Minotaur is critical for primary piRNA biogenesis

Abstract

Piwi proteins and their associated small RNAs are essential for fertility in animals. In part, this is due to their roles in guarding germ cell genomes against the activity of mobile genetic elements. piRNA populations direct Piwi proteins to silence transposon targets and, as such, form a molecular code that discriminates transposons from endogenous genes. Information ultimately carried by piRNAs is encoded within genomic loci, termed piRNA clusters. These give rise to long, single-stranded, primary transcripts that are processed into piRNAs. Despite the biological importance of this pathway, neither the characteristics that define a locus as a source of piRNAs nor the mechanisms that catalyze primary piRNA biogenesis are well understood. We searched an EMS-mutant collection annotated for fertility phenotypes for genes involved in the piRNA pathway. Twenty-seven homozygous sterile strains showed transposon-silencing defects. One of these, which strongly impacted primary piRNA biogenesis, harbored a causal mutation in CG5508, a member of the Drosophila glycerol-3-phosphate O-acetyltransferase (GPAT) family. These enzymes catalyze the first acylation step on the path to the production of phosphatidic acid (PA). Though this pointed strongly to a function for phospholipid signaling in the piRNA pathway, a mutant form of CG5508, which lacks the GPAT active site, still functions in piRNA biogenesis. We have named this new biogenesis factor Minotaur.

An atlas of chromatoid body components

“…Genetic mouse studies have also revealed that the intact CB is not required for piRNA production (Tanaka et al 2011). Moreover, the initial steps of primary piRNA biogenesis are compartmentalized on the surface of the mitochondria that contain the conserved primary piRNA components, including GAZS, GPAT2, and endoribonuclease Zucchini/ PLD6 (Ma et al 2009;Huang et al 2011;Watanabe et al 2011;Ipsaro et al 2012;Nishimasu et al 2012;Czech et al 2013;Handler et al 2013;Shiromoto et al 2013;Vagin et al 2013). These proteins were not detected in the CB pulldowns.…”

piRNA Biogenesis in Drosophila melanogaster

“…The molecular functions of protein components of Yb bodies remain largely unknown. Additionally, Zuc, as well as other proteins with unknown functions required for piRNA biogenesis, such as GASZ and Minotaur (mino), are anchored on the mitochondrial surface [91–93], suggesting that mitochondria also play a key role in piRNA precursor processing. The interplay between Yb bodies and mitochondria merits further investigation.…”

Posttranscriptional Regulation of Gene Expression by Piwi Proteins and piRNAs

“…MitoPLD/Zucchini, a mitochondrial outer-membrane protein, is a candidate nuclease involved in the processing of piRNA precursors or intermediates (Haase et al, 2010; Huang et al, 2011; Ipsaro et al, 2012; Murota et al, 2014; Nishimasu et al, 2012; Saito et al, 2010; Watanabe et al, 2011a). The piRNA biogenesis factors GASZ and GPAT2/Minotaur are also found in the mitochondrial outer-membrane (Czech et al, 2013; Handler et al, 2013; Ma et al, 2009; Shiromoto et al, 2013; Vagin et al, 2013), suggesting a link between the mitochondrial outer-membrane and piRNA biogenesis. Biochemical analysis is needed to determine the functions of these conserved factors in primary piRNA biogenesis.…”