Germ granule components regulate endosomal switching during the oocyte-to-embryo transition in Caenorhabditis elegans

The egg cytoplasm undergoes large-scale remodeling after fertilization. Here, we reveal that the germ granule component, HERD-1, is involved in selective degradation of maternal plasma membrane proteins after fertilization in Caenorhabditis elegans. HERD-1 is specifically expressed in the germline and mainly localized in a subtype of germ granules, Z granules. HERD-1 loss caused maternal plasma membrane protein accumulation in ubiquitin-positive early and late endosomal assemblies in the early embryos. The proteomic analysis showed that HERD-1 loss substantially reduced the protein levels of a subset of endolysosomal regulators such as ESCRT-0 components regulating the multivesicular body pathway. Defects in maternal membrane protein degradation in herd-1-deficient embryos were suppressed with the loss of DEPS-1 or PRG-1, which are required for germ granule organization and small-RNA biogenesis. These results suggest that several germ granule components maintain appropriate endolysosomal component levels via small RNA-mediated regulation during the oocyte-to-embryo transition, promoting endosomal switching toward embryogenesis. Germ granule components maintain protein levels of endolysosomal components via small RNA-mediated regulation during the oocyte-to-embryo transition, thereby ensuring spatiotemporal endosomal activation (endosomal switching) for proper embryogenesis.