eIF4E assembly into C. elegans germ granules is essential for its repressive function

Metazoan germ cells form intracellular germ granules, cytoplasmic RNA-protein condensates that contain a variety of RNAs and proteins essential for germline identity, maintenance, and fertility. P granules are a type of C. elegans germ granule proposed to be sites of mRNA repression. Proper P granule assembly is dependent on PGL-1 and its granule-forming protein relatives. Numerous RNA-binding proteins localize to P granules, like the eIF4E mRNA cap binding homolog, IFE-1. IFE-1 directly interacts with PGL-1 in vivo and in vitro. The molecular function of P granules remains enigmatic. Here, PGL-1 was molecularly dissected in vivo to determine protein regions required for P granule assembly, binding partner recruitment, and germ cell development. A specific region in the PGL-1 C-terminus was necessary and sufficient for IFE-1 recruitment to P granules and for fertility. IFE-1 RNA targets were identified, and reporters of top gene targets were repressed in the adult germline. This repression was dependent on PGL-1 and its IFE-1 binding peptide. These findings provide evidence that IFE-1 and P granules are a factor and site of mRNA repression, respectively. This repression required IFE-1 assembly into P granules, supporting the model that RNA-protein condensate assembly is necessary for its biological and biochemical functions.