The membrane receptor for polymeric immunoglobulin is structurally related to secretory component. Isolation and characterization of membrane secretory component from rabbit liver and mammary gland

The membrane receptor of epithelial cells which binds polymeric immunoglobulins and mediates their transepithelial translocation is antigenically related to secretory component (SC), a glycoprotein produced by the epithelial cells. In rabbit milk, secreted SC is found either bound to polymeric immunoglobulin A (IgA) or in a free form. It is comprised of a heterogeneous population of molecules with apparent Mr = 83,000 and 80,000 (upper doublet) and 58,000 and 55,000 (lower doublet). Membrane SC was isolated by immunoadsorption from deoxycholate-solubilized plasma membranes of rabbit liver and mammary gland. The purified proteins, heterogenous in size with apparent Mr = 120,000 and 116,000 (upper doublet) and 95,000 and 91,000 (lower doublet), were amphiphilic. One-dimensional peptide maps revealed extensive structural homology between both the upper doublets of the membrane and secreted SC, as well as between the lower doublets. This indicates that the high Mr membrane SC of hepatocytes and mammary cells are precursors of the lower Mr secreted SC found in bile and milk. All four membrane SC, purified in high yield from rabbit liver by immunoadsorption and preparative electrophoresis in the presence of sodium dodecyl sulfate, bound IgA dimer specifically. The affinity (Kd approximately equal to 10 nM) was similar to the one observed in free SC-IgA dimer interaction. Only the membrane proteins recognized by the anti-SC antibodies were able to bind polymeric IgA. In liver membrane preparations, the receptor was localized at the surface of smooth vesicles, using IgA dimer-biotin and avidin-gold. Our results indicate that the high Mr membrane SC is the plasma membrane receptor for polymeric immunoglobulin.