Ligand binding sites for a synthetic B cell growth and differentiation factor.

The mechanism of action of a group of synthetic lymphokine-like molecules, the C8-substituted guanine ribonucleosides, was studied. Among their pleiotropic effects on B cells are the increased expression of surface Ia antigens, induction of polyclonal immunoglobulin secretion, enhancement of thymus-dependent as well as thymus-independent antibody responses, and transmission of T cell-like differentiative signals to B cells. However, relatively little is known about their molecular mechanism of action. In the current article, the interaction of 8-bromo-guanosine (8BrGuo), a prototypical C8-substituted guanine ribonucleoside, with cellular components was examined. Rapidly exchangeable (free) and slowly exchangeable (bound) 8BrGuo pools exist within B cells. The bound nucleoside pool loses its ability to be retained by a boronate affinity resin (despite its resistance to metabolic processing) and localizes to the cytosol on sucrose density gradients. Binding affinity, ligand specificity, and cellular specificity of binding all correlate closely with observed functional properties of these molecules. Together, these data suggest that the binding interaction mediates the biologic activities of 8BrGuo, and that the binding site acts as a functional nucleoside receptor.