POLARIZED SORTING OF BETA-AMYLOID PRECURSOR PROTEIN AND ITS PROTEOLYTIC PRODUCTS IN MDCK CELLS IS REGULATED BY 2 INDEPENDENT SIGNALS

Progressive cerebral deposition of the amyloid (A beta) beta-protein i s an early and invariant feature of Alzheimer's disease. A beta is der ived by proteolysis from the membrane-spanning beta-amyloid precursor protein (beta APP). beta APP is processed into various secreted produc ts, including soluble beta APP (APP(s)), the 4-kD A beta peptide, and a related 3-kD peptide (p3). We analyzed the mechanisms regulating the polarized basolateral sorting of beta APP and its proteolytic derivat ives in MDCK cells. Deletion of the last 32 amino acids (residues 664- 695) of the beta APP cytoplasmic tail had no influence on either the c onstitutive similar to 90% level of basolateral sorting of surface bet a APP, or the strong basolateral secretion of APP(s), A beta, and p3. However, deleting the last 42 amino acids (residues 654-695) or changi ng tyrosine 653 to alanine altered the distribution of cell surface be ta APP so that similar to 40-50% of the molecules were inserted apical ly. In parallel, A beta was now secreted from both surfaces. Surprisin gly, this change in surface beta APP had no influence on the basolater al secretion of APP(s) and p3. This result suggests that most beta APP molecules which give rise to APP(s) in MDCK cells are cleaved intrace llularly before reaching the surface. Consistent with this conclusion, we readily detected intracellular APP(s) in carbonate extracts of iso lated membrane vesicles. Moreover, ammonium chloride treatment resulte d in the equal secretion of APP(s) into both compartments, as occurs w ith other non-membranous, basolaterally secreted proteins, but it did not influence the polarity of cell surface beta APP These results demo nstrate that in epithelial cells two independent mechanisms mediate th e polarized trafficking of beta APP holoprotein and its major secreted derivative (APP(s)) and that A beta peptides are derived in part from beta APP holoprotein targeted to the cell surface by a signal that in cludes tyrosine 653.