Analysis of the Molecular Function of Squash, a Crucial piRNA Component for Germline Genome Integrity

PIWI-interacting RNAs (piRNAs) repress the expression of moveable transposons (TEs) and protect the integrity of genome in animal gonads. In Drosophila germline cells, piRNAs are produced in membraneless granules called nuage by Aub and Ago3, known as PIWI family proteins. However, the function of another nuage component, Squash (Squ), remains elusive. Here, we confirmed loss function of squ could lead to the upregulation of TEs and accumulation of piRNA precursors 42AB and 38C on nuage. And squ depletion also results in a severe reduction of Ago3 protein and its bound piRNAs. Besides, we also found that Squ interacted with Spn-E and such the interaction is required for the TE repression and piRNA precursor processing. Based on these results, our findings identify Squ as a critical factor for heterotypic ping-pong amplification by maintaining Ago3 protein level and for piRNA precursor processing by modulating Spn-E helicase activity.