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

Each membrane antigen studied here showed a characteristic Triton X-100-insoluble fraction. This fraction was largely dependent on the existence of cell-cell contacts for B1 and B2: both antigens were totally extractable in cells with incomplete cell-cell contacts. Nelson and Veshnock (48) have recently reported that the development of an insoluble network of fodrin in the basolateral membrane is dependent on the establishment of cell-cell contacts and therefore can be blocked by incubation in 5 ~tM Ca ++. They have also presented evidence for a specific interaction between a basolateral marker of MDCK cells, Na+-K ÷ ATPase, and ankyrin (50), a protein known to form complexes with spectrin in the red cell (5, 9) . In this work, we have shown that the extractability of B1 and B2 is modified by conditions that affect the stability of the fodrin network (Table III) . Thus, in combination with Nelson and Veshnock's results, our data suggest that the polarization of several basolateral markers is linked to the formation of a basolateral fodrin network, with which they appear to interact. Whether this interaction is direct, through the cytoplasmic domain of the basolateral proteins, or indirect, via an intermediate membrane protein, is still open to study. Because insoluble fodrin is much more stable (49) , linkage to this network may result in a prolonged half-life for the membrane proteins that would contribute to their relative polarization to the basolateral membrane.