SIGNAL-DEPENDENT TRAFFICKING OF BETA-AMYLOID PRECURSOR PROTEIN-TRANSFERRIN RECEPTOR CHIMERAS IN MADIN-DARBY CANINE KIDNEY-CELLS

We have investigated the intracellular trafficking of a chimeric molec ule consisting of the cytoplasmic domain of the beta-amyloid precursor protein (APP) and the transmembrane region and external domain of the human transferrin receptor (TR) in Madin-Darby canine kidney cells. N ewly synthesized APP-TR chimeras are selectively targeted to the basol ateral surface by a tyrosine-dependent sorting signal in the APP cytop lasmic tail. APP-TR chimeras are then rapidly internalized from the ba solateral surface and a significant fraction (similar to 20-30%) are d egraded. Morphological studies show that APP-TR chimeras internalized from the basolateral surface are found in tubulo-vesicular endosomal e lements, internal membranes of multivesicular bodies, and lysosomes. A PP-TR chimeras are also found in 60-nm diameter vesicles previously sh own to selectively deliver wild-type TR to the basolateral surface; th is result is consistent with the fact that 90% of internalized chimera s that are not degraded are selectively recycled back to the basolater al surface. APP-TR chimeras internalized from the apical surface are s electively transcytosed to the basolateral surface underscoring the im portance of basolateral sorting in the endocytic pathway for maintaini ng the polarized phenotype. Tyr-653, an important element of the YTSI internalization signal in the APP cytoplasmic domain, is required for basolateral sorting in the biosynthetic and endocytic pathways. Howeve r, the structural features for basolateral sorting differ from those r equired for internalization.