| Understanding the specificity of sperm-egg recognition during mammalian fertilization has been a topic of intense study for many years, and yet the biochemical basis for this process is still largely unknown. Mammalian eggs are covered by thick glycoprotein coats, called zona pellucida (ZP), which contain glycans of varying structures that must be specifically recognized, and presumably cleaved, for fertilization to occur. Various experiments have implicated B1,4-galactosyltransferase 1 (B4GalT1) in the ability of mouse sperm to bind the ZP. Here I present evidence that additional members of the B4GalT family (specifically B4GalT3 and B4GalT4) are present on mouse and rat sperm, suggesting that these paralogs of BGalT1 might also be involved in fertilization.;B4GalT1 is expressed throughout the body and is thus unlikely to be solely responsible for specific binding of sperm to the ZP, unless it has a sperm-specific modifier. One known modifier of B4GalT1 activity is lactalbumin (LALBA), a paralog of lysozyme (LYZ) that is expressed specifically in mammary glands. LYZ binds and cleaves certain complex carbohydrates, whereas LALBA lacks enzymatic activity.;The more recent discovery of several LYZ-like transcripts in mammalian testes has changed our view of the LYZ gene family. To infer when these genes duplicated, we searched genomic databases, inferring their syntenic and phylogenetic relationships. These analyses suggested that most of the duplications happened early in mammalian evolution. We have studied the expression patterns of these LYZ-like genes and found them to be expressed predominantly in male reproductive tissues, including haploid sperm. Others have localized two of these proteins (LYZL4 and SPACA3) to the acrosome, and shown them to be involved in fertilization. Our analyses predict that some of the LYZ-like proteins can cleave complex carbohydrates, similar to LYZ, whereas others can only bind glycans.;In this dissertation, we propose a model for how these LYZ-like proteins, in combination with members of the B4GalT family, could be involved in sperm binding and cleaving of the glycans of the ZP in mammals. This model, unlike others proposed to date, may account for both the observed cell-specificity of sperm-egg binding and the species-specificity of this process. |
