No abstract

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.

The Sec1 family, a novel family of proteins involved in synaptic transmission and general secretion, is described. To date, 14 members of this family have been identified: four yeast proteins, Sec1, Sly1, Slp1/Vps33, and Vps45/Stt10; three nematode proteins, Unc‐18 and the homologues of Sly1 and Slp1; the Drosophila Rop; and six mammalian proteins, the rat Munc‐18/n‐Sec1/rbSec1A and rbSec1B, the mouse Munc‐18b/muSec1 and Munc‐18c, and the bovine Munc‐18 and mSec1. The mammalian proteins share 44–63% sequence identity with the nematode Unc‐18 and Drosophila Rop proteins and 20–29% with the yeast proteins and their nematode homologues. The Sec1 proteins are mostly hydrophilic and lack a transmembrane domain. Nevertheless, Sec1 proteins are found as membrane‐bound proteins. Some of them are also found as soluble, cytoplasmic proteins. Binding of the rat brain Sec1 to the presynaptic membrane may be due to strong interaction with syntaxin, an integral component of this membrane. The rat brain Sec1 is also bound to Cdk5, a neural cyclin‐dependent kinase. The Sec1 proteins play a positive role in exocytosis. Loss of function mutations in SEC1, SLY1, or SLP1 result in blocking of protein transport between distinct yeast subcellular compartments. Inactivation of unc‐18 and rop results in inhibition of neurotransmitter release and, in the case of rop, inhibition of general secretion as well. In addition, studies of Rop and n‐Sec1 indicate that they also play a negative role in synaptic transmission, mediated by their interaction with syntaxin. A working model addressing the dual regulative role of the Sec1 proteins in secretion is presented.

No abstract

Colorectal cancer is one of the most common malignancies in the western world, including Israel. An important step in progression includes induction of activating mutations in the protooncogene K-ras. This event is very frequent and is detected in about 40% of colorectal carcinomas. Previous studies of a variety of genetic disorders revealed unique gene mutation prevalence in Jewish populations, attributed both to differences in genetic background and to variability in environmental exposure. To determine the incidence and molecular subtypes of K-ras mutations in colorectal cancer in Israel, compared with other countries, DNA was isolated from a random collection of 105 colorectal carcinoma samples, and K-ras mutations were detected by an improved designed RFLP and direct sequencing. K-ras sporadic mutations in colorectal cancer in Israel are relatively frequent, with a higher fraction in codon 13 than reported thus far. Comparison with other countries shows a vast heterogeneity in terms of the relative abundance of the affected K-ras codon and in type and representation of specific mutations. The heterogeneous distribution found may be due to a variable genetic background and different environmental factors involved in the initiation and propagation of sporadic colorectal cancer.