Selective anchoring in the specific plasma membrane domain: a role in epithelial cell polarity

Previous work has shown that incubation in low calcium medium (~5 laM) prevents MDCK cells from forming complete cell-cell contacts (27) . Under these conditions A2 antigen (184 kD) was found completely polarized, while B1 (63 kD) was not (82) , and only very discontinuous tight junctional structures can be observed by ZO-1 monoclonal antibody fluorescence (76, 84) . In this work, those observations were confirmed and extended to two more membrane proteins: A1 was polarized in low calcium medium (Fig. 5 A) while B2 was not (Fig. 5 E) . Strikingly, both basolateral markers were almost totally extracted in 0.5 % Triton X-100 from monolayers developed in low calcium medium. Only a small plateau of cytoplasmic fluorescence was left after detergent extraction (Fig. 5, D and F ). This result was independently confirmed by RIA (not shown). On the other hand, the apical antigen A1 was resistant to TX-100 extraction in low calcium medium, and was preferentially removed from the basal (attached) surface (Fig. 5 B) . Thus, the cytoskeletal linkage of A1 is not dependent on the establishment of normal cell-cell contacts, as is the case with the basolateral antigens B1 and B2.