BETA-2-MICROGLOBULIN CO-DISTRIBUTES WITH THE HEAVY-CHAIN OF THE INTESTINAL IGG-FC RECEPTOR THROUGHOUT THE TRANSEPITHELIAL TRANSPORT PATHWAYOF THE NEONATAL RAT

Maternal IgG crosses the proximal small intestine of the suckling rat by receptor-mediated endocytosis and transepithelial transport, The Fc receptor resembles the major histocompatibility complex class I antig ens in that it consists of two subunits: a transmembrane glycoprotein (gp50) in association with beta(2)-microglobulin. We used immunofluore scence microscopy and quantitative immunogold cytochemistry to study t he subcellular distribution of the two subunits. In mature absorptive cells both subunits were colocalized in each of the membrane compartme nts that mediate transcytosis of IgG. IgG administered in situ apparen tly caused both subunits to concentrate within endocytic pits of the a pical plasma membrane, suggesting that ligand causes redistribution of receptors at this site. These results support a model for transport i n which IgG is transferred across the cell as a complex with both subu nits, During absorptive cell differentiation, gp50 and beta(2)-microgl obulin showed nearly identical patterns of increased expression that a ccompanied the development of the apical endocytic apparatus and termi nal web, However, absorptive cells in weanling rats expressed no detec table gp50 and only low levels of beta(2)-microglobulin in the Golgi r egion and on the basolateral plasma membrane where class I antigens wo uld likely reside, Thus, beta(2)-microglobulin has a novel distributio n unrelated to its function as a subunit of the class I antigens. The co-expression of the two receptor subunits is restricted to neonatal e pithelial cells engaged in IgG transport and is coordinately regulated during absorptive cell differentiation and during postnatal intestina l development.