Fig. 4. Surface labelling of CEACAM1-L and CEACAM1-S expressing MDCK cells. Untransfected MDCK cells and MDCK cells transfected with CEACAM1-L or CEACAM1-S were grown to confluence on permeable filters and incubated with anti-CEACAM1 Fab fragments from either the apical (A) or the basolateral (BL) compartments. The cells were solubilized, bound Fab fragments were retrieved, and complexed CEACAM1 was detected by immunoblotting. No immunoreactive proteins were detected in the untransfected cells. In the transfected cells, CEACAM1-L was retrieved both from the apical and the basolateral surfaces, whereas CEACAM1-S was retrieved only from the apical surfaces.