Structural studies of murine lymphocyte surface IgD.

Lymphocyte surface IgD was labeled with 125I by the lactoperoxidase technique and subjected to cleavage with trypsin or staphylococcal V8 protease. Tryptic cleavage resulted in Fab monomers consisting of one light chain disulfide bonded to an Fd fragment of m.w. 30,000 and an Fc fragment of m.w. 60,000, unreduced. Upon reduction, the tryptic Fc consisted of one labeled fragment of 16,000 daltons when digested to completion. Before completion of digestion, intermediates of 35,000 and 20,000 daltons were observed. Thus, in addition to cleavage at the hinge, trypsin causes additional cleavages in the Fc, within disulfide loops. Cleavage with staphylococcal V8 protease resulted in an Fc fragment that consisted of disulfide-bonded 20,000-dalton subunits (sFc) and Fab’ fragments made up of one Fd’ fragment (40,000 daltons) disulfide bonded to one light chain. The sFc fragment exhibited a marked anodal shift in electrophoretic mobility in the presence of sodium deoxycholate, and a marked cathodal shift in the presence of cetyl trimethyl ammonium bromide. The Fab’ fragment showed no such shift. These results indicate that a) the only inter-heavy chain disulfide bonds are situated within the last two domains, and b) the carboxy terminal 20,000 daltons of IgD contain a region that is capable of binding detergent and thus of interacting with membrane lipid.