The late chromatoid body component TSSK2 associates with translational machinery in elongating spermatids in mice

Spermatogenesis culminates in a dramatic morphological transformation, including a tight compaction of the chromatin and nuclear re-shaping that largely silences transcription. Due to transcriptional silencing, the production of the sperm-specific proteins needed for the morphological transformation requires active storage and translational regulation of mRNAs transcribed in earlier cell types. The germline-specific ribonucleoprotein (RNP) granule, the chromatoid body (CB), accumulates RNAs and has a role in RNA storage in early haploid cells (round spermatids). In late haploid cells (elongating spermatids), the CB is transformed to so-called late-CB, whose function in RNA regulation has remained elusive. Here we characterized the function of the late-CB by identifying proteins and RNAs interacting with the known late-CB marker, Testis- Specific Serine Kinase 2 (TSSK2). We showed that TSSK2 interacts with eukaryoti5 translation initiation factors and the late-CB associates with active, localized protein synthesis. Furthermore, we revealed an association of TSSK2 with a specific set of mRNAs that are enriched in polysome fractions in elongating spermatids, supporting the role of the late-CB in the temporally regulated translation. These results link the function of the late-CB to the RNA regulation during late spermatogenesis for the first time, providing important novel information about the RNA regulatory processes required for spermatogenesis and male fertility.