P granules are distinctive ribonucleoprotein organelles that are observed in germ cells at all stages of development. PGL-1, a putative RNA-binding protein, is associated with P granules at all developmental stages.
pgl-1 mutants contain defective P granules and are sterile, with both a maternal and a zygotic component to the sterility. Sterility is the result of defects in proliferation and gametogenesis and is highly sensitive to temperature. Using a yeast two-hybrid screen for proteins that interact with PGL-1, we identified IFE-1, one of the five isoforms of eukaryotic initiation factor 4E (eIF4E) present in C. elegans. eIF4E is the cap-binding component of the translation initiation complex.
ife-1 mRNA and protein are highly enriched in the C. elegans germline, and IFE-1::GFP (driven using the
pie-1-based germline-expression vector of Dunn, Reese, and Seydoux) is localized in P granules. Furthermore, native PGL-1 is retained on an mRNA cap affinity column, suggesting interaction with IFE-1 in vivo. By RNAi analysis, we found that IFE-1 is required for spermatogenesis in both hermaphrodites and males, and that the requirement is highly sensitive to temperature. In
ife-1(RNAi) worms grown at high temperature, germ cells enter spermatogenesis but fail to form mature sperm and as a result are sterile. IFE-1 is not required for oogenesis, since
ife-1(RNAi) hermaphrodites produce viable outcross progeny. Consistent with a primary role in spermatogenesis,
ife-1 mRNA levels are highest in regions of the gonad undergoing spermatogenesis. Our studies demonstrate tissue-specific expression and function of the IFE-1 isoform of eIF4E, and further show that this isoform is associated with P granules, perhaps via interaction with PGL-1, and is required for production of sperm but not oocytes. Finding that P granules contain an eIF4E lends support to the hypothesis that germ granules are involved in translational control in the germline.