Spermatozoa lacking TEX51 display hypofertility and defects in morphology.

Fertilization is essential for all sexually reproducing species, yet the complete molecular mechanism remains unclear. Here, we revisit a structure-based search of the AlphaFold human proteome to identify new members of the IST superfamily, a key family of proteins associated with fertilization. We discover that TEX51, predicted to form a 4-helix bundle, a ß-hinge, a pair of CXXC motifs and a transmembrane domain, shares structural similarities with IST superfamily proteins (IZUMO1, SPACA6, and TMEM95). Given the testis-specific expression of TEX51 and conservation across mammals, we hypothesize that TEX51 plays a role in fertilization. To investigate this, we create a Tex51-knockout (KO) mouse model that removes the complete Tex51 open reading frame and incidentally a part of the nearby lncRNA Gm44577. These KO males are hypofertile as shown by in vivo and in vitro analyses, despite showing normal sperm motility. Morphological abnormalities in the KO sperm include disjunction between the flagellum midpiece and principal piece, and an increased presence of cytoplasmic remnants. Electron microscopy further reveals defects in the anchoring of the acrosome to the nucleus. These findings expand the roles of IST superfamily proteins beyond functions in gamete interaction to broader aspects of reproduction such as sperm formation during spermiogenesis.