Fig. 8: Model: HRDE-2 promotes germ granule localization and small RNA binding for HRDE-1.

a HRDE-1 localization (green) and class of associated small RNAs are depicted for wild-type and various mutants. In wild-type, HRDE-1 associates with WAGO-class 22G-RNAs and localizes to the nucleus. In the hrde-2 mutant, HRDE-1 associates with CSR-class 22G-RNAs while still localizing to the nucleus. In the mut-2 mutant, HRDE-1 associates with both WAGO and CSR-class 22G-RNAs and localizes to both nucleus and germ granule. In the mut-2; hrde-2 mutant, HRDE-1 associates with CSR-class 22G-RNAs and localizes to the nucleus. In hrde-1(HK-AA) mutant, HRDE-1 remains unloaded, is excluded from the nucleus, and localizes to germ granules and the cytoplasm. In the hrde-1(HK-AA); hrde-2 mutant, HRDE-1 is unloaded and dispersed in the cytoplasm. b HRDE-2 is a component of SIMR foci, where it interacts with SIMR-1 and recruits unloaded HRDE-1. In SIMR foci, HRDE-1 loads WAGO-class 22G-RNAs, which are produced in adjacent Mutator foci by the RNA-dependent RNA polymerase RRF-1. Upon 22G-RNA loading, HRDE-1 is translocated to the nucleus, recognizes pre-mRNA complementary to the bound 22G-RNA, and recruits downstream NRDE pathway factors and methyltransferase to deposit the H3K9me3 silencing marker on HRDE-1 target genes and inhibit RNA polymerase II transcription. In contrast, CSR-class 22G-RNAs are produced in the cytosol by RNA-dependent RNA polymerase EGO-1, resulting in spatial separation from WAGO-class 22G-RNAs.