Acts via the piwi-interacting RNA (piRNA) metabolic process, which mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins and governs the methylation and subsequent repression of transposons (PubMed:26808625, PubMed:15817569, PubMed:17346786). Directly binds piRNAs, a class of 24 to 30 nucleotide RNAs that are generated by a Dicer-independent mechanism and are primarily derived from transposons and other repeated sequence elements (PubMed:16882972). In ovarian somatic cells, mediates silencing of transposable elements at the transcriptional level in a mael-dependent manner (PubMed:23159368, PubMed:28472469). Involved in silencing of long terminal repeat (LTR) retrotransposons in male germline (PubMed:15817569). In testis, regulates spermatogenesis together with Tudor-SN (PubMed:26808625). In germ cells, mediates silencing at both transcriptional and post-transcriptional levels and is involved in the maintenance of populations of primary and secondary piRNAs. Piwi-mediated transcriptional silencing is accompanied by the formation of His3 'Lys-9' trimethylated (H3K9me3) euchromatin and heterochromatin (PubMed:23434410, PubMed:23392610). In ovary, associates predominantly with antisense piRNAs that contain uridine at their 5' end. Association with sense piRNAs is also observed but to a lesser extent. Mediates a somatic signaling mechanism required for the maintenance of germline stem cells to produce and maintain a daughter germline stem cell (PubMed:9851978, PubMed:10631171, PubMed:9199372, PubMed:16949822). It is not essential for the further differentiation of the committed daughter cell (PubMed:9851978). Acts cell autonomously to promote germline stem cell division (PubMed:9851978, PubMed:10631171). Its role in stem cell maintenance does not seem to require nuclear localization. Required maternally for the posterior localization of osk and vas and for pole cell formation during oogenesis and early embryogenesis (PubMed:16949822). Together with Hop and Hsp83, mediates canalization, also known as developmental robustness, likely via epigenetic silencing of existing genetic variants and suppression of transposon-induced new genetic variation (PubMed:21186352). Shows RNA cleavage activity, although is not required for any of its known functions (PubMed:9199372, PubMed:16882972, PubMed:23297219). In the ovaries, forms a complex with nxf2, Panx and Nxt1 which acts as effectors of cotranscriptional transposon silencing (PubMed:31368590, PubMed:31384064). In the ovaries, part of a complex composed of at least Panx, nxf2, piwi and Nxt1 (PubMed:26472911, PubMed:26494711, PubMed:31368590, PubMed:31384064). The complex is knowns as Panx-induced co-transcriptional silencing (PICTS) complex, Panx-nxf2-dependent TAP/p15 silencing (Pandas complex), SFiNX (silencing factor interacting nuclear export variant) or piwi-Panx-nxf2-p15 (PPNP) complex (PubMed:26472911, PubMed:26494711, PubMed:31368590, PubMed:31384064). Interacts with vas; this interaction is RNA-independent (PubMed:16949822). Interacts with Dcr-1 and Fmr1; these interactions occur in polar granules (PubMed:16949822). Interacts (via N-terminal region) with CBX5 (via chromoshadow domain) (PubMed:17875665). Forms a complex with Hsp83 and Hop; probably Hop mediates the interaction between piwi and Hsp83 (PubMed:21186352). Forms a complex with Yb body components armi and fs(1)Yb; this interaction is required for proper piRNA loading and nuclear localization of piwi (PubMed:20966047). Interaction of Piwi and fs(1)Yb is likely to occur via armi (PubMed:20966047). Interacts (via the N-terminal region when unmethylated or symmetrically methylated at Arg-10) with papi (via Tudor domain) (PubMed:21447556, PubMed:29531043). Interacts with vret (PubMed:21831924). Interacts with Panx (PubMed:26472911, PubMed:26494711). Interacts with arx (PubMed:23913921, PubMed:23913922). Interacts with Tudor-SN (PubMed:26808625). Interacts with Nup358 (via N-terminus) (PubMed:29735528). Associates with the nuclear pore complex via interaction with Elys (PubMed:28472469). Interacts with thoc5; the interaction might be partly RNA-mediated (PubMed:28472469). Interacts with xmas-2 (PubMed:28472469). Component of polar granules. Present in the cytoplasm of the developing oocyte. At the mitotic cycle 11 translocates to the nucleus where it remains localized throughout germ-cell development and gonadogenesis. Localizes at genomic sites enriched for methylated H3K9. Interacts with chromatin at the nuclear pore complex (PubMed:28472469). Expressed in ovaries (at protein level) (PubMed:28472469, PubMed:31368590, PubMed:31384064). Expressed somatically in ovariole terminal filament cells, epithelial sheath cells, cap cells and follicle cells (at protein level). Expressed in nurse cells and oocytes in developing egg chambers (at protein level) (PubMed:29531043, PubMed:29735528). In embryos, accumulates in pole cells (at protein level). In larval and adult testis, expressed in a germinal proliferative center at the apical tip containing somatic hub cells and mitotically dividing germ stem cells (at protein level) (PubMed:26808625). Expressed both maternally and zygotically. Expressed in the germarium, at low levels during oogenesis stages 1-6, at a lower level during stages 7-9, strongly at stage 10, eventually accumulates in early embryos and later in development the expression decreases (at protein level). Symmetrically dimethylated, most likely by csul (PubMed:19377467, PubMed:29531043). Methylation at Arg-10 enhances binding to papi whereas methylation at Arg-7, Arg-9 or Arg-11 reduces binding affinity to papi (PubMed:29531043). Phosphorylated on serine and tyrosine residues in an Hsp83-dependent manner. Female mutants show normal ovarian development up to third instar larval stage (PubMed:9851978, PubMed:9199372, PubMed:26808625). However, adult mutant ovarioles contain germaria lacking germline cells and containing two normal or abnormal egg chambers as result of the failure of germline stem cell maintenance (PubMed:9851978, PubMed:9199372, PubMed:26808625). In adult testis, results in deregulation of transposon silencing (PubMed:26808625). Belongs to the argonaute family. Piwi subfamily. Extended N-terminus. Extended N-terminus. Extended N-terminus.