Post-translational processing of β-secretase (β-amyloid-converting enzyme) and its ectodomain shedding -: The pro- and transmembrane/cytosolic domains affect its cellular activity and amyloid-β production

Processing of the beta -amyloid precursor protein (beta APP) by beta- and gamma -secretases generates the amyloidogenic peptide A beta, a major factor in the etiology of Alzheimer's disease. Following the recent identification of the beta -secretase beta -amyloid-converting enzyme (BACE), we herein investigate its zymogen processing, molecular properties, and cellular trafficking. Our data show that among the proprotein convertase family members, furin is the major converting enzyme of pro-BACE into BACE within the trans-Golgi network of HK293 cells. While we demonstrate that; the 24-amino acid prosegment is required for the efficient exit of pro-BACE from the endoplasmic reticulum, it may not play a strong inhibitory role since we observe that pro-BACE can produce significant quantities of the Swedish mutant beta APP(sw) beta -secretase product C99, BACE is palmitoylated at three Cys residues within its transmembrane/cytosolic tail and is sulfated at mature N-glycosylated moieties, Data with three different antibodies show that a small fraction of membrane-bound BACE is shed into the medium and that the extent of ectodomain shedding is palmitoylation-dependent. Overexpression of full-length BACE causes a significant increase in the production of C99 and a decrease in the alpha -secretase product APPs alpha. Although there is little increase in the generation of A beta by full-length BACE, overexpression of either a soluble form of BACE (equivalent to the shed form) or one lacking the prosegment leads to enhanced A beta levels. These findings suggest that the shedding of BACE may play a role in the amyloidogenic processing of beta APP.