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. Author manuscript; available in PMC: 2013 Nov 19.
Published in final edited form as: Annu Rev Genet. 2011 Sep 19;45:10.1146/annurev-genet-110410-132541. doi: 10.1146/annurev-genet-110410-132541

Figure 1. Molecular functions of Piwi proteins.

Figure 1

(A) In Drosophila, Piwi is a nuclear protein that binds Heterochromatin Protein 1a (HP1a) and is required for H3K9 (Lysine 9 of Histone 3) methylation and heterochromatin formation (10, 84). (B) In the mouse, Miwi2 protein is enriched in the nucleus and is required for de novo DNA methylation in male germ cells (4). (C) Piwi proteins in the nuage participate in a ping-pong mechanism that simultaneously produces new piRNAs and degrades active transposon mRNAs, see Figure 2 for details (9, 41). (D) The Drosophila Piwi proteins Aub and Ago3 are required for the deadenylation and degradation of maternal mRNAs during the maternal-to-zygotic transition (101). (E) In Drosophila, mouse, and Xenopus, 10–35% of piRNAs are processed from the 3′UTR of select mRNAs. This may represent a mechanism for regulating these mRNAs in cis. In addition, the resulting piRNAs may regulate gene expression in trans (100). (F) Mili and Miwi associate with polysomes in the mouse male germline to positively regulate gene expression (40, 117). (G) In Drosophila, Hsp90 and Hop form a complex with Piwi, which leads to Piwi phosphorylation and is required for Piwi-dependent canalization in the nucleus (36).