Although great progress has been made in describing in detail receptor-ligand interactions in a wide variety of biological systems, knowledge about such interactions that mediate cell-cell adhesion, where the "receptor" is present on one cell and the "ligand" is on another, is much less developed. Fertilization is a specialized case of cell-cell adhesion that has a well-defined and profound endpoint, formation of a zygote. Two systems that have been studied extensively in this respect are the sea urchin and the mouse. A central issue to be resolved in these systems is how, following interaction at the extracellular coat, the sperm moves to the next stage, interacts and fuses with the egg plasma membrane. Remarkably, the frog, although used very extensively and productively to study basic phenomena related to early development, has received relatively little attention with respect to the mechanism of gamete interaction during fertilization. The frog system, perhaps better than any of the other biological systems under study, has the potential to lead to elucidation of the staged interactions of the sperm and the egg that culminate in fusion of the two gametes. This is the case because 1 ) the initial interaction of the sperm with a glycoprotein in the frog extracellular coat known as the vitelline envelope (VE) has been characterized and 2) the relationship between the VE and the plasma membrane in the frog has been well studied ultrastructurally by others. In this proposal a glycoprotein (gp69/64) found in the VE and shown to be involved in sperm binding will be further studied with respect to the structure of its polypeptide chain and its oligosaccharide chains. The role of each structural element of this glycoprotein in sperm binding and how this process is developmentally regulated will be studied. Preliminary evidence has been obtained for the presence of a high molecular weight glycoprotein in the plasma membrane that cross reacts with antibody to the VE glycoprotein , gp69/64. A study of the transition from VIE binding to plasma membrane binding and the molecular basis of the latter event will be under taken. To do this, eggs from which a portion of the VE has been removed to expose a "window" of the plasma membrane, as well as eggs completely denuded of their VE, will be studied for sperm binding. Finally, fusion of the plasma membrane of the sperm and egg will be investigated and the possible role of the cross-reactive egg plasma membrane protein in this event will be examined. It is anticipated that these studies will bring new insight into the multistep interactions between sperm and egg that culminate in fertilization.
