Germline factors, TDRD and Piwi, colocalize with Vasa on the mitotic apparatus during the embryogenesis of the sea urchin

Germline factors are thought to function exclusively in the germline, providing the unique characteristics of germ cells. However, recent studies suggest that some of these factors may also be expressed and function outside the germline. One such example includes Vasa, a DEAD-box RNA helicase that appears to control localized translation on the spindle, facilitating efficient protein synthesis during embryogenesis of the sea urchin. However, it remains unclear if other germline factors are also involved in this process. In this study, we investigated the localization dynamics of Vasa's partners in the germline, such as Tudor-domain-containing proteins (TDRDs) and P-element-induced wimpy testis proteins (Piwis). Among TDRDs tested in this study, we found that TDRD7 is enriched on the spindle and forms granules with Vasa during early embryogenesis. Vasa and TDRD7 recruited each other when the expression of either was forced at the membrane, suggesting their interaction with each other. TDRD7 mutants lacking the N-terminal eLOTUS domain or the central intrinsically disordered region exhibited reduced granule formation, which also compromised their recruitment to Vasa. In contrast, PiwiL1/2 and PiwiL3 showed enrichment at the perinuclear region and the spindle, yet were never recruited to Vasa or TDRD7 when either was expressed at the membrane. These results suggest that a group of germline factors is present and may dynamically interact with each other on the spindle, contributing to somatic cell regulation in the sea urchin embryo.