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

In this work, we have used four monoclonal antibodies directed against cellular plasma membrane antigens of MDCK cells. The key features of these antibodies and their antigens are illustrated in Table I . Two of them react with apical membrane proteins ($8/1M13 and $2/2G1) and will be hereafter referred to as A1 and A2, respectively. The other two react with basolateral proteins ($2/3G2 and $7/5G23) and will be referred to as B1 and B2. The antigens for all four of them showed membrane associated fluorescence images in semi-thin frozen sections (see Figs. 3, 4, and 5). A1, B1, and B2 behaved as integral membrane proteins according to both of the following criteria: (a) <14% was extractable in 0.1 M NaECO3, pH 11 (26) , and (b) >70% partitioned with the detergent phase when the cells were extracted with Triton X-114 (reference 7). A2, the best polarized of all four antigens, although resistant to alkali extraction, partitioned pref-erentiaUy with the water phase during Triton X-114 phase separation. Other integral membrane proteins, including the acetylcholine receptor (40) , uvomorulin (28) , and the platelet Ib glycoprotein (14) have been shown to behave like A2 during Triton X-114 phase separation. On the other hand, mutants in the p62/E2 protein, an integral membrane protein of Semliki Forest virus have been shown to be an exception to the high pH criterion (15) .