Lectin-erythrocyte interaction with external transmembrane glycophorin saccharides controlling membrane internal...

Human red blood cell (RBC, erythrocyte) membranes have internal protein skeletons that govern the cells' distinctive discocyte-echinocyte morphology (shape) changes, seen in conventional microscopy. Glycophorin, the cell's transmembrane protein, presents all of its saccharides outside the cell. The protein sector of glycophorin is linked inside to the RBC cytoskeleton, enabling lectins binding to the external saccharides to gain profound control over internal cytoskeleton behavior, expressed by governance of the visibly seen cell shape. Critical lectin binding stoichiometries ((125)I-labeled lectins) equate to the number of glycophorin monomers per RBC, 7 x 10(5) copies/cell. Wheat germ agglutinin lectin (sialic acid specific) binds to glycophorin's outermost (exo) saccharides and exerts tight control over the cell's morphology. Removal of sialic acid groups (desialation) exposes the endosaccharides of glycophorin, enabling peanut agglutinin and Osage orange lectins to gain equally tight control over the RBC's morphology behavior in simple stoichiometric ratios, bound lectin molecules/glycophorin receptor. Thus, lectin specificities for saccharides are sharply in register with the glycophorin external saccharide composition, the sequence along the chains, and the number of copies of protein (stoichiometry). These relationships were determined via RBC shape change equilibria and also via shape change rates. Rate data are somewhat laborious to determine, but are exquisitely sensitive to lectin specificities and in very small lectin concentrations. Both classes of data enable these interactions to be analyzed in lectin and RBC concentrations approximately 100-fold smaller than agglutinating levels.