PEEL-1 and ZEEL-1 are a toxin and antidote pair that mediate an intraspecific hybrid incompatibility in C. elegans. We are interested in determining the mechanism of PEEL-1 toxicity and ZEEL-1 anti-toxicity. PEEL-1 is expressed paternally and delivered to embryos via sperm. In the absence of embryonically expressed ZEEL-1, PEEL-1 leads to embryonic arrest at a the two-fold stage. Ectopically expressed PEEL-1 causes a cell-autonomous necrotic cell death that does not depend on the apoptotic caspase CED-3. To identify factors required for PEEL-1 toxicity, we performed a forward genetic screen for suppressors of lethality caused by heat-shock induced expression of PEEL-1. We isolated six independent suppressors belonging to four different complementation groups. Extent of suppression ranges from partial suppression of lethality primarily in larval stages to full suppression of lethality at all stages. We molecularly identified two genes required for PEEL-1 toxicity. Two partial hs::
peel-1 suppressors carry missense mutations in the gene
nxf-1, encoding a nuclear-export factor required for general export of mRNAs out of the nucleus. Though null mutants in
nxf-1 are lethal, our
nxf-1 missense mutants are indistinguishable from wild-type, suggesting that the
peel-1 mRNA may be particularly dependent on the function of this RNA export factor. Two strong PEEL-1 suppressors carry mutations (missense and deletion) in the gene F47B7.1, encoding a 59 amino acid transmembrane protein that belongs to a family of similar small transmembrane proteins previously uncharacterized in C. elegans. Transgenic expression of F47B7.1 rescues the
peel-1 suppression phenotype of the deletion mutation
yak103, confirming the gene identification. We also show that
yak103 suppresses the embryonic lethal phenotype caused by endogenous PEEL-1 protein supplied from sperm, demonstrating that F47B7.1 is required for the toxic effects of both ectopically and endogenously expressed PEEL-1. We are using GFP constructs to determine the expression pattern and cellular localization of F47B7.1. We are also investigating whether mutations in F47B7.1 affect localization of ectopically expressed GFP-tagged PEEL-1.