Figure 5.

Redistribution of plasma membrane receptors requires endocytosis. (A) Comparison of endocytosis-competent and endocytosis-defective receptors. Filter-grown MDCK cells expressing either the LDLR (top row) or a nonendocytosed mutant of the LDLR (LDLR 18YA, bottom row) were fixed and stained for surface receptor with C7 IgG. Both receptors were expressed basolaterally (left column). After Lat B treatment for 30 min (right column), the wild-type receptor was partially redistributed to the apical membrane. In contrast, the mutant receptor was not redistributed by Lat B treatment, except in a few isolated examples (bottom row, right column). (B) Quantitative assay of receptor polarity after Lat B treatment. The polarity of the TfnR, LDLR, and LDLR-18YA was assessed by binding ¹²⁵I-Tfn, or ¹²⁵I-C7 IgG to the apical or basolateral surfaces of filter-grown MDCK cells. Background levels were determined from binding to nontransfected cells. Values are derived by summing pairs of wells and expressing to counts bound to each surface as a percentage of the total bound to an apical/basolateral pair. All assays were in triplicate. Lat B treatment for 30 min resulted in depolarization of the TfnR and LDLR but not of LDLR-18YA.