Interaction with mLin-7 alters the targeting of endocytosed transmembrane proteins in mammalian epithelial cells

To investigate the targeting mechanism for proteins bound to the mammalian Lin-7 (mLin-7) PDZ domain, we created receptor protein chimeras composed of the carboxyl-terminal amino acids of LET-23 fused to truncated nerve growt h factor receptor/P75. mLin-7 bound to the chimera with a wild-type LET-23 carboxyl-terminal tail (P75t-Let23WT), but not a mutant tail (P75t-Let23MUT ). In Madin-Darby canine kidney (MDCK) cells, P75t-Let23WT localized to the basolateral plasma membrane domain, whereas P75t-Let23MUT remained apical. Furthermore, mutant mLin-7 constructs acted as dominant interfering protei ns and inhibited the basolateral localization of P75t-Let23WT. The mechanis ms for this differential localization were examined further, and, initially , we found that P75t-Let23WT and P75t-Let23MUT were delivered equally to th e apical and basolateral plasma membrane domains. Although basolateral rete ntion of P75t-Let23WT, but not P75t-Let23MUT, was observed, the greatest di fference in receptor localization was seen in the rapid trafficking of P75t -Let23WT to the basolateral plasma membrane domain after endocytosis, where as P75t-Let23MUT was degraded in lysosomes, indicating that mLin-7 binding can alter the fate of endocytosed proteins. Altogether, these data support a model for basolateral protein targeting in mammalian epithelial cells dep endent on protein-protein interactions with mLin-7, and also suggest a dyna mic role for mLin-7 in endosomal sorting.