SPE-13 is a small sperm membrane protein required for fertilization in caenorhabditis elegans that interacts and colocalizes with other fertilization synapse molecules
Creators: name: Zuo, Yamei
Publisher: name: No Publisher Supplied
Year: 2024

description: Fertilization is a critical process in which two gametes recognize each other and fuse to form a zygote, initiating the development of a new organism. While the overall cellular events of fertilization are well documented, the underlying genetic pathways and molecular mechanisms remain poorly understood across species. In C. elegans, the spe-13 gene has been identified as a member of the spe-9 class genes, which are crucial for successful fertilization. Mutations in this class of gene result in sperm that exhibit normal morphology and motility but fail to fertilize wild-type oocytes after direct contact in the spermatheca. Hermaphrodites with loss-of-function mutations in spe-13 are sterile. However, their fertility can be rescued by mating with wild-type (N2) males, indicating that the fertility defects in spe-13 mutants are sperm-specific. Similar to other spe-9 class mutants, spe-13 mutants undergo normal spermatogenesis, and their sperm accumulate in the spermatheca but fail to fertilize oocytes.Through Whole Genome Sequencing (WGS), we recently identified spe-13 as the gene R06A10.5. SPE-13 is a small protein consisting of 130 amino acids, with a predicted transmembrane domain and a large cytoplasmic tail. Our gene expression analysis revealed that SPE-13 is exclusively expressed in sperm, supporting the hypothesis that SPE-13 is located on the sperm membrane and plays a critical role in fertilization. Through CRISPR/Cas9, we generated a SPE-13::GFP strain to visualize its subcellular localization. Initial analysis using a membranous organelle (MO) marker, SPE-38, indicated that SPE-13 localizes to the MOs in spermatids. In activated spermatozoa, SPE-13 indeed localizes to the plasma membrane as predicted.Moreover, our studies revealed that the localization of SPE-13 to the plasma membrane involves two distinct assembly steps, each requiring specific members of the SPE-9 class proteins. In the first step, SPE-45, SPE-42, and SPE-49 are crucial for trafficking SPE-13 from the endoplasmic reticulum (ER)/Golgi complex to the MOs in primary spermatocytes. Without these proteins, SPE-13 fails to be incorporated into the MOs and instead remains in the residual body. In the second step, which occurs during sperm activation (spermiogenesis or post-miotic sperm differentiation), SPE-9, SPE-36, SPE-38, and SPE-51 are required for the redistribution of SPE-13 from the MOs to the plasma membrane in spermatozoa. In the absence of any of these proteins, SPE-13 remains trapped in the fused MOs and does not reach the plasma membrane, suggesting that these proteins are essential for SPE-13’s proper redistribution during sperm activation. Together, our findings demonstrate that SPE-13 is located on the sperm surface and is a necessary component for the formation of a functional fertilization synapse complex to ensure successful fertilization. Our study, hence, identifies a new component of the fertilization synapse in C. elegans, enhancing our understanding of the complexity of this structure.