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

To study the distribution of the TX-100-insoluble and -soluble fractions, MDCK cell monolayers were extracted, fixed, frozen sectioned, and processed for immunofluorescence with A1 and BI monoclonals (Fig. 3) . Both markers showed a relatively "imperfect" polarity with small but detectable levels of mAb binding to the "incorrect" surface (i.e., the basolateral surface for an apical marker; Fig. 3 , A and C, arrows). The extraction with TX-100 resulted in the disappearance of B1 fluorescence from the apical surface and of A1 fluorescence from the basolateral plasma membrane domain (Fig. 3 , B and D, arrows). Strikingly, antigen in the correct domain was relatively unextracted, suggesting binding to the submembrane cytoskeleton. Similar results were observed for two more basolateral markers: B2 and B2-m, the physiological endogenous light chain of class I major histocompatibility antigens, DLA (not shown).