Modulation of Ago2 Loading by Cyclophilin 40 Endows a Unique Repertoire of Functional miRNAs during Sperm Maturation in Drosophila.

In gene silencing, Hsp90 chaperone machinery assists Argonaute (Ago) binding and unwinding of silencing small RNA (sRNA) duplexes. This enables the formation of effector RNA-induced silencing complex (RISC) that often displays cargo preferences. Hence, in Drosophila, microRNAs (miRNAs) and small-interfering RNAs (siRNAs) are differentially sorted into Ago1-RISC and Ago2-RISC, respectively. Here, we identify fly Cyclophilin 40 (Cyp40) as a testis-specialized Hsp90 co-chaperone essential for spermatogenesis and for modulating Ago2-RISC formation. We show that testis-distinctive Ago-sorting and strand-selection mechanisms accumulate a unique set of miRNAs on Ago2. Cyp40 interacts with duplex-incorporating Ago2 through Hsp90 in vitro and selectively promotes the build-up of Ago2-bound miRNAs, but not endogenous siRNAs, in vivo. Moreover, one of Cyp40-dependent Ago2-sorted miRNAs is required for late spermatogenesis, unraveling the physiological relevance of the unconventional yet conserved Drosophila miRNA-Ago2 sorting pathway. Collectively, these results identify RISC-regulatory roles for Hsp90 machinery and, more generally, highlight the tissue-specific adaptation of sRNA pathways through chaperone diversification.