Carbohydrates and fertilization: an overview.

Initial sperm-egg binding in mammals involves recognition of glycosylated proteins of egg zonae by glycosylated proteins on sperm surfaces. Egg zona protein structure is relatively simple, and has been strongly conserved. Species specificity must reside in the carbohydrate modifications on the egg surface, and in the co-ordinated assembly of a unique cohort of sperm proteins at capacitation. Fruitful advances have been made along four lines. Oligosaccharide structures capable of binding spermatozoa have been dissected by in-vitro synthesis and binding experiments, informed by the general advance of knowledge of protein glycosylation processes. Site-specific mutagenesis of zona proteins and their expression in tissue culture have identified glycosylation sites involved in species-specific sperm binding. Antibody and lectin labelling studies show a continuing process of remodeling of glycosylated sperm surface epitopes within a set of stable compartments during epididymal transit and capacitation of spermatozoa. Characterization of sperm-egg binding proteins from a variety of mammalian species shows that a different set of effectors induce acrosome reactions in each species, with each set including one or more sugar-recognizing proteins. Sequencing of some of these effectors suggests that each group may form a supermolecular complex to induce a species-specific acrosome reaction, with the functional activities distributed in a species limited or non-limited manner among the individual proteins.