The presenilin proteins (PS1 and PS2)1,2and their interacting partners nicastrin3, aph-1 (refs4, 5) and pen-2 (ref.5) forma series of high-molecular-mass, membrane-bound protein complexes6-8that are necessary for γ-secretase and ε-secretase cleavage of selected type 1 transmembrane proteins, including the amyloid precursor protein9, Notch10and cadherins11. Modest cleavage activity can be generated by reconstituting these four proteins in yeast andSpodoptera frugiperda(sf9) cells12-14. 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 γ and ε 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)6-8. To address these questions we undertook a search for presenilin-interacting proteins that differentially affected γ- and ε-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 γ-secretase cleavage without affecting ε-secretase activity.