1. Departments of Medicine, Laboratory Medicine and Pathobiology, and Medical Biophysics, and Centre for Research in Neurodegenerative Diseases, University of Toronto, Toronto, Ontario, Canada M5S 3H2
2. Institut de Pharmacologie Moléculaire et Cellulaire, UMR6097CNRS/UNSA, Equipe labellisée FRM, 06560 Valbonne, France
3. Department of Pathology, Albert Einstein College of Medicine, Bronx, New York, New York 10461, USA, and The Litwin-Zucker Research Center, North Shore-LIJ Institute for Medical Research, Manhasset, New York 11030, USA
4. Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94143, USA, and Proteomics Core, University Health Sciences Center, Aurora, Colorado 80045, USA
5. Department of Medicine (Division of Neurology), Toronto Western Hospital Research Institute, University Health Network, Toronto, Ontario, Canada M5G 1Z5
6. *These authors contributed equally to this work
7. †Present addresses: Shiga University, Oitsu, Japan (H.H.); Brain and Heart Centre, Hyogo, Japan (T.K.); Tokyo Medical and Dental University, Tokyo, Japan (N.S.)

Correspondence to: Peter St George-Hyslop^1,5 Correspondence and requests for materials should be addressed to P.StG.-H. (Email: p.hyslop@utoronto.ca).

Abstract

The presenilin proteins (PS1 and PS2)^{1, 2} and their interacting partners nicastrin³, aph-1 (refs 4, 5) and pen-2 (ref. 5) form a series of high-molecular-mass, membrane-bound protein complexes^{6, 7, 8} that are necessary for -secretase and -secretase cleavage of selected type 1 transmembrane proteins, including the amyloid precursor protein⁹, Notch¹⁰ and cadherins¹¹. Modest cleavage activity can be generated by reconstituting these four proteins in yeast and Spodoptera frugiperda (sf9) cells^{12, 13, 14}. However, a critical but unanswered question about the biology of the presenilin complexes is how their activity is modulated in terms of substrate specificity and/or relative activities at the and sites. A corollary to this question is whether additional proteins in the presenilin complexes might subsume these putative regulatory functions. The hypothesis that additional proteins might exist in the presenilin complexes is supported by the fact that enzymatically active complexes have a mass that is much greater than predicted for a 1:1:1:1 stoichiometric complex (at least 650 kDa observed, compared with about 220 kDa predicted)^{6, 7, 8}. To address these questions we undertook a search for presenilin-interacting proteins that differentially affected - and -site cleavage events. Here we report that TMP21, a member of the p24 cargo protein family, is a component of presenilin complexes and differentially regulates -secretase cleavage without affecting -secretase activity.

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