Evidence that the CH2 domain of IgG contains the recognition unit for binding by the fetal rabbit yolk sac membrane receptor
Johanson, R.A.; Shaw, A.R.; Schlamowitz, M.
Journal of Immunology 126(1): 194-199
1981
ISSN/ISBN: 0022-1767 PMID: 6778915 Document Number: 179079
The maternofetal transport of IgG in rabbits is mediated by an IgG-specific receptor on the yolk sac membrane (YSM). To localize the receptor recognition unit (RRU) in IgG, fragments of IgG were prepared by controlled enzymatic digestions and were purified under mild, nondenaturing conditions. By using in vitro assay systems that measured the binding of 125I-labeled IgG or IgG fragments to the receptor, the relative molar bindings of IgG and Facb (IgG minus the CH3 domain) were found to be, respectively, 100 and 162%. Fab and pFc', which contain no CH2 domain, showed no significant binding (3 and 5%, respectively). The data showing that Facb binds better than IgG suggest that removal of the CH3 domain allows the RRU to be more readily accessible to the receptor. Mild reduction and alkylation was carried out to split the inter-H chain disulfide bonds of IgG and Facb. Under the conditions used, the 2 halves of the reduced and alkylated IgG remained associated through noncovalent forces. Changes in the hinge region tertiary structure were indicated by increases in the hydrodynamic volume and the susceptibility to papain digestion. In the case of reduced and alkylated Facb, the half molecules freely dissociated. Despite these changes, the reduced and alkylated IgG and Facb retained most of the binding activity of the parent proteins (i.e., 83 and 75%, respectively). The integrity of the inter-H chain disulfide bond apparently is not essential for expression of RRU activity. The principal, if not the only, RRU recognized by the IgG-specific YSM receptor is localized in the CH2 domain.