Mutant forms of presenilin (PS) 1 and 2 and amyloid precursor protein (APP) lead to familial Alzheimer's disease. Several reports indicate that PS may modulate APP export from the endoplasmic reticulum (ER). To develop a test of this possibility, we reconstituted the capture of APP and PS1 in COPII (coat protein complex II) vesicles formed from ER membranes in permeabilized cultured cells. The recombinant forms of mammalian COPII proteins were active in a reaction that measures coat subunit assembly and coated vesicle budding on chemically defined synthetic liposomes. However, the recombinant COPII proteins were not active in cargo capture and vesicle budding from microsomal membranes. In contrast, rat liver cytosol was active in stimulating the sorting and packaging of APP, PS1, and p58 (an itinerant ER to Golgi marker protein) into transport vesicles from donor ER membranes. Budding was stimulated in dilute cytosol by the addition of recombinant COPII proteins. Fractionation of the cytosol suggested one or more additional proteins other than the COPII subunits may be essential for cargo selection or vesicle formation from the mammalian ER membrane. The recombinant Sec24C specifically recognized the APP C-terminal region for packaging. Titration of Sarla distinguished the packaging requirements of APP and PS1. Furthermore, APP packaging was not affected by deletion of PS1 or PS1 and 2, suggesting APP and PS1 trafficking from the ER are normally uncoupled.

γ-Secretase is responsible for proteolytic maturation of signaling and cell surface proteins, including amyloid precursor protein (APP). Abnormal processing of APP by γ-secretase produces a fragment, Aβ42, that may be responsible for Alzheimer's disease (AD). The biogenesis and trafficking of this important enzyme in relation to aberrant Aβ processing is not well defined. Using a cell-free reaction to monitor the exit of cargo proteins from the endoplasmic reticulum (ER), we have isolated a transient intermediate of γ-secretase. Here, we provide direct evidence that the γ-secretase complex is formed in an inactive complex at or before the assembly of an ER transport vesicle dependent on the COPII sorting subunit, Sec24A. Maturation of the holoenzyme is achieved in a subsequent compartment. Two familial AD (FAD)–linked PS1 variants are inefficiently packaged into transport vesicles generated from the ER. Our results suggest that aberrant trafficking of PS1 may contribute to disease pathology.