat expression was detected in the larval central nervous system including the brain and the ventral ganglion, in two glands ± the ring gland and the salivary gland, and in parts of the foregut ± the gastric caecae and the proventriculus. A second brat-like gene was found in D. melanogaster, and homologs were identi®ed in the nematode, mouse, rat, and human. Accumulated data suggests that Brat may regulate proliferation and di erentiation by secretion/transportmediated processes.

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