A DC-STAMP domain within C. elegans sperm protein SPE-42 is required for fertilization

Abstract

New life requires a sperm and an egg to progress through stages of recognition, binding and fusion once they meet at the site of fertilization. Relatively little is known regarding the common molecular mechanisms of fertilization that unite all metazoans as well as the divergences that are necessary for species specificity. The nematode C. elegans has been instrumental in the discovery of several sperm and egg genes that are required for fertilization. Spermatogenesis defective (spe) gene spe-42 functions at the time gametes meet. spe-42 mutant sperm look and behave like wild type sperm, but fail to fertilize oocytes. The spe-42 family is present in all organisms that use sperm and eggs and thus may be evolutionarily as old as the sperm and egg system itself. SPE-42 is predicted to be a six-pass sperm plasma membrane protein and contains three essential domains, a large extracellular domain, a RING finger domain, and a dendritic cell-specific transmembrane protein (DC-STAMP) domain. The original DC-STAMP protein is required for cell-cell fusion events unrelated to fertilization such as fusion of preosteoclasts into osteoclasts. The presence of DC-STAMP domains in these otherwise unrelated protein families suggests that this domain is involved in the mediation of membrane fusion. Amino acids within the SPE-42 DC-STAMP domain that showed conservation among the many DC-STAMP domains analyzed were mutated to explore the effect on protein activity. One amino acid was shown to be absolutely essential for function, 2 amino acids nearly erased function and 4 showed mild effects on function. One triple amino acid substitution had no effect on protein function. We also showed that the homologous C. briggsae SPE-42 DC-STAMP domain is functional within C. elegans SPE-42 DC-STAMP domain. These results support our hypothesis that the DC-STAMP domain is required for SPE- 42 function and suggest it is also critical for membrane fusion events mediated by the canonical DC-STAMP protein.