A role for BYN-1/bystin in cellular uptake and clearance of residual bodies in the <i>Caenorhabditis elegans</i> germline

GLH/Vasa/DDX4 helicases are core germ-granule proteins that promote germline development and fertility. A yeast-two-hybrid screen using Caenorhabditis elegans GLH-1 as bait identified BYN-1, the homolog of human bystin/BYSL. In humans, bystin promotes cell adhesion and invasion in gliomas, and, with its binding partner trophinin, triggers embryonic implantation into the uterine wall. C. elegans embryos do not implant and lack a homolog of trophinin, but both trophinin and GLH-1 contain unique decapeptide phenylalanine-glycine (FG)-repeat domains. In germ cells, we find endogenous BYN-1 in the nucleolus, partitioned away from cytoplasmic germ granules. However, BYN-1 enters the cytoplasm during spermatogenesis to colocalize with GLH-1. Both proteins become deposited in residual bodies (RBs), which are then engulfed and cleared by the somatic gonad. We show that BYN-1 acts upstream of CED-1 to drive RB engulfment, and that removal of the FG-repeat domains from GLH-1 and GLH-2 can partially phenocopy byn-1 defects in RB clearance. These results point to an evolutionarily conserved pathway whereby cellular uptake is triggered by the cytoplasmic mobilization of bystin/BYN-1 to interact with proteins harboring FG-repeats.