The C. elegans
sel-12 gene was identified as a suppressor of a
lin-12 gain-of-function allele and found to encode a functional homolog of human presenilin (1, 2). Presenilin was also identified as a gene implicated in familial early-onset Alzheimer's disease. It is now believed that presenilin mediates transmembrane cleavage of LIN-12/Notch proteins, a critical step in signal transduction by these receptors, as well as transmembrane cleavage of beta-amyloid precursor protein, a critical step in the generation of a peptide that can cause Alzheimer's disease. Hermaphrodites carrying a
sel-12 loss-of-function mutation are unable to lay eggs. The Egl phenotype of
sel-12 null mutants resembles that of
lin-12 partial loss-of-function mutants, and it has been proposed that reduction in
lin-12 activity during Pi cells determination is the basis for the
sel-12(-) Egl phenotype (3). A screen for suppressors of the Egl phenotype of
sel-12(-) mutants was conducted (4) and defined at least four spr genes (for suppressor of presenilin). We have undertaken the genetic and molecular characterisation of
spr-1. Our genetic evidence suggests that
spr-1 does not bypass the need for presenilin activity, as
spr-1 is unable to suppress the
sel-12 mutant phenotype when the activity of the other C. elegans presenilin,
hop-1, is removed. Furthermore, genetic interactions with various loss- or gain-of- function alleles of
lin-12, as well as with allele of
glp-1 (the other C. elegans LIN-12/Notch gene), suggest that
spr-1 may play a general role in LIN-12/Notch activity. We mapped
spr-1 between single nucleotide polymorphisms using the Hawaiian strain CB4856 (5) and identified the ORF corresponding to
spr-1 by sequencing the predicted ORFs in the region (5). Our analysis of cDNAs and RACE products revealed a more extensive ORF than originally predicted by Genefinder. The SPR-1 protein based on the corrected ORF appears to have orthologs in humans and Drosophila. The human ortholog of SPR-1 has been shown to be involved in transcriptional repression. Our finding that SPR-1 is a nuclear protein is consistent with a role for SPR-1 in transcriptional repression as well. We have evidence that
spr-1 appears to function in Pi cells to negatively regulate the LIN- 12/Notch pathway: expression of SPR-1 under the control of
cog-2 regulatory sequences restores the Egl defect of
spr-1;
sel-12 hermaphrodites. Furthermore, the human homolog has some ability to replace SPR-1 in this assay. At the meeting, we will discuss how a protein involved in transcriptional repression may function in LIN-12/Notch signalling, and why a gene encoding such a protein might be identified in genetic screens for suppression of the Egl phenotype of
sel-12(-).