Domain-functional analyses of PIWIL1 and PABPC1 indicate their synergistic roles in protein translation via 3'-UTRs of meiotic mRNAs.

Translational regulation plays a central role during post-meiotic development of male germ cells. Previous studies suggested that P-element induced wimpy testis like 1 (PIWIL1), a PIWI-interacting RNA (piRNA) binding protein that is critical for sperm development, participates in the maintenance and translational regulation of post-meiotic mRNAs in haploid spermatids. However, how PIWIL1 regulates protein translation remains largely unclear. Using biochemical assays, we show here that PIWIL1 utilizes different domains to interact with post-meiotic mRNAs and Poly-A binding protein cytoplasmic 1 (PABPC1), a general protein translation regulator. PIWIL1 binds 3'-untranslated regions (3'-UTRs) of several spermiogenic mRNAs via its N-terminal domain, whereas its interactions with PABPC1 are mediated through its N- and C-terminal domains in an RNA-dependent manner. Using a heterologous cell system, we analyzed its effects on protein translation via luciferase reporter assay and sucrose gradient sedimentation. It was found that PIWIL1 augments protein translation with PABPC1 in the presence of 3'-UTRs of post-meiotic mRNAs. While both the N-terminal RNA recognition motif (RRM) domain and the central linker region of PABPC1 stimulate translation, only the PIWI Argonaute and Zwille (PAZ) domain of PIWIL1 positively affects translation of reporter mRNAs. Interestingly, the PAZ domain was found absent from polysomal fractions, in contrast to the N- and C-terminal domains of PIWIL1. Taken together, the results suggest that PIWIL1 interacts with various partners using different domains and participates in translational regulation partly through 3'-UTRs. It will be of interest to further explore how PIWIL1 elicit its versatile functions, including translational regulation of post-meiotic mRNAs through intrinsic structural changes and extrinsic signals during mouse spermiogenesis under more physiological settings.