Core component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junctions on mRNAs (Probable). Positively regulates the nuclear export of spliced mRNAs including the sex determination gene tra-2 (PubMed:23149939). Promotes oogenesis and represses germline masculinization and spermatogenesis (PubMed:10656761, PubMed:14706697, PubMed:23149939). Required for embryonic development, possibly through positively regulating the expression of pie-1 (PubMed:30279189). Not required for the spatial patterning of pie-1 in embryos (PubMed:30279189). Heterodimer with rnp-4/RBM8A (PubMed:14706697). Part of the mRNA splicing-dependent exon junction complex (EJC) complex; the core complex contains CASC3, EIF4A3, mag-1/MAGOH and rnp-4/RBM8A (Probable). Expressed in the soma and germline of hermaphrodites. Expressed at all stages of development in hermaphrodites (PubMed:10656761). More abundant in embryos than in larvae and adults (PubMed:10656761). First expressed in embryos prior to morphogenesis, and after morphogenesis begins, primarily expressed in the head and tail regions (PubMed:10656761). RNAi-mediated knockdown results in embryonic lethality due to developmental arrest during morphogenesis (PubMed:30279189). RNAi-mediated knockdown in the syncytial gonads of adult hermaphrodites results in some inviable embryos and then eventually no egg laying (PubMed:10656761). RNAi-mediated knockdown at the L1 larval stage results in the accumulation of unspliced mRNAs including ama-1 and ife-4 in the cytoplasm, and specifically in the production of an irregular unspliced form of the sex determining protein tra-2, which accumulates in the cytoplasm and inhibits the regular tra-2 isoforms a and c reducing their expression and disrupting their nuclear localization (PubMed:23149939). RNAi-mediated knockdown in L4 hermaphrodite larvae, results in no viable embryos and egg laying ceases due to masculinization of the germline (also called a mog phenotype) with animals producing undifferentiated germ cells and sperm instead of oocytes (PubMed:10656761, PubMed:14706697). In these animals, sperm are distributed throughout the germline, including the distal tip region (PubMed:10656761). RNAi-mediated knockdown does not result in masculinization of the germline in some animals, but these animals still do not produce oocytes with germlines appearing disorganized (PubMed:10656761). RNAi-mediated knockdown reduces the expression of pie-1 in P2 blastomeres (PubMed:30279189). Belongs to the mago nashi family.