Density-dependent lectin-glycan interactions as a paradigm for conditional regulation by posttranslational modifications

Mice with null mutations in specific Golgi glycosyltrans-ferases show evidence of glycan compensation where missing carbohydrate epitopes are found on biosyntheti-cally related structures. Repetitive saccharide sequences within the larger glycan structures are functional epitopes recognized by animal lectins. These studies provide the first in vivo support for the existence of a feedback system that maintains and regulates glycan epitope density in cells. Receptor regulation by lectin-glycan interactions and the Golgi provides a mechanism for the adaptation of cell surface receptors and solute transporters in response to environmental cues and intracellular signaling. We suggest that other posttranslational modification systems might have similar conditional features regulated by density-dependent ligand-epitope interactions.