Functional expression of FcRn in polarized T84 cells. T84 cells grown on collagen-coated Transwells were cell surface–biotinylated and solubilized at either pH 6.0 or pH 8.0 lysates were incubated with Sepharose beads (N) or beads coupled to IgG (IgG). Immunoprecipitated proteins were analyzed by SDS-PAGE and avidin blot. A 12-kDa band consistent with the mobility of β₂M (small arrow) and a 45-kDa band consistent with the mobility of human FcRn α chain (large arrow) were immunoprecipitated by IgG-coupled beads at pH 6.0 but not at pH 8.0.