UC San Diego

Fertilization is a multi-step process that involves contact, binding, and fusion of sperm and eggs. In most deuterostomes, sperm must undergo a process called the acrosome reaction (AR) before fusion can occur. The AR is induced by contact between the egg and sperm, resulting in a complex signaling cascade, which leads to the exocytosis of an acrosomal vesicle found at the tip of the sperm head and the polymerization of actin to form the acrosomal process. This process results in a newly exposed membrane that fuses with the plasma membrane of the egg.

In the sea urchin, Strongylocentrotus purpuratus, wheat germ agglutinin (WGA) is capable of blocking the egg jelly induced AR. WGA is a lectin that binds N-acetyl-D-glucosamine and sialic acid. One WGA-binding protein, receptor for egg jelly 1 (suREJl) has been identified as binding to the AR-inducing substance, fucose sulfate polymer of egg jelly. Also, monoclonal antibodies to suREJl are capable of inducing the AR. The focus of this dissertation is to identify and characterize other WGA-binding proteins in an effort to identify the important cell surface receptors involved in the sea urchin AR.

Chapter I introduces the glycobiology of fertilization in deuterostomes. Chapter II describes the known members of the sea urchin REJ family, suREJl, suREJ2 and suREJ3. These are large multi domain membrane receptors related to the polycystic kidney disease proteins, polycystin-1 and polycystin-2. suREJl is known to be a involved in the AR, and indirect evidence indicates suREJ3's involvement, as is discussed in chapter III. Chapter IV identifies another WGA-binding protein as an ATP-binding cassettte (ABC) transporter belonging to the human subfamily A. Members of this subfamily are involved in lipid and cholesterol transport. It is known that cholesterol must be removed from mammalian sperm before they can acrosome react. suABCA may have a similar function. Chapter VI gives general conclusions about these newly discovered proteins and gives suggestions for future work. Appendix A describes another putative suREJ protein, suREJX, and Appendix B demonstrates the presence of REJ proteins in sea urchin embryos.