The P granules are the C. elegans germ granules, RNA granules specific to the germline. In the embryo, P granules segregate asymmetrically with the P lineage for inheritance by the primordial germ cells (PGCs) Z2 and Z3. P granules are thought to regulate RNAs with critical germline functions, such as the Nanos homolog nos-2, but the function and structure of embryonic P granules are not well understood. We previously reported that embryonic P granules are heterogeneous organelles consisting of an inner core and an outer scaffold (Wang et al., 2014). The inner core contains the PGL and GLH class of RNA-binding proteins that are found in P granules throughout development. The outer scaffold contains a novel class of intrinsically-disordered and serine-rich proteins (MEGs) that associate with P granules only in embryos.
Surprisingly, our genetic analyses suggest that the MEG proteins mediate the essential function of P granules in embryos independently of the PGL/GLH cores. Embryos lacking the MEGs do not assemble PGL/GLH granules, do not express NOS-2 (or express very low levels), exhibit PGC defects characteristic of embryos lacking Nanos activity, and are sterile. In contrast, embryos that do not form PGL/GLH granules still form RNA-positive MEG granules, express NOS-2, and are fertile. We conclude that the intrinsically-disordered MEGs mediate the essential functions of P granules during embryogenesis. We speculate that the MEGs may be functional homologues of other intrinsically disordered proteins that scaffold RNA granules in germ cells, such as vertebrate Bucky ball. .
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