THE MECHANISM BY WHICH PROTEOLYSIS ENHANCES THE LIGAND-BINDING ACTIVITY OF GUINEA-PIG TYPE-II FC RECEPTOR FOR IGG (FC-GAMMA-RIIB)

We have previously shown that the ligand-binding activity of type II F c receptor for IgG (Fc gamma RIIB) on guinea pig peripheral blood poly morphonuclear leukocytes is very low and dramatically increases after treatment of the cells with proteolytic enzymes. In the present study, we analyzed the mechanism of this augmentation. We found that the pro tease treatment failed to enhance the binding of monomeric IgG to Fc g amma RIIB, increased the binding of small immune complexes (IC) prepar ed under antigen-excess conditions only modestly, but markedly enhance d the binding of large IC prepared under antibody-excess conditions. T hese results suggest that proteolysis increases the ligand-binding avi dity but not the intrinsic affinity of Fc gamma RIIB, Confocal laser s canning microscopy revealed that the mobility of Fc gamma RIIB on the cell surface was increased after protease treatment. In addition, tran sfection experiments indicated that the effect of proteolysis on IC bi nding to CHO cells expressing guinea pig Fc gamma RIIB was strongly de pendent on the receptor density. Finally, we demonstrated that the tra nsmembrane and cytoplasmic domains of Fc gamma RIIB were not involved in the proteolysis-induced augmentation of IC binding. Together our re sults suggest that the mobility of Fc gamma RIIB, which may be restric ted due to the association of the ectodomain of the receptor with unkn own membrane proteins, is enhanced by proteolysis, allowing the recept ors to bind multivalent ligands more readily and hence with higher avi dity.