Biogenesis and migration of the sperm cytoplasmic droplet require ARRDC5 and TEX38.

The maturation of spermatids to fertilization-competent sperm is a sophisticated process required for sexual reproduction, and biogenesis of the cytoplasmic droplet (CD) is a key event. Here, we demonstrate that two proteins with testis-specific expression, arrestin domain-containing 5 (ARRDC5) and testis expressed 38 (TEX38), physically interact, are required for normal sperm maturation, and play a critical role in CD formation and function. In the CDs of Arrdc5-/- or Tex38-/- mice, a key aspect of saccular element biogenesis is deficient, the proteome is abnormal, and migration from the neck down the flagellum does not occur. Furthermore, comparative proteomic profiling between sperm CDs and spermatids from knockouts and wild-type mice suggests that ARRDC5 and TEX38 facilitate cargo loading during CD biogenesis and deposition of proteins such as translocase of outer mitochondrial membrane 40 (TOMM40) from the CD to the sperm mitochondrial sheath during epididymal maturation. This protein-delivery mechanism is likely a key event in enabling sperm motility and therefore fertilization capacity.