Apoptosis causes the coordinated dismantling of the protein synthesis machinery. In mammals, the protease caspase-3 cleaves a factor of the translation complex called eIF4G. This modification induces the synthesis of pro-apoptotic proteins such as Apaf-1, while general protein synthesis is suppressed. In the C. elegans gonad, apoptosis occurs naturally to maintain the viability of sibling germ cells that will mature as oocytes. We demonstrate that worms express multiple isoforms of eIF4G from a single gene (
ifg-1). The isoforms of IFG-1 (
p170 and
p130) differ structurally and in association with mRNA cap complexes. Loss of the long
p170 isoform induced the expression of the Apaf-1 ortholog CED-4 early in dying oocytes. We have depleted IFG-1
p170 in worms containing
ced-1:gfp, which decorates apoptotic corpses, and either
ced-3 or
ced-4 loss-of-function alleles. Upon depletion of IFG-1
p170, there was virtually complete suppression of germ cell death in worms lacking CED-3 or CED-4. While the number of fertile oocytes increased, embryonic lethality was still observed as previously described. This indicates that there are non-apoptotic functional roles for the short IFG-1
p130 that can be exercised in the absence of
p170 for late oogenesis. These results also suggest that germ cells can use a component of the translational apparatus as a novel mechanism to alter their fate by upstream activation of the caspase cascade. Concomitantly, during apoptosis Apaf-1 has been shown to be preferentially translated through cap-independent synthesis by short isoforms of human eIF4G. We are currently exploring similar translational control mechanisms involving CED-4 during the cap-independent (
p130) induction of physiological germ cell death. These studies address changes in the availability and/or translational efficiency of
ced-4 mRNA. We have also used a conditional mutant strain that alters
ifg-1 splicing resulting in nonsense mRNA. IFG-1
p170 and
p130 were markedly depleted in these worms, which were viable at 20 deg C. Growth at 25 deg C, however, resulted in gonad degeneration, arrested embryonic development, and F1 sterility. At this non-permissive temperature, we will determine if the protein synthesis mechanism is similarly altered, and its influence on germ cell proliferation and death. Ultimately, our findings support a model in which a balance between
p170 and
p130 isoforms maintain growth-promoting protein synthesis while preventing the induction of CED-4 in order to preserve oocytes destined to mature. (Supported by grants MCB-0321017 and MCB-0842475 from the NSF and IRG 5-89812 from the American Cancer Society).