P granule assembly and asymmetric segregation in Caenorhabditis elegans

P granules are the germline ribonucleoprotein condensates of Caenorhabditis elegans. In adult germ cells they sit on the cytoplasmic face of nuclear pore clusters, where newly exported mRNAs pass through them. In oocytes and embryos they are cytoplasmic and are inherited by the germline founder cell. The condensate is built in layers. The PGL proteins PGL-1 and PGL-3 (with PGL-2 after embryogenesis) self-associate and bind RNA to form the liquid core. The Vasa-family DEAD-box helicases GLH-1 and GLH-4 (with GLH-2) gather at nuclear pores independently of the PGLs and are needed for the PGLs to reach the nuclear periphery. DEPS-1 is a perinuclear P granule protein that is needed for PGL-1 localization and GLH-1 accumulation, and that binds the Piwi protein PRG-1. In the zygote, the intrinsically disordered proteins MEG-3 and MEG-4 condense with mRNA on the surface of PGL droplets. This shell stabilizes the granules, recruits about 500 mRNAs, and makes the granules co-segregate with the germline. Segregation to the posterior blastomere uses two regulatory inputs. First, PAR polarity sets up an anterior-high gradient of the RNA-binding proteins MEX-5 and MEX-6. MEX-5 competes with PGL-3 and MEG-3 for RNA, so granules dissolve in the anterior and condense in the posterior. Second, the DYRK kinase MBK-2 phosphorylates MEG-1 and MEG-3 to promote granule disassembly, and the PP2A phosphatase with its PPTR-1/PPTR-2 B56 regulatory subunits reverses this to promote assembly. MEG-1 and MEG-2 scaffold a second germ plasm condensate. These germline P-bodies carry the DDX6 helicase CGH-1 and other decapping and deadenylation regulators, and they are needed to specify the germline founder cell. Loss of P granule components causes temperature-sensitive or complete sterility, and germ cells that lack P granules can switch to somatic fates.

Boundary. This is a concrete C. elegans module about how the P granule condensate is built, anchored and positioned. It covers the PGL core, the Vasa/GLH and DEPS-1 perinuclear layer, the MEG-3/MEG-4 embryonic shell, the MEX-5/MEX-6 gradient that dissolves anterior granules, and the MBK-2/PP2A-PPTR phosphorylation switch on MEG proteins. The germline P-body (MEG-1/MEG-2 with CGH-1 and CAR-1) is an optional companion part. It is a separate condensate (PMID:36196602, PMID:18692039) that merges with P granules in P4 and shares the MBK-2 input. Earlier work described MEG-1 and MEG-2 as P granule components (PMID:18202375), and the meg-1 and meg-2 reviews still carry P granule assembly terms. Here they are placed in the P-body part to follow the later two-condensate model. The small RNA machinery that lives in or next to P granules is out of scope: the Argonautes PRG-1, CSR-1 and WAGO-1, the Z granule protein ZNFX-1, and the Mutator foci protein MUT-16. Those proteins use the granule but do not build or position it. DEPS-1 is kept because it is needed for PGL-1 localization and GLH-1 accumulation. Its PRG-1 binding is recorded as a role, not as a separate silencing part. Anchoring. The perinuclear anchor is the GLH layer. GLH proteins gather at nuclear pores without the PGLs, and the PGLs need GLH-1 or GLH-4 to reach the nuclear periphery (PMID:33579952, PMID:18430929). Physical pore contacts are not modeled as a molecular function, because the gene reviews do not assert one. The nucleoporin NPP-10 and the Vasa paralog GLH-3 have no reviews and are left out. DEPS-1 is perinuclear and needed for PGL-1 localization, but no cached primary paper shows it binding nuclear pore proteins. It is therefore modeled as a perinuclear scaffold, not as a pore anchor. Phosphatase. PPTR-1 (UniProtKB:O18178) follows its gene review: a B56 regulatory subunit with protein phosphatase activator activity (GO:0072542), part of the PP2A complex (GO:0000159), acting in P granule assembly by dephosphorylating MEG-1/MEG-3, redundantly with PPTR-2 and opposite to MBK-2. Its second core role, restraining AKT-1 in insulin/IGF-1 signaling, is outside this module. PPTR-2 has only unreviewed TrEMBL entries spread over several isoforms. It is named in the text but not given an annoton. The PP2A catalytic and scaffold subunits (LET-92, PAA-1) are not modeled. MF choices follow the gene reviews' core_functions. PGL-1 carries identical protein binding (N-terminal dimerization) because its review does not use condensate scaffold activity. PGL-3, PGL-2, DEPS-1, MEG-2, MEG-3 and MEG-4 carry molecular condensate scaffold activity (GO:0140693). MEG-1 carries molecular adaptor activity. PGL-1 and PGL-3 also have guanosine-specific RNase activity (PMID:26787882), but no assembly role has been shown for it, so it is not used here. MEX-5 and MEX-6 also repress translation of maternal mRNAs. That role belongs to embryonic patterning and is not modeled here.