CD23 FC-EPSILON-RII AND ITS SOLUBLE FRAGMENTS CAN FORM OLIGOMERS ON THE CELL-SURFACE AND IN SOLUTION/

Human CD23 (also known as Fc epsilon RII) is a 45 000 MW glycoprotein with homology to C-type animal lectins. It is involved in B-cell diffe rentiation and IgE regulation, and is naturally cleaved to give solubl e products of 37 000, 33 000, 29 000, 25 000 and 16 000 MW. Previous w ork has suggested that the region between the transmembrane sequence a nd the extracellular lectin head is capable of forming an alpha-helica l coiled coil, one of the main consequences of which would be formatio n of dimers or trimers. Here we present protein-protein cross-linking data showing that CD23 forms trimers on the cell surface and hexamers in solution, and we use several different fragments to determine the r egions of the protein involved in this self-association. The region of the putative coiled coil is indeed responsible for trimerization, wit h additional interactions between the lectin heads resulting in the fo rmation of hexamers observed in solution.