Sperm activation involves the rapid transition of a round, immotile spermatid into a polarized, motile sperm capable of fertilization. This transition in cell fate does not involve transcriptional changes, because the nucleus is not accessible. Although a large number of genes involved in sperm activation have been characterized in C. elegans, the signaling pathway that mediates this transition is not well-defined. We identified the
zipt-7.1 gene, which encodes a ZIP (Zrt/ Irt-like protein), using a reverse genetic approach. A deletion of the gene caused a partially penetrant sterile phenotype in hermaphrodites and reduced the fertility of males. The sterile hermaphrodites laid unfertilized oocytes and had mostly inactive sperm. Since the sterile phenotype of the hermaphrodites could be rescued by crossing with wild-type males, it must have been caused by a sperm defect. A similar phenotype was observed in mutants of the related species C. tropicalis. Finally, WT spermatids could be activated in vitro with pronase, trypsin or zinc, but
zipt-7.1(lf) spermatids were less responsive to pronase and trypsin and had almost no response to zinc. Consistent with these results, RNAi showed that ZIPT-7.1 functions in the germ line, and GFP::ZIPT-7.1 was expressed in spermatocytes. Sperm activation in C. elegans depends on two genetic pathways - the
spe-8 pathway and the
try-5 protease pathway. Epistasis studies suggest that
zipt-7.1 functions downstream of
spe-6 in the
spe-8 pathway. To understand the mechanism of
zipt-7.1-dependent sperm activation, we used molecular approaches. Transcription of
zipt-7.1 was regulated by environmental zinc levels, and
zipt-7.1 mutant sperm displayed reduced amounts of labile zinc, suggesting that it regulates zinc levels in sperm. Expression of ZIPT-7.1 in mammalian cells promoted zinc uptake, showing that it can transport zinc. Homologues of ZIPT-7.1 in other species localize to ER-Golgi, and ZIPT-7.1 expressed in human cell lines localized to the Golgi and lysosomes. Based on these results, we propose that ZIPT-7.1 is localized to vesicles in spermatids, and that a sperm activation signal results in the activation of its transporter activity and the release of zinc stores into the cytoplasm. This cytoplasmic zinc constitutes a second messenger that promotes activation. Our results provide the first identification of a zinc transporter that mediates sperm activation, and may have broad relevance, since zinc has been implicated in sperm function in a variety of animals.