Presenilins are a family of multiple trans-membrane proteins, specific to plants and animals, that are predominantly localized to the ER and Golgi. Mutations in the two human presenilins dominantly lead to early onset Alzheimer's disease by increasing the production of the 42 amino acid variant of the beta-amyloid peptide. Presenilins also play an essential role in Notch-type signaling throughout the animal kingdom. Studies in a number of systems implicate presenilins in the cleavage of Notch type receptors after ligand binding, but how directly these genes affect this cleavage is unclear. C. elegans has two somatically expressed presenilin genes,
sel-12 and
hop-1 , which appear to have partially redundant functions.
hop-1 mutants have no obvious defects however, mutations
sel-12 result in an egg-laying defect. At least two developmental defects underlie the
sel-12 Egl defect, the most penetrant of which appears to be a mispatterning of the sex muscles that control egg-laying (see abstract by Eimer et al. ). All defects seen in
sel-12 mutants or
hop-1;
sel-12 double mutants resemble those seen in mutants with reduced
lin-12 or
glp-1 signaling. This, along with the fact that mutations in
sel-12 suppress
lin-12(gf) mutations, suggest that the most important, and perhaps only, role of presenilins in C. elegans is to regulate Notch-type signaling. In order to better understand the biological and biochemical role of presenilins in animals, we have screened over 75, 000 mutagenized haploid genomes for suppressors of the Egl defect of
sel-12(
ar171) mutants using either EMS, UV/TMP or a mutator strain. We have identified 26 recessive strong suppressor mutations that fall into at least 6 complementation groups. However most mutations lie in only three genes which we call
ses-1,
ses-2 and
ses-3 (for suppressor of the egg-laying defect of
sel-12 ). These strong suppressor mutations completely suppress the egg-laying defect and restore largely wild-type brood sizes to both
sel-12(
ar131) and
sel-12(
ar171) mutants. This indicates that the suppressors are not allele specific and that they must correct both
sel-12 developmental defects. On their own, suppressor mutations display no obvious developmental or behavioral abnormalities, indicating that these mutations are specific suppressors of the
sel-12 defects. We have cloned
ses-1 by a positional cloning strategy and found that
ses-1 encodes for a novel protein with no close similarity to any sequence in the publicly available databases. It contains several C2H2 zinc finger domains and two predicted nuclear localization signals that suggest that the protein may function as a transcription factor. Consistent with this, a Myc-tagged fusion protein is localized to the nucleus in SF9 insect cells. Only one of the seven
ses-1 alleles is clearly non null, suggesting that the function of
ses-1 must be severely reduced or even eliminated to see suppression. All of this evidence suggests that
ses-1 may function as a transcriptional repressor, that acts genetically downstream of
sel-12 . Since another
sel-12 suppressor gene identified by the Greenwald lab,
spr-2 , is also a nuclear gene that may affect transcription (1), we have been looking for possible targets of ses gene regulation by Northern analysis. We have examined the expression of many genes involved in
lin-12 and
glp-1 signaling in various ses gene mutant strains. Although we have found no clear differences in transcript levels of any gene tested using mixed stage RNA, we have preliminary evidence that
ses-2 and
ses-3 may suppress
sel-12 defects by up-regulating the expression of
hop-1 specifically in the L1 and L2 larval stages. This suggests that we may not be able to bypass completely the need for presenilins in C. elegans . At the meeting, the further characterization of
ses-1 and the other ses genes will be presented. (1) Wen C, Levitan D, Li X, Greenwald I (2000). PNAS 97, 14524-9