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Biogenesis of 
-secretase early in the secretory pathway
Correspondence to Randy W. Schekman: schekman{at}berkeley.edu
-Secretase is responsible for proteolytic maturation of signaling and cell surface proteins, including amyloid precursor protein (APP). Abnormal processing of APP by -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 -secretase. Here, we provide direct evidence that the -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.
Abbreviations used in this paper: APP, amyloid precursor protein; COPII, coat protein complex II; CTF, C-terminal fragment; ERGIC, ER-to-Golgi intermediate compartments; FAD, familial Alzheimer's disease; FL, full length; NTF, N-terminal fragment; PS, presenilin; SIC, semi-intact cell; WT, wild type.
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